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space junk

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space junk

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space junk

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What if there was a black hole in your pocket?

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space junk

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Black Holes Explained – From Birth to Death

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What Does The Inside Of A Black Hole Look Like?

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This video describes how astronomers have accurately measured the diameter of the faraway dwarf planet Eris for the first time by catching it as it passed in front of a faint star. This event was seen at the end of 2010 by telescopes in Chile, including the TRAPPIST telescope at ESO's La Silla Observatory.

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10 Amazing Facts About Dwarf Planet Eris

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Eris is the most massive and second-largest dwarf planet known in the Solar System.

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Planet Song for Kids/Solar System Songs for Children/Eris Song for Kids

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Learn about our dwarf planet Eris in this amazing animation and awesome song. And don't forget to sing along. Facebook: https://www.facebook.com/kidslearningtube Subscribe: http://www.youtube.com/c/kidslearningtube Tweet Us: https://twitter.com/learningtube Instagram: https://instagram.com/kidslearningtube Add us on Google+: https://plus.google.com/+KidsLearning...

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To the Galaxy and Beyond!

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Our galaxy, the Milky Way, is typical: it has hundreds of billions of stars, enough gas and dust to make billions more stars, and about six times as much dark matter as all the stars and gas put together. And it's all held together by gravity.

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Stars are the most widely recognized astronomical objects, and represent the most fundamental building blocks of galaxies. The age, distribution, and composition of the stars in a galaxy trace the history, dynamics, and evolution of that galaxy. Moreover, stars are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen, and oxygen, [...]

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The Milky Way Galaxy is our home galaxy in the universe. It is a fairly typical barred spiral with four major arms in its disk, at least one spur, and a newly discovered outer arm. The galactic centre, which is located about 26,000 light-years from Earth, contains at least one supermassive black hole (called Sagittarius A*), and is crossed by a bar. The Milky Way began forming around 12 billion years ago and is part of a group of about 50 galaxies called the Local Group. The Andromeda Galaxy is part of this group as are numerous smaller galaxies, including the Magellanic Clouds. The Local Group itself is part of a larger gathering of galaxies called the Virgo Supercluster of galaxies.

Milky Way Galaxy Profile

Facts about the Milky Way

The Milky Way began as a series of dense regions in the early universe not long after the Big Bang. The first stars to form were in globular clusters that still exist. They are among the oldest stars formed in the Milky Way region.

The Milky Way has grown by merging with other galaxies through time. It is currently acquiring stars from a very small galaxy called the Sagittarius Dwarf Spheroidal, as well as gobbling up material from the Magellanic Clouds.

The Milky Way moves through space at a velocity of about 552 kilometres per second (343 miles per second) with respect to the Cosmic Microwave Background radiation.

The Milky Way's central core contains a supermassive black hole. It is commonly referred to as Sagittarius A*. It contains the mass of about 4.3 million Suns.

The stars, gas and dust of the Milky Way all orbit the centre at a rate of about 220 kilometres per second. This constant rate for all stars at different distances from the core implies the existence of a shell of dark matter surrounding our galaxy.

Our galaxy will collide with Andromeda Galaxy in about 5 billion years. Some astronomers refer to our two galaxy as a binary system of giant spirals.

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The Milky Way Galaxy is our home galaxy in the universe. It is a fairly typical barred spiral with four major arms in its disk, at least one spur, and a newly discovered outer arm. The galactic centre, which is located about 26,000 light-years from Earth, contains at least one supermassive black hole (called Sagittarius ...

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This Galaxy shouldn't even be there!!!!

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Peering back in time to find the very earliest objects in the universe, an international team of astronomers has discovered a galaxy that shouldn't be there at all.

The problem, the scientists report Monday in Nature, is that while the tiny galaxy dates from just 700 million years or so after the big bang, it's far more dusty than something this young and small has any right to be.

It's surprising, says Daniel Marrone, a University of Arizona expert on galaxy formation who wasn't involved in the research, because although dust is essential for the formation of planets and other solid material, the cosmos started out with no dust at all. In the aftermath of the big bang, the universe consisted only of hydrogen and helium gas (along with dark matter, but that's invisible).

The dusty galaxy is just one of the recent surprises astronomers have found. "Last week," says Marrone, "we learned of an incredibly massive black hole in the early universe. Now we have this average galaxy with significant amounts of dust. We've had this cartoon picture of the early universe, but it's clear that we really don't know what's going on."

What Astronomers Think They Know

The gas in the post-big bang universe condensed to form the first stars, which forged heavier elements, including carbon, silicon, and oxygen, then died and released those elements into space. It was from these that the first particles of dust formed.

Those first stars had already lived and died by the time this newly discovered galaxy, known as A1689-zD1, was up and running, so the universe was hardly dust free at that point. But most of the dust should have been in large, bright galaxies that formed lots of stars. A1689-zD1 is relatively small and dim-no bigger than the Large Magellanic Cloud, a dwarf galaxy that orbits the Milky Way.

Few early galaxies have been spotted, because even the brightest among them is far away and thus relatively dim. To get around that problem, astronomers use the magnifying effect of gravity. Einstein's general theory of relativity says the gravity from an object closer to Earth can warp light rays from a more distant one. (Einstein thought we'd never actually observe this so-called gravitational lensing, but he was dead wrong about that.)

In this case, the Nature paper's lead author, Darach Watson, of the deliciously named Dark Cosmology Centre at the University of Copenhagen, used the Very Large Telescope (VLT) in Chile's Atacama Desert to look at a massive cluster of galaxies called Abell 1689. The cluster's gravity magnified A1689-zD1 by a factor of nine, says Watson, which allowed him and several colleagues to gauge its distance from Earth and thus how long ago its light began traveling in our direction.

The VLT can see starlight but not dust, so Watson called on a colleague who was using the dust-sensitive ALMA radio telescope (see "Cosmic Dawn" in National Geographic magazine). "She had a look," says Watson, "and bingo!"

Like Marrone, Watson was taken aback by how dusty A1689-zD1 turned out to be. In bigger galaxies with more massive, short-lived stars, dust can pile up quickly since giant stars explode violently as supernovae after just a few million years. In smaller galaxies, most of the dust should emerge from the more gentle deaths of smaller stars that live for billions of years. So maybe A1689-zD1's dust comes from supernovae after all, says Watson. "But they'd have to produce the maximum possible dust," he says, to account for what ALMA sees, "and the dust can't be destroyed."

The only way to find out for sure is to uncover more galaxies like A1689-zD1. Unfortunately, they're extremely difficult to find-and the one other example astronomers have is almost entirely dust free, so it's impossible to say which type is more common. "We don't have any other candidates at this point," says Watson.

With ALMA, completed two years ago, and a new generation of giant optical telescopes now under construction, however, they're likely to find more of these galaxies before long. (See " Cosmic Vision: Telescopes.") The very early universe doesn't make a lot of sense at the moment-but that's almost certainly going to change.

Follow Michael D. Lemonick on Twitter.

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Astronomers Find a Dusty Galaxy That Shouldn't Exist

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Peering back in time to find the very earliest objects in the universe, an international team of astronomers has discovered a galaxy that shouldn't be there at all. The problem, the scientists report Monday in Nature , is that while the tiny galaxy dates from just 700 million years or so after the big bang, it's far more dusty than something this young and small has any right to be.

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Our galaxy is nothing compared to the universe :)

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"Space is big," said Douglas Adams. "You just won't believe how vastly, hugely, mind-boggingly big it is." But why must this be so? And why does our Universe exhibit such tremendous scale, from the very tiny to the extremely large? Here are some possible answers.

From Micro to Macro

It's hard to wrap our heads around the size of our galaxy, let alone the Universe. At 100,000 light-years across, it would take the New Horizons space probe- the fastest object ever launched by humans -some 1,844,000,000 years to travel from one side of the Milky Way to the other (it's currently moving away from Pluto at 58,536 km/h or 36,373 mph).

But there are structures even larger than single galaxies. Back in 2013, astronomers discovered a concentration of galaxy clusters stretching some 10-billion light-years across. There are also cosmic filaments to consider-massive strands of rarefied and highly ionized gas which stretch like spider webs across the observable Universe linking galaxy clusters across billions upon billions of light-years. And then there's the Universe as a whole, an expanse of expanding space that appears to be about 92 billion light-years in diameter. And that's just the observable Universe.
Universe Today: How Big is the Universe?
But when it comes to size, there's more than the universe's cosmic vastness to consider. There's also its tremendous scale. Heading in the opposite direction, the microscopic world is similarly vast in scope. There are roughly 9.6 x 10 ²⁴ molecules in a single glass of water, while the human body is comprised of about 7 x 10 ²⁷ atoms, where the size of a single atom is about 10⁻⁸ centimeters. But once we get down to the Planck length, roughly 10⁻³³ centimeters, we enter into the realm of the absurdly small. Beyond this scale no meaningful measurements can be made, and classical physics breaks down.

From our perspective as humans, this vast range, from the smallest to the largest, is beyond extreme. It's inconceivable. But why should this be? Why can't the Universe be packed down into a more "manageable" size? Why the degree of micro, and why such macro? To us, this seems like a tremendous waste of space.

Anthropic Bias

One person who has given this subject considerable attention is theoretical cosmologist Sean Carroll, a research professor in the Department of Physics at the California Institute of Technology. As he explained to me, these are certainly interesting questions-but they're not necessarily the right questions to be asking.

"Whenever we seem surprised or confused about some aspect of the universe, it's because we have some pre-existing expectation for what it 'should' be like, or what a 'natural' universe might be," Carroll told io9. "But the universe doesn't have a purpose, and there's nothing more natural than Nature itself-so what we're really trying to do is figure out what our expectations should be."

Carroll says it's not surprising that humans are small compared to the universe, but big compared to atoms. That's a feature of our existence with an obvious anthropic explanation.

"Complex structures can only form on in-between scales, not at the very largest or very smallest sizes," he says. "Given that living organisms are going to be complex, it's no surprise that we find ourselves at an in-between size compared to the universe and compared to elementary particles."

Which is both a good and obvious point. The Universe appears the way it does to to us on account of our position as biological organisms. We're evolutionarily biased to perceive the Universe the way we do, both as creatures that formed in the "in-between scales," and owing to our psychological and cognitive predispositions, including our inability to comprehend large numbers and sizes beyond a certain point.

Chandra Observatory astrophysicist Jonathan McDowell, who works out of the Harvard-Smithsonian Center for Astrophysics, extends this line of thinking even further by invoking the anthropic principle, which states that observations of the Universe must be both consistent and compatible with the conscious life that observes it.

"It is the vast numbers of elementary particles and of space-time geometry quanta that give the illusion of continuity and the emergence of complexity," McDowell told io9. "We will eventually discover that our laws of physics are natural byproducts of the behaviour of rearrangements of 10-to-the-very-large-power particles."

In other words, the Universe has to be this way, otherwise we wouldn't be here to observe it. Moreover, our "perception" of the Universe is hideously limited, and even illusory, as McDowell points out.

Crafted by the Constants

And further to McDowell's point, we must also consider the laws of the Universe. According to Brian Koberline, an astrophysicist at the Rochester Institute of Technology, the apparent size of the Universe is a function of the strength of different physical constants.

"If they were different the 'scale' of the universe would be different," he told io9.
A hydrogen atom's orbital structure (credit: APS/Alan Stonebraker)
As an example, Koberlein points to the strength of the electromagnetic field-a physical law that has determined how large creatures like us can be; if it were weaker, atoms would be larger. Or, had the universe sprung into existence with a stronger gravity, it would have collapsed back into a dense state a long time ago.

Koberlein says that the speed of light is also a factor-a built-in speed limit that will preclude us from perceiving the Universe as anything but an inconceivably massive expanse of space.

"It limits how far we can travel in our lifetime, and even whether we can reach some of the most distant galaxies-which we can't," he says. "All of these physical constants determine the scale of a universe like ours."

The "Preposterous" Universe

There's also something very interesting-and even bizarre-about the size of the Universe as it pertains to particle-physics scales. The Planck scale is much, much smaller than the level of the atom. According to Sean Carroll, the difference between these two numbers is puzzling-an issue that's related to the " hierarchy problem" of particle physics. At the same time, however, the scale of the universe is roughly 10 ²⁹ centimeters across, which is enormous by any scale of microphysics. Carroll says it's perfectly reasonable to ask why.

"Part of the answer is that 'typical' configurations of stuff, given the laws of physics as we know them, tend to be very close to empty space," says Carroll. "That's a feature of general relativity, which says that space is dynamical, and can expand and contract. So you give me any particular configuration of matter in space, and I can find a lot more configurations where the same collection of matter is spread out over a much larger volume of space. So if we were to 'pick a random collection of stuff' obeying the laws of physics, it would be mostly empty space. Which our universe is, kind of."

But as Carroll points out, even empty space has a natural length scale, which is set by the cosmological constant, which is the value of the energy density of the vacuum of space. Back in 1988, physicists discovered that the cosmological constant is not quite zero, although it's very small. The length scale that it establishes-which is roughly the distance over which the curvature of space becomes appreciable owing to the cosmological constant-just happens to be the size of the universe we see today. Interestingly, because the cosmological constant is inversely proportional to this length scale, Carroll says that the question, "Why is the cosmological-constant length scale so large?" is the same as, "Why is the cosmological constant so small?"

All this raises two rather important questions. First, the Universe is expanding, but the length scale associated with the cosmological constant is a constant-so why are they roughly equal today? This is what cosmologists refer to as the " coincidence problem." Secondly, why is the cosmological constant scale so enormously larger than the Planck scale, or even the atomic scale for that matter? Carroll says there are no widely-accepted answers to either of these questions.

The Universe is so big, says Carroll, because the cosmological constant is so small. As to why the cosmological constant is so small, that remains a scientific mystery.
Note: An earlier version of this article incorrectly described "a smaller gravity" to collapse an early version of the universe. It should have read stronger. Note 2: An earlier version of this article incorrectly noted the scale of the universe at roughly 10²⁷ centimeters across. It's actually closer to 10²⁹ centimeters across.
Contact the author at george@io9.com and @dvorsky. Top image: Messier 15, an ancient globular cluster located 35,000 light-years away. It's one of the densest clusters ever discovered. Credit: NASA/ESA.

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Why is the Scale of the Universe so Freakishly Large?

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"Space is big," said Douglas Adams. "You just won't believe how vastly, hugely, mind-boggingly big it is." But why must this be so? And why does our Universe exhibit such tremendous scale, from the very tiny to the extremely large? Here are some possible answers.

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More about Galaxies colliding enjoy! :)

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Many galaxies are members of groups or clusters. Since groups and clusters contain so many galaxies relatively close together, it should not be surprising that galaxies sometimes collide with each other. In fact, the Milky Way Galaxy is colliding with the Sagittarius Dwarf Galaxy right now (see the SDSS First Discoveries for more information).

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Why do meteors make such big Craters?  :D

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by Gemma Lavender, 4 June 2013

How do space rocks make such large dents in celestial bodies?

The reason why craters are made on Earth is because, when these chunks of rock pass through the atmosphere, they are too large to be slowed down and smash into the surface at a velocity of around 7.5 to 12.5 miles per second. These solid pieces of debris that have broken off from asteroids or comets, called meteorites, often create impact craters on the surface of terrestrial planets and moons.

Two types of crater are often made by debris that crashes through the Earth's atmosphere - some are bowl-shaped, whereas some are of similar appearance but contain a raised central peak along with a risen rim which appears as a ring-shaped structure from a bird's-eye view. However a meteorite doesn't have to be large to make a great impact - the Barringer Crater in Arizona (pictured above), which has a diameter of almost a mile (1.6 kilometres) across, was made by a chunk of space rock about 0.02 to 0.03 miles (30 to 50 metres) in diameter. It is the speed of these pieces of rock that determine the crater's size. But that's not all.

The angle at which a meteorite impacts the ground, as well as the type of material it smashes into, are important factors. By studying impact craters, planetary scientists are able to piece together how the crater happened. In fact, simulations have been made that investigate such a scenario - LCOGT and Faulkes Telescope's Impact Simulator or Purdue University's Impact Earth are such examples. Give them a try!
Image credit: Shane Torgerson

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Why do some meteorites create such huge craters? | Space Facts - Astronomy, the Solar System & Outer Space | All About Space Magazine

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by Gemma Lavender, 4 June 2013 How do space rocks make such large dents in celestial bodies? The reason why craters are made on Earth is because, when these chunks of rock pass through the atmosphere, they are too large to be slowed down and smash into the surface at a velocity of around 7.5 to 12.5 miles per second.

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EMMA

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The Moon Of Mars - Phobos - Alien Mothership

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Uploaded by KryptoniteOne on 2015-12-15.

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Really good website if your looking into Galaxys :)

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COSMOS - The SAO Encyclopedia of Astronomy | COSMOS

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Cosmos is a unique astronomy reference written by research astronomers. Our encyclopedia entries are for a general audience who wish to know detailed information on a wide range of astronomical topics. Cosmos is an evolving resource, with new entries being added all the time. Click here- for information on the production of Cosmos.

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The Cassini mission is sending us better and better data and images of just what's happening on the surface of Saturn's moon Titan. And it's beginning to look awfully familiar...

The latest data NASA researchers have been pouring over shows new details about the strange lakes and seas that trickle across Saturn's frigid moon, Titan. It also draws comparisons between the only other interstellar body found to have similarly liquid lakes and seas on its surface-our very own Earth. Unlike our watery planet, though, Titan's lakes and seas are made up of pure liquid methane.

But how do the seas stay filled up with all that methane? One potential new explanation takes the liquid cycle we see here on Earth, tweaks it slightly to account for Titan's own conditions, and comes up with something pretty familiar: Rain.

Of course, instead of being made up of liquid water, that rain is also made up of pure liquid methane. Still, it is rain which, as it falls, fills up the lakes. These lakes then create Titan's shorelines, which look very like our own as you can see in this flyover visualization:

Researchers even suspect the weather along those Titan shorelines behaves a lot like the shores along our own seas, with temperatures along them influenced by the temperature in the lake.

But although the liquid cycle coupled with Titan's nitrogen-heavy atmosphere may look a lot like Earth, there's plenty of differences to separate them. For instance: Titan's almost total lack of oxygen, the freezing temperatures, and the pesky fact that its liquid methane filling up those seas instead of water. Still, it's quite a familiar sight to see in some incredibly strange terrain.

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Saturn's Moon Titan Is Looking Even More Earth-Like

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The Cassini mission is sending us better and better data and images of just what's happening on the surface of Saturn's moon Titan. And it's beginning to look awfully familiar... The latest data NASA researchers have been pouring over shows new details about the strange lakes and seas that trickle across Saturn's frigid moon, Titan.

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NASA's Mars Reconnaissance Orbiter took this image of the larger of Mars' two moons, Phobos, from a distance of about 6,800 kilometers (about 4,200 miles).

Credit: NASA/JPL/University of Arizona

The Martian moon Phobos orbits only a few thousand miles above the Red Planet's surface. Its proximity to its planet is one of the reasons astronomers were unable to see the satellite until the late 19th century. In fact, the moon is getting closer to Mars over the centuries, and eventually will either break up or be pulled into the Martian surface.

In the early 17th century, German astronomer Johannes Kepler proposed that Mars might host two moons, given that it lay between the Earth and Jupiter, which were known to have one and four satellites. No evidence of such moons could be found. Although most people thought that Mars had no moons, American astronomer Asaph Hall performed a methodical study from the U.S. Naval Observatory in Washington, D.C., searching closer to the planet than previous surveys.

After searching without success, a frustrated Hall was about to give up, but his wife, Angelina, urged him to continue. The next night, Aug. 12, 1877, he discovered the moon that would later be known as Deimos. Six days later, he found Phobos as well. The two moons lay so close to the planet that they were hidden by the glare of Mars. Asteroid-size, they are also two of the smallest moons in the solar system, with the larger Phobos 7.24 times as massive as its companion, Deimos.

In Greek mythology, Phobos was one of the sons of the god of war Ares (Mars to the Romans). The twin sons attended their father in battle. Phobos means fear (as in phobia), while Deimos means flight (especially in the sense of fleeing after a defeat), according to NASA.

NASA's Mars rover Curiosity snapped this photo of the larger Martian moon Phobos during a Mars sky observing session. Phobos is Mars' larger moon, but only 14 miles across. Image released Sept. 26, 2012.

Credit: NASA/JPL

It took almost another century for scientists to begin to understand the two tiny Martian moons. In 1971, NASA's Mariner 9 spacecraft became the first manmade satellite to orbit another planet. Images from the craft revealed that both Phobos and Deimos have lumpy, potato-like shapes, rather than spherical like most moons. Observations of Phobos were limited by the tidal locking of the moon to the planet, resulting in the same side always facing outward.

As the exploration of continued, scientists were able to learn more information about the satellites circling Mars. The Viking orbiters flew by in the late 1970s. The Soviet Phobos 2 mission, NASA's Mars Global Surveyor, and the European Mars Express all provided more clues about the two curious moons. Rovers from the planet's surface even got in on the act, with Spirit, Opportunity, and Curiosity all providing images from the ground. NASA's MAVEN (Mars Atmosphere and Volatile Evolution mission) has also imaged the moon to try to learn more about Phobos' composition.

In 2011, Russia attempted to send a mission to the Martian moon called Phobos-Grunt, which was supposed to come back to Earth in 2014 bearing a small sample of the moon. The spacecraft, however, was marooned in Earth orbit due to issues with its rocket. Phobos-Grunt fell back to Earth in early 2012 and crashed into the Pacific Ocean.

NASA may not be finished with Phobos yet. The space agency is considering the Phobos Surveyor mission, which would deploy small, hedgehog-like probes to the surface of the moon. The mission is funded under NASA's Innovative Advanced Concepts program, which looks at concepts that could be decades away from flight.

In 2024, the Japan Aerospace Exploration Agency (JAXA) plans to launch the Mars Moons eXploration (MMX) mission to visit both Phobos and Deimos. MMX will land on the surface of Phobos and collect samples to be returned to Earth in 2029.

"Understanding how Phobos and Deimos formed has been a goal of the planetary science community for many years," David Lawrence, of the Applied Physics Laboratory, said in a statement. Lawrence leads the team to develop one of the instruments for MMX.

In 2016, a low-cost Mars orbiter mission called PADME (Phobos And Deimos and Mars Environment) was proposed to visit the moons. Among its major aims is to characterize where Deimos and Phobos came from - whether they were created at the same time as Mars, or captured from the asteroid belt, or somewhere else. PADME competed in NASA's Discovery Program but lost out to the Psyche and Lucy missions to visit asteroids.

Some human spaceflight proponents have also suggested NASA should land people on Phobos before Mars; NASA hopes to have a human mission sent there by the 2030s. But future explorers may be in for a shock. Powerful solar eruptions could charge regions of the Martian moon to hundreds of volts, potentially affecting electronic equipment.

"We found that astronauts or rovers could accumulate significant electric charges when traversing the night side of Phobos - the side facing Mars during the Martian day," William Farrell of NASA's Goddard Space Flight Center, Greenbelt, Maryland, said in a statement.

"While we don't expect these charges to be large enough to injure an astronaut, they are potentially large enough to affect sensitive equipment, so we would need to design spacesuits and equipment that minimizes any charging hazard."

The examination of Phobos and its companion revealed more than their odd, non-spherical shapes. The two moons are dark gray in color, and heavily cratered. The moons are some of the darkest, least reflective objects in the solar system.

After observing the pair, scientists concluded that they were made of material similar to Type I or II carbonaceous chondrites, the material that makes up asteroids and dwarf planets. The composition and odd shape led some scientists to conclude that Phobos and Deimos came from the asteroid belt, with Jupiter's gravity long ago nudging them into orbit around Mars.

However, scientists aren't certain that the asteroid belt is the source of the moons' birth. Both have a nearly circular orbit unusual for captured objects. The thin atmosphere of Mars would have a hard time providing the necessary braking to settle the pair into their present-day orbits. Similarly, the moons are not as dense as objects in the asteroid belt.

Instead, the moons may have formed as satellites around Mars, with dust and rock drawn together by gravity.

A more violent birth may have occurred by collision. A large impactor smashing into the red planet could have sent pieces flying into the air, where gravity may have drawn them together. An existing moon might also have been destroyed, creating the rubble that later formed Phobos and Deimos.

A recent proposal combines the last two possibilities. According to researchers, a collision once scattered debris into a ring around Mars. As Phobos approaches the red planet, it will be torn into a ring again, scientists predict.

"Solving the riddle of how Mars' moons came to be will help us better understand how planets formed around our sun and, in turn, around other stars," Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate (SMD), said in a statement.

Traveling only 3,700 miles (6,000 kilometers) above the Martian surface, Phobos flies around the Red Planet three times a day. Crossing the sky in about four hours, the moon appears to rise in the west and set in the east.

Many science fiction films feature large moons dominating alien skies, but despite its proximity to its planet, Phobos is only a third as wide as the full moon seen from Earth. On the other hand, Mars dominates the horizon of Phobos, taking up a fourth of the sky.

But Phobos won't zip around Mars forever. The doomed moon is spiraling inward at a rate of 1.8 centimeters (seven-tenths of an inch) per year, or 1.8 meters (about 6 feet) each century. Within 50 million years, the moon will either collide with its parent planet or be torn into rubble and scattered as a ring around Mars.

The unusual characteristics of Phobos, including its decaying orbit, led some scientists in the 1950s and 1960s to conclude that it was artificial. One prominent proponent was the science adviser to President Dwight D. Eisenhower. Further examination revealed that the moon bears stronger resemblance to a rubble pile than an artificial satellite, and images sent back from orbiting craft show that it formed in nature.

The larger of the two moons, Phobos has a diameter of 10 by 14 by 11 miles (17 by 22 by 18 km). The surface is covered with a dusty powder 3 feet (1 meter) thick, likely caused by meteor bombardment.

A large impact crater dominates the moon. Stretching nearly 6 miles (9.5 km), Stickney Crater covers most of the surface. The impact that formed it likely caused many secondary impacts, as rocks flew up and fell back to Phobos. The crater bears the maiden name of Hall's encouraging wife.

The moon is marred by long grooves. In 2015, a NASA-led study suggested that the grooves on Phobos are actually early signs of the moon breaking apart due to tidal stresses induced by Mars' gravity.

"We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves," Terry Hurford, a research assistant at NASA's Goddard Space Flight Center in Maryland, who led a study about stretch marks, said in a statement.

Another theory stated that the grooves could be remnants of a huge impact that left behind the crater Stickney, but the grooves did not center on this crater. Another explanation is the moon's grooves could come from material displaced from Mars that hits Phobos' surface, which was espoused in an Open University study published in 2014.

Temperatures vary on Phobos, reaching highs of 25 degrees Fahrenheit (minus 4 degrees Celsius) during the day, while nights can be as cold as minus 170 degrees F (minus 112 degrees C).

The moon is so small that a 150-pound person standing on its surface would weigh only two ounces.

Radius of moon: 6.9 miles (11.1 km)

Semi-major axis around Mars (distance from planet's center): 5,826 miles (9,376 km)

Closest approach: 5,738 miles (9,234 km)

Farthest approach: 5,914 miles (9,518 km)

Orbit eccentricity: 0.0151

Orbit inclination: 1.075 degrees

Time to make one orbit: 7.65 hours

Mass: 1.0659 x 10¹⁶ kg

Density: 1.872 g/cm³

Surface gravity: 0.0057m/s²

Escape velocity: 25 mph (41 km/h)

Follow Nola Taylor Redd at @NolaTRedd, Facebook, or Google+. Follow us at @Spacedotcom, Facebook or Google+.

Additional reporting by contributor Elizabeth Howell.

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Phobos: Facts About the Doomed Martian Moon

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The Martian moon Phobos orbits only a few thousand miles above the Red Planet's surface. Its proximity to its planet is one of the reasons astronomers were unable to see the satellite until the late 19th century. In fact, the moon is getting closer to Mars over the centuries, and eventually will either break up or be pulled into the Martian surface.

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EMMA

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IntroductionA solar system is a star and all of the objects that travel around it-planets, moons, asteroids, comets and meteoroids. Most stars host their own planets, so there are likely tens of billions of other solar systems in the Milky Way galaxy alone. Solar systems can also have more than one star. These are called binary star systems if there are two stars, or multi-star systems if there are three or more stars.

The solar system we call home is located in an outer spiral arm of the vast Milky Way galaxy. It consists of the Sun (our star) and everything that orbits around it. This includes the eight planets and their natural satellites (such as our moon), dwarf planets and their satellites, as well as asteroids, comets and countless particles of smaller debris. Explore Our Solar System ›

Did You Know

Five spacecraft have achieved enough velocity to travel beyond the boundaries of our solar system. Voyager 1 crossed into interstellar space in 2012. Voyager 2 and New Horizons are still active and will eventually transition to the space between the stars. Pioneers 10 and 11 also have reached escape velocity, but both spacecraft have been inactive for many years

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IntroductionA solar system is a star and all of the objects that travel around it-planets, moons, asteroids, comets and meteoroids. Most stars host their own planets, so there are likely tens of billions of other solar systems in the Milky Way galaxy alone. Solar systems can also have more than one star.

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Pluto's moons are even stranger and more intriguing than scientists imagined, a new study reveals.

The Pluto system consists of four tiny satellites - Nix, Hydra, Kerberos and Styx - orbiting a "binary planet" comprised of Pluto and its largest moon Charon, which, at 750 miles (1,207 kilometers) in diameter, is nearly half as wide as the dwarf planet itself.

This binary setup has profoundly influenced the orbits of the four small moons, injecting chaos into their movements - as depicted in a newly released animation of tumbling Nix - in ways not fully appreciated until now, the study suggests. [ Photos of Pluto and Its Moons]

"It's a very strange place to live in if you are orbiting a binary planet," lead author Mark Showalter, of the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, told Space.com.

Showalter and co-author Douglas Hamilton, of the University of Maryland, analyzed images of the Pluto system taken by NASA's Hubble Space Telescope between 2005 and 2012. (During this period, the observatory discovered all four of the dwarf planet's tiny known moons; Showalter led the teams that detected Kerberos and Styx.)

The two researchers used these photos - which captured brightness variations of the moons over time - and computer models to characterize the four small satellites and their orbits in unprecedented detail.

For example, Showalter and Hamilton derived new brightness and size estimates for the moons. They found that Nix and Hydra are likely about as bright as Charon, which reflects roughly 40 percent of the light that hits it.

Before the new study, "we didn't really know how big Nix and Hydra were, because we didn't know how bright they were," Showalter said. "And now, based on our analysis, we actually do pretty well know how big and how bright they are, so there are a lot of uncertainties about the properties of Nix and Hydra that have gone away now."

The Hubble images suggest that Hydra is around 28.2 miles (45.4 km) across, while Nix has a diameter of 24.6 miles (39.6 km) or so. Kerberos and Styx, meanwhile, are probably about 15.4 miles (24.8 km) and 4.2 miles (6.8 km) wide. (These latter two moons are tougher to characterize, because they are fainter than Nix and Hydra.)

These diameter estimates assume a spherical shape for the moons, which is likely not the reality; most if not all of the four tiny satellites are probably ellipsoidal, researchers said.

The duo's analysis also suggests that Kerberos is much darker than its fellow satellites, with a reflectivity of just 4 percent. Whereas Nix and Hydra are basically "dirty snowballs" in terms of reflectivity, "Kerberos is a charcoal briquet," Showalter said. [ Pluto's 5 Moons Explained (Infographic)]

This result "took us completely by surprise, because everybody has been assuming all along that the moons would be pretty similar," he added. "They all probably formed at the same time; they all are made of the same stuff."

Astronomers think Pluto's four small moons were formed from the debris scattered by a long-ago giant impact between a proto-Pluto object and a proto-Charon. Perhaps the proto-Charon was a very dark body, and Kerberos is a relatively pristine piece of this original impactor, Showalter said, though he stressed that this idea is mere speculation.

These illustrations of Pluto's moon Nix show how the orientation of the moon changes unpredictably while it orbits the Pluto-Charon system.

Credit: NASA, ESA, M. Showalter (SETI Inst.), G. Bacon (STScI)

Showalter and Hamilton also determined that Styx, Nix and Hydra are linked by a "resonance," a sort of gravitational sweet spot in which orbits of multiple celestial bodies are related by a ratio of two whole numbers. A similar three-body resonance is found among the Jupiter moons Io, Europa and Ganymede, the researchers said.

"The resonant relationship between Nix, Styx and Hydra makes their orbits more regular and predictable, which prevents them from crashing into one another," Hamilton said in a statement. "This is one reason why tiny Pluto is able to have so many moons."

But there is also quite a bit of chaos in the Pluto system, imparted by the complex and shifting gravitational field of the Pluto-Charon binary.

For instance, Showalter and Hamilton found that Nix and Hydra exhibit chaotic rather than synchronous rotation, meaning they don't always keep the same side facing Pluto-Charon - and that it's very tough to predict their rotational movement. (Nearly every other moon in the solar system, including Earth's, is a synchronous rotator.)

"If you lived on Nix, you would not know if the sun is coming up tomorrow; it is that extreme," Showalter said, adding that models suggest that Styx and Kerberos are chaotic rotators as well. "You'd have days where the sun rises in the east and sets in the north."

Such findings could help researchers better understand the many alien planets that orbit binary stars, researchers said.

"We are learning that chaos may be a common trait of binary systems," Hamilton said. "It might even have consequences for life on planets orbiting binary stars."

The new study was published online in the journal Nature today (June 3), just six weeks ahead of the first-ever Pluto flyby. On July 14, NASA's New Horizons spacecraft will zoom within 7,800 miles (12,500 km) of the dwarf planet's surface.

New Horizons should get good looks at the surfaces of Nix and Hydra, and the probe's observations will likely reveal how dark Kerberos is, Showalter said.

Data from the flyby, as well as continued long-term monitoring by Hubble and NASA's James Webb Space Telescope, which is scheduled to launch in 2018, could end up bringing Pluto's moons into sharp focus, he added.

"We've got pieces to start fitting together, and maybe eventually out of this we'll get a whole formation scenario for the [Pluto] system," Showalter said.

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Pluto's Moons Are Even Weirder Than Thought

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Pluto's moons are even stranger and more intriguing than scientists imagined, a new study reveals. The Pluto system consists of four tiny satellites - Nix, Hydra, Kerberos and Styx - orbiting a "binary planet" comprised of Pluto and its largest moon Charon, which, at 750 miles (1,207 kilometers) in diameter, is nearly half as wide as the dwarf planet itself.

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These close views of Pluto's four smaller moons were captured by NASA's New Horizons probe during its historic July 2015 flyby. Data from New Horizons suggests that at least two (and maybe even all four) were born from mergers of even smaller moons.

Credit: NASA/JHUAPL/SwRI

Pluto, the dwarf planet that was once considered the ninth planet, has a growing entourage of satellites. The tiny world has five moons of varying size in orbit around it that tumble and dance in a strange and chaotic pattern.

"The way I would describe this system is not just chaos, but pandemonium," Mark Showalter, a co-investigator on the New Horizons mission, said at a press conference last November.

To the surprise of many, NASA's New Horizons mission did not turn up any additional moons during its historical flyby.

Pluto has one very large moon that is almost half the planet's size. Discovered in 1978, it was named Charon after the demon who ferried souls to the underworld in Greek mythology. (Pluto is named for the god of the underworld.) The huge size of Charon (750 miles or 1,200 kilometers in diameter) sometimes leads scientists to refer to Pluto and Charon as a double dwarf planet or binary system. Pluto's diameter is 1,473 miles (2,370 km).

Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney, Australia. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation - a Pluto day - also takes 6.4 Earth days. This means Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking.

NASA's New Horizons spacecraft obtained this high-resolution enhanced color view of Pluto's moon Charon just before the closest approach on July 14, 2015.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

While Charon is mostly gray, its northern pole has a reddish tint. Most of the moon is covered with water-ice, but the red region is likely made up of shredded pieces of Pluto's atmosphere. The moon has a surprisingly young surface, suggestive that it once housed an ocean in its interior, one that would have long ago frozen.

Compared with most of solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun. Also, Pluto's rotation is retrograde compared to the other worlds - it spins backwards, from east to west.

In 2005, as scientists photographed Pluto with NASA's Hubble Space Telescope in preparation for the New Horizons mission - the first spacecraft to visit Pluto and the Kuiper Belt - they discovered two other tiny moons of Pluto, now dubbed Nix and Hydra. These are two to three times farther away from Pluto than Charon.

While NASA's New Horizons mission was able to capture significant details about Charon during its 2015 flyby, it was only able to view the other four moons from a distance. Still, it provided the best look at the satellites to date. The spacecraft revealed details as small as 2 miles (3 km) on Nix and 0.7 miles (1.2 km) on Hydra.

The mission also provided the best measurements for the tiny moons. Nix is estimated to be 26 miles (42 km) long and 22 miles (36 km) wide, while Hydra is 34 miles (55 km) in length.

Hydra has a shape resembling a mitten or a rubber duck with at least two large craters. The moon resembles Comet 67P/Churyumov-Gerasimenko, which Europe's Rosetta mission has been orbiting since August 2014. Scientists think that Comet 67P may have formed when two separate bodies merged, and the same process may have played a role in the formation of Hydra.

Hydra was the nine-headed mythological serpent that guarded Pluto's realm.

The irregularly shaped Nix looks much like a jellybean, with a surprisingly big impact crater. It is not as bright as Pluto, though brighter than Charon. In Greek mythology, Nyx was the goddess of the night and mother of Charon. The spelling was changed to Nix, the Egyptian goddess of night and darkness, to avoid confusion with two asteroids of the same name.

"Nix isn't very large, and there is a very fine line between an impact that will make a crater that big and one that will break Nix apart," New Horizons team member Simon Porter, of the Southwest Research Institute in Boulder, Colorado, wrote in a blog post.

"So, either Nix was very lucky in surviving that collision, or it's a fragment of an older moon that was somehow destroyed."

All four of the small moons appear to be coated by relatively pure water-ice.

Scientists using Hubble discovered a fourth moon, Kerberos, in 2011. New Horizons revealed that the moon has two lobes, one about 5 miles (8 km) wide and the other about 3 miles (5 km) in diameter. Like Hydra, it may have formed from the merger of two separate objects.

Kerberos orbits between Nix and Hydra. Despite its small size, Hubble observations revealed that it exerts a strong gravitational influence on the neighboring moons. Before New Horizons arrived, scientists thought the moon might be large but dark, keeping it obscured. Instead, the moon is small and bright. The moon was named for the three-headed dog in Greek mythology (Cerberus to the Romans).

The fifth moon, spotted in 2012, is named Styx after the river separating the living from the dead. It is estimated to be irregular in shape and 4.5 miles (7 km) wide. It is in a 59,030-mile (95,000-km) diameter circular orbit around Pluto that is assumed to lie in the same plane as Pluto's other known moons.

Scientists anticipated the discovery of more moons when New Horizons arrived at Pluto, but none were spotted.

Pluto's entire moon system is thought to have formed by a collision between Pluto and a similar-size body early in the history of the solar system. The collision threw out material that coalesced into the family of satellites orbiting Pluto.

New Horizons found that the four moons were surprisingly chaotic. Hydra is the fastest-rotating, spinning once every 10 hours during its 38-day orbit of Pluto, or 89 times every orbit.

"If Hydra were spinning much faster, material would fly off its surface due to the centrifugal force," Showalter said in a statement from NASA.

The other three moons rotate between 6 to 10 times during their orbit, which is still surprisingly fast. In addition to that, Nix is tilted on its axis by 132 degrees and rotating backward.

Such rapid rotations are surprising because the gravity of Pluto should slow down the spin of the smaller moons.

"The question is, why [don't they] slow down? It's not so much why [are they] so fast. ... There's clearly something fundamental about the dynamics of the system that we do not understand," Showalter said in the statement.

Additional reporting by Nola Taylor Redd, Space.com Contributor. Follow Nola on Twitter @NolaTRedd or Google+. Follow us at @Spacedotcom, Facebook or Google+. Originally published on Space.com.

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Pluto's Moons | Five Satellites of Pluto

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Pluto, the dwarf planet that was once considered the ninth planet, has a growing entourage of satellites. The tiny world has five moons of varying size in orbit around it that tumble and dance in a strange and chaotic pattern.

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So Little We Are........ Music: X-ray Dog - Here Comes The King

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Max

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A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon.

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National Geographic | Astrobiology & Space Travel

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Dark Energy | Universe Science Documentary

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The Secret of Black Hole - Full Documentary(2016)

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The Secret of Black Hole - Full Documentary(2016)

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Max

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Is there any possible way to take a black hole and terraform it to be a place we could actually live?

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Max

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Watch a NASA animation zooming in on a black hole as you listen to black hole sounds. The sounds are X-ray data from GRS 1915+105, translated into audible pulsations by MITs Edward Morgan. http://www.msnbc.msn.com/id/24111670/wid/11915829 This black hole is a near-extremal Kerr black hole.

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Black Holes Explained – From Birth to Death

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Space Travel Through Black Holes & Wormholes

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Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube.

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National Geographic | Astrobiology & Space Travel

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Space Travel Through Black Holes & Wormholes

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Black Hole, Wormhole & White Hole Facts (#Mind Blowing Documentary)

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In this edition of My Universe documentaries, we are going to take you through a journey where you can learn all about the black holes, wormholes and white . In this The Mysterious Universe documentary, we are going to present you the Supermassive Black Hole i.e., the biggest black hole in the universe.

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Space Travel Through Black Holes & Wormholes - Space Documentary

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You've heard of black holes but have you heard of white holes? Thoughty2 tells you exactly what a white hole is and wether they actually exist!

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Milky Way Galaxy Facts & Secrets | Best Universe Science

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Cosmic Holes -  Black Holes, White Holes & Worm Holes Facts ( Documentary) | Best Universe Science

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☞ Watching Movies : https://youtu.be/W3uhGxhPigg ☞ Playlish BUS : https://goo.gl/KiMy7l ☞ Please subscribe : https://goo.gl/y4m5kS ☞ Like Fanpage :https://www.facebook.com/BestUniverseScience ☞ Twitter : https://twitter.com/BUSDocumentary Today, we know black holes exist, and now scientists are trying to confirm that other holes lurk in hyperspace. Our infinite cosmos could contain a variety of "holes" such as black, white, "mini" and wormholes.

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Monster Of The Cosmos - Black Holes Documentary

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SUBSCRIBE FOR MORE DOCUMENTARY FILMS HD UPDATES: http://www.youtube.com/subscription_center?add_user=worlddocumentarych A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.

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PBS Genius By Stephen Hawking episode 1- Can We Time Travel ?

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PBS Genius By Stephen Hawking episode 1- Can We Time Travel ? Join Stephen Hawking as he challenges three ordinary people to determine if time travel is possible. The fascinating journey of discovery features DeLoreans, atomic clocks, a giant black hole and a large swath of New York City.

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NASA's historic Parker Solar Probe mission will revolutionize our understanding of the Sun, where changing conditions can propagate out into the solar system, affecting Earth and other worlds. Parker Solar Probe will travel through the Sun's atmosphere, closer to the surface than any spacecraft before it, facing brutal heat and radiation conditions-and ultimately providing humanity with the closest-ever observations of a star.

The Sun, at the heart of our solar system, is a yellow dwarf star, a hot ball of glowing gases. Its gravity holds the solar system together, keeping everything from the biggest planets to the smallest particles of debris in its orbit. Electric currents in the Sun generate a magnetic field that is carried out through the solar system by the solar wind-a stream of electrically charged gas blowing outward from the Sun in all directions.

The connection and interactions between the Sun and Earth drive the seasons, ocean currents, weather, climate, radiation belts and aurorae. Though it is special to us, there are billions of stars like our Sun scattered across the Milky Way galaxy. Explore the Sun ›

Ten Things to Know About the Sun

Pop Culture

Pop Culture

The Sun has inspired mythological stories in cultures around the world, including those of the ancient Egyptians, the Aztecs of Mexico, Native American tribes of North and South America, the Chinese, and many others.

In more recent times, the Sun adorns everything from album covers, such as Sublime's iconic 1992 debut, to packages of raisins, while it influences stories in comics, theatrical films and everything in between.

If you're Superman (or a fellow Kryptonian), your powers are heightened by the yellow glow of our Sun, and you can even dispose of dangerous materials like Superboy once did, by hurling them into the Sun. And in the 2007 film Sunshine, the Sun is dying, leaving Earth in a state of deep freeze. To save humanity, a crewed spacecraft is on its way to reignite the Sun with a bomb, though things don't go quite as planned.

Resources

Science Mission Directorate: Heliophysics Missions

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NASA's historic Parker Solar Probe mission will revolutionize our understanding of the Sun, where changing conditions can propagate out into the solar system, affecting Earth and other worlds. Parker Solar Probe will travel through the Sun's atmosphere, closer to the surface than any spacecraft before it, facing brutal heat and radiation conditions-and ultimately providing humanity with the closest-ever observations of a star.

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What is a Planet?

This seemingly simple question doesn't have a simple answer. Everyone knows that Earth, Mars and Jupiter are planets. At least, they are for now. Both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them-a vigorous debate that continues to this day. The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:

1. It must orbit a star (in our cosmic neighborhood, the Sun).

2. It must be big enough to have enough gravity to force it into a spherical shape.

3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.

Discussion-and debate-will continue as our view of the cosmos continues to expand. › More

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This seemingly simple question doesn't have a simple answer. Everyone knows that Earth, Mars and Jupiter are planets. At least, they are for now. Both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them-a vigorous debate that continues to this day.

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The Nine Planets is an encyclopedic overview with facts and information about mythology and current scientific knowledge of the planets, moons, and other objects in our solar system and beyond.

The 9 Planets in Our Solar System

The Five Dwarf Planets

Other Objects in The Solar System

Earth Science course

Excel High School has an online high school Earth Science course. Learn more about the course details here.

Northgate Academy offers Earth science and astronomy as online homeschooling courses.

Washington Technical Institute is an accredited college that offers online paralegal program and more.

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The Nine Planets of The Solar System | Eight Planets Without Pluto

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An overview of the history, mythology and current scientific knowledge of the planets, moons and other objects in our solar system.

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The existence of such objects was first suggested as far back as the late 1700s. However, it was Karl Schwarzschild (1873-1916), a German astronomer, who basically developed the modern idea for a black hole.

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Sailors have their krakens and their sea serpents. Physicists have white holes: cosmic creatures that straddle the line between tall tale and reality. Yet to be seen in the wild, white holes may be only mathematical monsters. But new research suggests that, if a speculative theory called loop quantum gravity is right, white holes could be real-and we might have already observed them.

A white whole is, roughly speaking, the opposite of a black hole. "A black hole is a place where you can go in but you can never escape; a white hole is a place where you can leave but you can never go back," says Caltech physicist Sean Carroll. "Otherwise, [both share] exactly the same mathematics, exactly the same geometry." That boils down to a few essential features: a singularity, where mass is squeezed into a point of infinite density, and an event horizon, the invisible "point of no return" first described mathematically by the German physicist Karl Schwarzschild in 1916. For a black hole, the event horizon represents a one-way entrance; for a white hole, it's exit-only.

There is excellent evidence that black holes really exist, and astrophysicists have a robust understanding of what it takes to make one. To imagine how a white hole might form, though, we have to go out on a bit of an astronomical limb. One possibility involves a spinning black hole. According to Einstein's general theory of relativity, the rotation smears the singularity into a ring, making it possible in theory to travel through the swirling black hole without being crushed. General relativity's equations suggest that someone falling into such a black hole could fall through a tunnel in space-time called a wormhole and emerge from a white hole that spits its contents into a different region of space or period of time.

Though mathematical solutions to those equations exist for white holes, "they're not realistic," says Andrew Hamilton, an astrophysicist at the University of Colorado at Boulder. That is because they describe universes that contain only black holes, white holes and wormholes-no matter, radiation or energy. Indeed, previous research, including Hamilton's, suggests that anything that falls into a spinning black hole will essentially plug up the wormhole, preventing the formation of a passage to a white hole.

But there's a light at the end of the wormhole, so to speak. General relativity, from which Hamilton draws his predictions, breaks down at a black hole's singularity. "The energy density and the curvature become so large that classical gravity is not a good description of what's happening there," says Stephen Hsu, a physicist at Michigan State University in East Lansing. Perhaps a more complete model of gravity-one that works as well on the quantum scale as it does on large ones-would negate the instability and allow for white holes, he says.

Indeed, a unified theory that merges gravity and quantum mechanics is one of the holy grails of contemporary physics. Applying one such theory, loop quantum gravity, to black holes, theorists Hal Haggard and Carlo Rovelli of Aix-Marseille University in France have shown that black holes could metamorphose into white holes via a quantum process. In July, they published their work online.

Loop quantum gravity proposes that space-time is made up of fundamental building blocks shaped like loops. According to Haggard and Rovelli, the loops' finite size prevents a dying star from collapsing all the way down into a point of infinite density, and the shrinking object rebounds into a white hole instead. This process may take just a few thousandths of a second, but thanks to the intense gravity involved, the effects of relativity make the transformation appear to take much, much longer to anyone watching from afar. That means that minuscule black holes born in the infant universe could "now be ready to pop off like firecrackers," forming white holes, according to a report in Nature. Some of the explosions astronomers thought were supernovae may actually be the wails of newborn white holes.

The black-to-white conversion could resolve a nettlesome conundrum known as the black hole information paradox. The notion that information can be destroyed is anathema in physics, and general relativity says that anything, including information, that falls into a black hole can never escape. These two statements are not at odds if black holes simply act as locked safes for any information they slurp up, but Stephen Hawking showed 40 years ago that black holes actually evaporate over time. That led to the disturbing possibility that the information contained within them could be lost too, triggering a debate that rages to this day.

But if a black hole instead turns into a white hole, then "all the information is recovered," says Haggard. "We are quite excited about this mechanism because it avoids so many of the thorny issues that surround this discussion."

The new work is preliminary, however, and it is far from clear whether loop quantum gravity is an accurate description of reality. The only glimpse we get of white holes might turn out to be those we model in labs and kitchen sinks. But Carroll says that's okay. Just thinking about these possibly mythical cosmic creatures can improve physicists' intuition, "even if the real world is messy and not like those exact situations," he says. "That's the way in which white holes are very useful."

Go Deeper
Editor's picks for further reading

Inside Black Holes
Physicist Andrew Hamilton is your guide on a visual journey inside a variety of kinds of black holes.

Nautilus: "White Holes" Could Exist-But That Doesn't Mean They Do
Matthew Francis on white holes and their implications for the symmetry of time.

Ask an Astronomer: What is a white hole?
Astronomer Karen Masters provides an introduction to the physics of white holes.

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Sailors have their krakens and their sea serpents. Physicists have white holes: cosmic creatures that straddle the line between tall tale and reality. Yet to be seen in the wild, white holes may be only mathematical monsters. But new research suggests that, if a speculative theory called loop quantum gravity is right, white holes could be real-and we might have already observed them.

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An artist's concept illustrates what the Mars rover Curiosity will look like on the Red Planet.

Credit: NASA/JPL-Caltech

The Mars Science Laboratory and its rover centerpiece, Curiosity, is the most ambitious Mars mission yet flown by NASA. The rover's primary mission is to find out if Mars is, or was, suitable for life. Another objective is to learn more about the red planet's environment.

[For the latest news about the mission, follow Space.com's Mars Science Lab Coverage.]

Curiosity's size allows it to carry a host of scientific experiments to zap, analyze and take pictures of any rock within reach of its 7-foot (2 meters) arm. Curiosity is about the size of a small SUV. It is 9 feet 10 inches long by 9 feet 1 inch wide (3 m by 2.8 m) and about 7 feet high (2.1 m). It weighs 2,000 lbs. (900 kilograms). Curiosity's wheels have a 20-inch (50.8 cm) diameter.

Engineers at NASA's Jet Propulsion Laboratory designed the rover to roll over obstacles up to 25 inches (65 centimeters) high and to travel about 660 feet (200 meters) per day. The rover's power comes from a multi-mission radioisotope thermoelectric generator, which produces electricity from the heat of plutonium-238's radioactive decay.

The $2.5-billion MSL spacecraft launched from Cape Canaveral, Florida, on Nov. 26, 2011, and arrived on Mars on Aug. 6, 2012, after a daring landing sequence that NASA dubbed " Seven Minutes of Terror." Because of Curiosity's weight, NASA determined that the past method of using a rolling method with land bags would probably not work. Instead, the rover went through an extremely complicated sequence of maneuvers to land.

From a fiery entry into the atmosphere, a supersonic parachute needed to deploy to slow the spacecraft down. NASA officials said the parachute would need to withstand 65,000 lbs. (29,480 kg) to break the spacecraft's fall to the surface.

Under the parachute, MSL let go of the bottom of its heat shield so that it could get a radar fix on the surface and figure out its altitude. The parachute could only slow MSL to 200 mph (322 kph), far too fast for landing. To solve the problem, engineers designed the assembly to cut off the parachute, and use rockets for the final part of the landing sequence.

About 60 feet (18 m) above the surface, MSL's "skycrane" deployed. The landing assembly dangled the rover below the rockets using a 20-foot (6 m) tether. Falling at 1.5 mph (2.4 kph), MSL gently touched the ground about the same moment the skycrane severed the link and flew away, crashing into the surface.

NASA personnel tensely watched the rover's descent on live television. When they received confirmation that Curiosity was safe, engineers pumped fists and jumped up and down in jubilation.

News of the landing spread through social media, such as Twitter and Facebook, and traditional outlets such as newspapers and television. One engineer became famous because of the Mohawk he sported on landing day.

The rover has a few tools to search for habitability. Among them is an experiment that bombards the surface with neutrons, which would slow down if they encounter hydrogen atoms: one of the elements of water.

Curiosity's 7-foot arm can pick up samples from the surface and cook them inside the rover, sniffing the gases that come out of there and analyzing them for clues as to how the rocks and soil formed.

The Sample Analysis of Mars instrument, if it does pick up evidence of organic material, can double-check that. On Curiosity's front, under foil covers, are several ceramic blocks infused with artificial organic compounds. [ Related: Curiosity Rover Finds Methane on Mars]

Curiosity can drill into each of these blocks and place a sample into its oven to measure its composition. Researchers will then see if organics appear that were not supposed to be in the block. If so, scientists will likely determine these are organisms hitchhiking from Earth.

High-resolution cameras surrounding the rover take pictures as it moves, providing visual information that can be compared to environments on Earth. This was used when Curiosity found evidence of a streambed, for example.

Curiosity's prime mission is to determine if Mars is, or was, suitable for life. While it is not designed to find life itself, the rover carries a number of instruments on board that can bring back information about the surrounding environment.

Scientists hit something close to the jackpot in early 2013, when the rover beamed back information showing that Mars had habitable conditions in the past.

Powder from the first drill samples that Curiosity obtained included the elements of sulfur, nitrogen, hydrogen, oxygen, phosphorus and carbon, which are all considered "building blocks" or fundamental elements that could support life. While this is not evidence of life itself, the find was still exciting to the scientists involved in the mission.

"A fundamental question for this mission is whether Mars could have supported a habitable environment," stated Michael Meyer, lead scientist for NASA's Mars Exploration Program. "From what we know now, the answer is yes."

Scientists also detected a huge spike in methane levels on Mars in late 2013 and early 2014, at a level of about 7 parts per billion (compared to the usual 0.3 ppb to 0.8 ppb). This was a notable finding because in some circumstances, methane is an indicator of microbial life. But it can also point to geological processes. In 2016, however, the team determined the methane spike was not a seasonal event. There are smaller background changes in methane, however, that could be linked to the seasons.

Curiosity also made the first definitive identification of organics on Mars, as announced in December 2014. Organics are considered life's building blocks, but do not necessarily point to the existence of life as they can also be created through chemical reactions.

"While the team can't conclude that there was life at Gale crater, the discovery shows that the ancient environment offered a supply of reduced organic molecules for use as building blocks for life and an energy source for life," NASA stated at the time.

Initial results released at the Lunar and Planetary Science conference in 2015 showed scientists found complex organic molecules in Martian samples stored inside the Curiosity rover, but using an unexpected method.

Vapors from a "wet chemistry" experiment filled with a fluid called MTBSTFA (N-methyl-N-tert-butyldimethylsilyl-trifluoroacetamide) contaminated a gas-sniffing analysis instrument shortly after Curiosity landed.

Since the scientists knew the collected samples were already reacting with the vapor, they eventually derived a way to seek and preserve the organics after extracting, collecting and analyzing the vapor. Analyzing the exact composition of the organics will take more time.

Besides hunting for habitability, Curiosity has other instruments on board that are designed to learn more about the environment surrounding it. Among those goals is to have a continuous record of weather and radiation observations to determine how suitable the site would be for an eventual human mission.

Curiosity's Radiation Assessment Detector runs for 15 minutes every hour to measure a swath of radiation on the ground and in the atmosphere. Scientists in particular are interested in measuring "secondary rays" or radiation that can generate lower-energy particles after it hits the gas molecules in the atmosphere. Gamma rays or neutrons generated by this process can cause a risk to humans. Additionally, an ultraviolet sensor stuck on Curiosity's deck tracks radiation continuously.

In December 2013, NASA determined the radiation levels measured by Curiosity were manageable for a crewed Mars mission in the future. A mission with 180 days flying to Mars, 500 days on the surface and 180 days heading back to Earth would create a dose of 1.01 sieverts, Curiosity's Radiation Assessment Detector determined. The total lifetime limit for European Space Agency astronauts is 1 sievert, which is associated with a 5-percent increase in fatal cancer risk over a person's lifetime.

The Rover Environmental Monitoring Station measures the wind's speed and chart its direction, as well as determining temperature and humidity in the surrounding air. By 2016, scientists were able to see long-term trends in atmospheric pressure and air humidity. Some of these changes occur when the winter carbon-dioxide polar caps melt in the spring, dumping huge amounts of moisture into the air.

Curiosity ran into its first major problem in February 2013, when a computer glitch punted the roving laboratory into safe mode for a few days. The issue caused an interruption of normal science activities, but did not impact the rover's long-term health.

But a more long-term problem has been the state of the rover's wheels. While some damage was expected, by 2014 controllers were making accommodations in the rover's routing to slow down the appearance of dings and holes.

"They are taking damage. That's the surprise we got back at the end of last year," said Jim Erickson, Curiosity project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California in a July 2014 interview. "We always expected we would get some holes in the wheels as we drove. It's just the magnitude of what we're seeing that was the surprise."

In September 2014, Curiosity arrived at its science destination, Mount Sharp (Aeolis Mons) shortly after a NASA science review said the rover should do less driving and more searching for habitable destinations. It is now carefully evaluating the layers on the slope as it moves uphill. The goal is to see how the climate of Mars changed from a wet past to the drier, acidic conditions of today.

"I think the principal recommendation of the panel is that we drive less and drill more," Curiosity project scientist John Grotzinger said during a news conference at the time. "The recommendations of the review and what we want to do as a science team are going to align because we have now arrived at Mount Sharp."

NASA pioneered a new drilling technique at Mount Sharp in February 2015 to begin operations at a lower setting, a requirement for working with the soft rock in some of the region. (Previously, a rock sample shattered after being probed with the drill.)

Additional resources

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The Mars Science Laboratory and its rover centerpiece, Curiosity, is the most ambitious Mars mission yet flown by NASA. The rover's primary mission is to find out if Mars is, or was, suitable for life. Another objective is to learn more about the red planet's environment.

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In studying our Solar System over the course of many centuries, astronomers learned a great deal about the types of planets that exist in our universe. This knowledge has since expanded thanks to the discovery of extrasolar planets, many of which are similar to what we have observed here at home.

For example, while hundreds of gas giants of varying size have been detected (which are easier to detect because of their size), numerous planets have also been spotted that are similar to Earth - aka. "Earth-like". These are what is known as terrestrial planets, a designation which says a lot about a planet how it came to be.

Definition:

Also known as a telluric or rocky planet, a terrestrial planet is a celestial body that is composed primarily of silicate rocks or metals and has a solid surface. This distinguishes them from gas giants, which are primarily composed of gases like hydrogen and helium, water, and some heavier elements in various states.

The term terrestrial planet is derived from the Latin "Terra" (i.e. Earth). Terrestrial planets are therefore those that are "Earth-like", meaning they are similar in structure and composition to planet Earth.

Composition and Characteristics:

All terrestrial planets have approximately the same type of structure: a central metallic core composed of mostly iron, with a surrounding silicate mantle. Such planets have common surface features, which include canyons, craters, mountains, volcanoes, and other similar structures, depending on the presence of water and tectonic activity.

Terrestrial planets also have secondary atmospheres, which are generated through volcanism or comet impacts. This also differentiates them from gas giants, where the planetary atmospheres are primary and were captured directly from the original solar nebula.

Terrestrial planets are also known for having few or no moons. Venus and Mercury have no moons, while Earth has only the one (the Moon). Mars has two satellites, Phobos and Deimos, but these are more akin to large asteroids than actual moons. Unlike the gas giants, terrestrial planets also have no planetary ring systems.

Solar Terrestrial Planets:

All those planets found within the Inner Solar System - Mercury, Venus, Earth and Mars - are examples of terrestrial planets. Each are composed primarily of silicate rock and metal, which is differentiated between a dense, metallic core and a silicate mantle. The Moon is similar, but has a much smaller iron core.

Io and Europa are also satellites that have internal structures similar to that of terrestrial planets. In the case of the former, models of the moon's composition suggest that the mantle is composed primarily of silicate rock and iron, which surrounds a core of iron and iron sulphide. Europa, on the other hand, is believed to have an iron core that is surrounded by an outer layer of water.

Dwarf planets, like Ceres and Pluto, and other large asteroids are similar to terrestrial planets in the fact that they do have a solid surface. However, they differ in that they are, on average, composed of more icy materials than rock.

Extrasolar Terrestrial Planets:

Most of the planets detected outside of the Solar System have been gas giants, owing to the fact that they are easier to spot. However, since 2005, hundreds of potentially terrestrial extrasolar planets have been found - mainly by the Kepler space mission. Most of these have been what is known as " super-Earths " (i.e. planets with masses between Earth's and Neptune's).

Examples of extrasolar terrestrial planets include Gliese 876 d, a planet that has a mass 7 to 9 times that of Earth. This planet orbits the red dwarf Gliese 876, which is located approximately 15 light years from Earth. The existence of three (or possibly four) terrestrial exoplanets was also confirmed between 2007 and 2010 in the Gliese 581 system, another red dwarf roughly 20 light years from Earth.

The smallest of these, Gliese 581 e, is only about 1.9 Earth masses, but orbits very close to the star. Two others, Gliese 581 c and Gliese 581 d, as well as a proposed fourth planet ( Gliese 581 g) are more-massive super-Earths orbiting in or close to the habitable zone of the star. If true, this could mean that these worlds are potentially habitable Earth-like planets.

The first confirmed terrestrial exoplanet, Kepler-10b - a planet with between 3 and 4 Earth masses and located some 460 light years from Earth - was found in 2011 by the Kepler space mission. In that same year, the Kepler Space Observatory team released a list of 1235 extrasolar planet candidates, including six that were "Earth-size" or "super-Earth-size" (i.e. less than 2 Earth radii) and which were located within their stars' habitable zones.

Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range. As of January, 2013, 2740 planet candidates have been discovered.

Categories:

Scientists have proposed several categories for classifying terrestrial planets. Silicate planets are the standard type of terrestrial planet seen in the Solar System, which are composed primarily of a silicon-based rocky mantle and a metallic (iron) core.

Iron planets are a theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass. Planets of this type are believed to form in the high-temperature regions close to a star, and where the protoplanetary disk is rich in iron. Mercury is possible example, which formed close to our Sun and has a metallic core equal to 60-70% of its planetary mass.

Coreless planets are another theoretical type of terrestrial planet, one that consists of silicate rock but has no metallic core. In other words, coreless planets are the opposite of an iron planet. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common. Though the Solar System has no coreless planets, chondrite asteroids and meteorites are common.

And then there are Carbon planets (aka. "diamond planets"), a theoretical class of planets that are composed of a metal core surrounded by primarily carbon-based minerals. Again, the Solar System has no planets that fit this description, but has an abundance of carbonaceous asteroids.

Until recently, everything scientists knew about planets - which included how they form and the different types that exist - came from studying our own Solar System. But with the explosion that has taken place in exoplanet discovery in the past decade, what we know about planets has grown significantly.

For one, we have come to understand that the size and scale of planets is greater than previously thought. What's more, we've seen for the first time that many planets similar to Earth (which could also include being habitable) do in fact exist in other Solar Systems.

Who knows what we will find once we have the option of sending probes and manned missions to other terrestrial planets?

Universe Today has articles on smallest terrestrial exoplanet and gas planets. For the latest information on confirmed extrasolar planets, be sure to check out the Kepler's Planet Candidates.

For a full list of all confirmed and potential planets, consult the Extrasolar Planet Encyclopaedia.

Astronomy Cast has episodes on the terrestrial planets including Mars, and an interview with Darin Ragozzine, one of the Kepler Space Mission scientists.

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What is a Terrestrial Planet? - Universe Today

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In studying our Solar System over the course of many centuries, astronomers learned a great deal about the types of planets that exist in our universe. This knowledge has since expanded thanks to the discovery of extrasolar planets, many of which are similar to what we have observed here at home.

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Most ancient cultures saw pictures in the stars of the night sky. The earliest known efforts to catalogue the stars date to cuneiform texts and artifacts dating back roughly 6000 years. These remnants, found in the valley of the Euphrates River, suggest that the ancients observing the heavens saw the lion, the bull, and the scorpion in the stars. The constellations as we know them today are undoubtedly very different from those first few--our night sky is a compendium of images from a number of different societies, both ancient and modern. By far, though, we owe the greatest debt to the mythology of the ancient Greeks and Romans.

The earliest references to the mythological significance of the Greek constellations may be found in the works of Homer, which probably date to the 7th century B.C. In the Iliad, for instance, Homer describes the creation of Achilleus's shield by the craftsman god Hephaistos:

On it he made the earth, and sky, and sea, the weariless sun and the moon waxing full, and all the constellations that crown the heavens, Pleiades and Hyades, the mighty Orion and the Bear, which men also call by the name of Wain: she wheels round in the same place and watches for Orion, and is the only one not to bathe in Ocean (Iliad XVIII 486-490).
At the time of Homer, however, most of the constellations were not associated with any particular myth, hero, or god. They were instead known simply as the objects or animals which they represented--the Lyre, for instance, or the Ram. By the 5th century B.C., however, most of the constellations had come to be associated with myths, and the Catasterismi of Eratosthenes completed the mythologization of the stars. "At this stage, the fusion between astronomy and mythology is so complete that no further distinction is made between them"--the stars were no longer merely identified with certain gods or heroes, but actually were perceived as divine (Seznec, 37-40).
Despite the many mentions of the stars in Greek and early Roman texts, by far the most thorough star catalogue from ancient times belongs to the Roman Ptolemy of Alexandria, who grouped 1022 stars into 48 constellations during the 2nd century A.D. Although Ptolemy's Almagest does not include the constellations which may only be seen from the southern hemisphere, it forms the basis for the modern list of 88 constellations officially designated by the International Astronomical Union (Pasachoff, 134-135). The influence of both the Greek and Roman cultures may be plainly seen; the myths behind the constellations date back to ancient Greece, but we use their Latin names.
The Major Constellations: Andromeda | Aquarius | Aries |Cancer | Capricornus | Cassiopeia | Cepheus | Cetus | Corona Borealis | Cygnus | Draco | Eridanus | Gemini | Hercules | Hydra | Leo | Libra | Lyra | Orion | Perseus | Pisces | Sagittarius |Scorpius | Taurus | Ursa Major |Ursa Minor | Virgo
Mythology, of course, influenced the naming of many objects in the night sky, not just the constellations. The planets all bear names from Roman mythology which reflect their characteristics: Mercury, named for the speedy messenger god, revolves fastest around the sun; Venus, named for the goddess of love and beauty, shines most brightly; Mars, named for the god of war, appears blood-red; Jupiter, named for the single most important god, is the largest planet in our solar system. Even the names of the Galilean moons of Jupiter (the four largest, which may be seen with even a small telescope) are drawn from mythology. Io, Europa, Ganymede, and Callisto were all desired--and taken by force--by Jupiter. It is ironic that the mythological characters mythological women the king of the gods so ardently pursued now revolve around him.
Works Cited and Consulted
Cathy Bell
cmbell (at) comfychair (dot) org
originally for the Princeton University course CLA 212.

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Most ancient cultures saw pictures in the stars of the night sky. The earliest known efforts to catalogue the stars date to cuneiform texts and artifacts dating back roughly 6000 years. These remnants, found in the valley of the Euphrates River, suggest that the ancients observing the heavens saw the lion, the bull, and the scorpion in the stars.

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The planets of the solar system as depicted by a NASA computer illustration. Orbits and sizes are not shown to scale.

Credit: NASA

A gas giant is a large planet composed mostly of gases, such as hydrogen and helium, with a relatively small rocky core. The gas giants of our solar system are Jupiter, Saturn, Uranus and Neptune. These four large planets, also called jovian planets after Jupiter, reside in the outer part of the solar system past the orbits of Mars and the asteroid belt. Jupiter and Saturn are substantially larger than Uranus and Neptune, and each pair of planets has a somewhat different composition.

Although there are only four large planets in our solar system, astronomers have discovered thousands outside of it, particularly using NASA's Kepler Space Telescope. These exoplanets (as they are called) are being examined to learn more about how our solar system came to be.

Jupiter is the largest planet in our solar system. It has a radius almost 11 times the size of Earth. It has 50 known moons and 17 waiting to be confirmed, according to NASA. The planet is mostly made of hydrogen and helium surrounding a dense core of rocks and ice, with most of its bulk likely made up of liquid metallic hydrogen, which creates a huge magnetic field. Jupiter is visible with the naked eye and was known by the ancients. Its atmosphere consists mostly of hydrogen, helium, ammonia and methane. [ Related: Planet Jupiter: Facts About Its Size, Moons and Red Spot]

Saturn is about nine times Earth's radius and is characterized by large rings; how they formed is unknown. It has 53 known moons and nine more awaiting confirmation, according to NASA. Like Jupiter, it is mostly made up of hydrogen and helium that surround a dense core and was also tracked by ancient cultures. Its atmosphere is similar to Jupiter's. [ Related: Planet Saturn: Facts About Saturn's Rings, Moons & Size]

Uranus has a radius about four times that of Earth's. It is the only planet tilted on its side, and it also rotates backward relative to every planet but Venus, implying a huge collision disrupted it long ago. The planet has 27 moons, and its atmosphere is made up of hydrogen, helium and methane, according to NASA. It was discovered by William Herschel in 1781. [ Related: Planet Uranus: Facts About Its Name, Moons & Orbit]

Neptune also has a radius about four times that of Earth's. Like Uranus, its atmosphere is mostly made up of hydrogen, helium and methane. It has 13 confirmed moons and an additional one awaiting confirmation, according to NASA. It was discovered by several people in 1846. [ Related: Planet Neptune: Facts About Its Orbit, Moons & Rings]

Super-Earths: Scientists have found a multitude of "super-Earths" (planets between the size of Earth and Neptune) in other solar systems. There are no known super-Earths in our own solar system, although some scientists speculate there may be a "Planet Nine" lurking in the outer reaches of our solar system. Scientists are studying this category of planets to learn whether super-Earths are more like small giant planets or big terrestrial planets.

Astronomers think the giants first formed as rocky and icy planets similar to terrestrial planets. However, the size of the cores allowed these planets (particularly Jupiter and Saturn) to grab hydrogen and helium out of the gas cloud from which the sun was condensing, before the sun formed and blew most of the gas away.

Since Uranus and Neptune are smaller and have bigger orbits, it was harder for them to collect hydrogen and helium as efficiently as Jupiter and Saturn. This likely explains why they are smaller than those two planets. On a percentage basis, their atmospheres are more "polluted" with heavier elements such as methane and ammonia because they are so much smaller.

Scientists have discovered thousands of exoplanets. Many of these happen to be "hot Jupiters," or massive gas giants that are extremely close to their parent stars. (Rocky worlds are more abundant in the universe, according to estimates from Kepler.) Scientists speculate that large planets may have moved back and forth in their orbits before settling into their current configuration. But how much they moved is still a subject of debate.

There are dozens of moons around the giant planets. Many formed at the same time as their parent planets, which is implied if the planets rotate in the same direction as the planet close to the equator (such as the huge Jovian moons Io, Europa, Ganymede and Callisto.) But there are exceptions.

One moon of Neptune, Triton, orbits the planet opposite to the direction Neptune spins - implying that Triton was captured, perhaps by Neptune's once larger atmosphere, as it passed by. And there are many tiny moons in the solar system that rotate far from the equator of their planets, implying that they were also snagged by the immense gravitational pull.

Jupiter: NASA's Juno spacecraft arrived at the planet in 2016 and has already made several discoveries. It studied the planet's rings, which is difficult to achieve since they are far subtler than Saturn's. Juno discovered that the particles influencing the auroras of Jupiter are different than those on Earth. It also revealed insights about the atmosphere, such as finding snow emanating from high-altitude clouds. Meanwhile, scientists using the Hubble Space Telescope have made detailed studies of Jupiter's Great Red Spot, watching it shrink and intensify in color.

Saturn: The Cassini spacecraft wrapped up more than a dozen years of observation at Saturn in 2017. But the science Cassini performed is still very much in progress, as scientists analyze work from its many years at Saturn. In its latter months, the mission examined Saturn's gravity and magnetic fields, looked at the rings from a different angle than before, and plunged into the atmosphere deliberately (a move that will reveal more about the structure of the atmosphere.)

Uranus: The storms of Uranus are a frequent target for both professional telescopes and amateur astronomers, who monitor how they evolve and change over time. Scientists are also interested in learning about the structure of its rings, and what its atmosphere is made of. Uranus may also have several Trojan asteroids (asteroids in the same orbit as the planet); the first was found in 2013.

Neptune: Storms on Neptune are also a popular observing target, and in 2018 those observations again bore fruit; work from the Hubble Space Telescope showed that a long-standing storm is now shrinking. The researchers noted the storm is dissipating differently than what their models expected, which shows that our understanding of Neptune's atmosphere still requires refinement.

Exoplanets: Many ground telescopes search for exoplanets. There are also several active space missions performing exoplanet research, including Kepler, the Hubble Space Telescope and the Spitzer Space Telescope. A flurry of new missions is also planned: the NASA Transiting Exoplanet Survey Satellite (TESS) in 2018, the NASA James Webb Space Telescope in 2020, the European Space Agency's PLAnetary Transits and Oscillations of stars (PLATO) in 2026 and ESA's Atmospheric Remote-sensing Infrared Exoplanet Large-survey mission (Ariel) in 2028.

Additional resources

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Gas Giants: Facts About the Outer Planets

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A gas giant is a large planet composed mostly of gases, such as hydrogen and helium, with a relatively small rocky core. The gas giants of our solar system are Jupiter, Saturn, Uranus and Neptune. These four large planets, also called jovian planets after Jupiter, reside in the outer part of the solar system past the orbits of Mars and the asteroid belt.

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The Milky Way Galaxy is an immense and very interesting place. Not only does it measure some 120,000-180,000 light-years in diameter, it is home to planet Earth, the birthplace of humanity. Our Solar System resides roughly 27,000 light-years away from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust particles called the Orion Arm.

But within these facts about the Milky Way lie some additional tidbits of information, all of which are sure to impress and inspire. Here are ten such facts, listed in no particular order:

1. It's Warped:

For starters, the Milky Way is a disk about 120,000 light years across with a central bulge that has a diameter of 12,000 light years (see the Guide to Space article for more information). The disk is far from perfectly flat though, as can be seen in the picture below. In fact, it is warped in shape, a fact which astronomers attribute to the our galaxy's two neighbors -the Large and Small Magellanic clouds.

These two dwarf galaxies - which are part of our "Local Group" of galaxies and may be orbiting the Milky Way - are believed to have been pulling on the dark matter in our galaxy like in a game of galactic tug-of-war. The tugging creates a sort of oscillating frequency that pulls on the galaxy's hydrogen gas, of which the Milky Way has lots of (for more information, check out How the Milky Way got its Warp).

2. It Has a Halo, but You Can't Directly See It:

Scientists believe that 90% of our galaxy's mass consists of dark matter, which gives it a mysterious halo. That means that all of the "luminous matter" - i.e. that which we can see with the naked eye or a telescopes - makes up less than 10% of the mass of the Milky Way. Its halo is not the conventional glowing sort we tend to think of when picturing angels or observing comets.

In this case, the halo is actually invisible, but its existence has been demonstrated by running simulations of how the Milky Way would appear without this invisible mass, and how fast the stars inside our galaxy's disk orbit the center.

The heavier the galaxy, the faster they should be orbiting. If one were to assume that the galaxy is made up only of matter that we can see, then the rotation rate would be significantly less than what we observe. Hence, the rest of that mass must be made up of an elusive, invisible mass - aka. " dark matter " - or matter that only interacts gravitationally with "normal matter".

To see some images of the probable distribution and density of dark matter in our galaxy, check out The Via Lactea Project.

3. It has Over 200 Billion Stars:

As galaxies go, the Milky Way is a middleweight. The largest galaxy we know of, which is designated IC 1101, has over 100 trillion stars, and other large galaxies can have as many as a trillion. Dwarf galaxies such as the aforementioned Large Magellanic Cloud have about 10 billion stars. The Milky Way has between 100-400 billion stars; but when you look up into the night sky, the most you can see from any one point on the globe is about 2,500. This number is not fixed, however, because the Milky Way is constantly losing stars through supernovae, and producing new ones all the time ( about seven per year).

4. It's Really Dusty and Gassy:

Though it may not look like it to the casual observer, the Milky Way is full of dust and gas. This matter makes up a whopping 10-15% of the luminous/visible matter in our galaxy, with the remainder being the stars. Our galaxy is roughly 100,000 light years across, and we can only see about 6,000 light years into the disk in the visible spectrum. Still, when light pollution is not significant, the dusty ring of the Milky Way can be discerned in the night sky.

The thickness of the dust deflects visible light (as is explained here) but infrared light can pass through the dust, which makes infrared telescopes like the Spitzer Space Telescope extremely valuable tools in mapping and studying the galaxy. Spitzer can peer through the dust to give us extraordinarily clear views of what is going on at the heart of the galaxy and in star-forming regions.

5. It was Made From Other Galaxies:

The Milky Way wasn't always as it is today - a beautiful, warped spiral. It became its current size and shape by eating up other galaxies, and is still doing so today. In fact, the Canis Major Dwarf Galaxy is the closest galaxy to the Milky Way because its stars are currently being added to the Milky Way's disk. And our galaxy has consumed others in its long history, such as the Sagittarius Dwarf Galaxy.

6. Every Picture You've Seen of the Milky Way Isn't It:

Currently, we can't take a picture of the Milky Way from above. This is due to the fact that we are inside the galactic disk, about 26,000 light years from the galactic center. It would be like trying to take a picture of your own house from the inside. This means that any of the beautiful pictures you've ever seen of a spiral galaxy that is supposedly the Milky Way is either a picture of another spiral galaxy, or the rendering of a talented artist.

Imaging the Milky Way from above is a long, long way off. However, this doesn't mean that we can't take breathtaking images of the Milky Way from our vantage point!

7. There is a Black Hole at the Center:

Most larger galaxies have a supermassive black hole (SMBH) at the center, and the Milky Way is no exception. The center of our galaxy is called Sagittarius A*, a massive source of radio waves that is believed to be a black hole that measures 22,5 million kilometers (14 million miles) across - about the size of Mercury's orbit. But this is just the black hole itself.

All of the mass trying to get into the black hole - called the accretion disk - forms a disk that has 4.6 million times the mass of our Sun and would fit inside the orbit of the Earth. Though like other black holes, Sgr A* tries to consume anything that happens to be nearby, star formation has been detected near this behemoth astronomical phenomenon.

8. It's Almost as Old as the Universe Itself:

The most recent estimates place the age of the Universe at about 13.7 billion years. Our Milky Way has been around for about 13.6 billion of those years, give or take another 800 million. The oldest stars in our the Milky Way are found in globular clusters, and the age of our galaxy is determined by measuring the age of these stars, and then extrapolating the age of what preceded them.

Though some of the constituents of the Milky Way have been around for a long time, the disk and bulge themselves didn't form until about 10-12 billion years ago. And that bulge may have formed earlier than the rest of the galaxy.

9. It's Part of the Virgo Supercluster:

As big as it is, the Milky Way is part of an even larger galactic structures. Our closest neighbors include the Large and Small Magellanic Clouds, and the Andromeda Galaxy - the closest spiral galaxy to the Milky Way. Along with some 50 other galaxies, the Milky Way and its immediate surroundings make up a cluster known as the Local Group.

And yet, this is still just a small fraction of our stellar neighborhood. Farther out, we find that the Milky Way is part of an even larger grouping of galaxies known as the Virgo Supercluster. Superclusters are groupings of galaxies on very large scales that measure in the hundreds of millions of light years in diameter. In between these superclusters are large stretches of open space where intrepid explorers or space probes would encounter very little in the way of galaxies or matter.

In the case of the Virgo Supercluster, at least 100 galaxy groups and clusters are located within it massive 33 megaparsec (110 million light-year) diameter. And a 2014 study indicates that the Virgo Supercluster is only a lobe of a greater supercluster, Laniakea, which is centered on the Great Attractor.

10. It's on the move:

The Milky Way, along with everything else in the Universe, is moving through space. The Earth moves around the Sun, the Sun around the Milky Way, and the Milky Way as part of the Local Group, which is moving relative to the Cosmic Microwave Background (CMB) radiation - the radiation left over from the Big Bang.

The CMB is a convenient reference point to use when determining the velocity of things in the universe. Relative to the CMB, the Local Group is calculated to be moving at a speed of about 600 km/s, which works out to about 2.2 million km/h. Such speeds stagger the mind and squash any notions of moving fast within our humble, terrestrial frame of reference!

We have written many interesting articles about the Milky Way for Universe Today. Here's 10 Interesting Facts about the Milky Way, How Big is the Milky Way?, What is the Closest Galaxy to the Milky Way?, and How Many Stars Are There in the Milky Way?

For many more facts about the Milky Way, visit the Guide to Space, listen to the Astronomy Cast episode on the Milky Way, or visit the Students for the Exploration and Development of Space at seds.org.

cosmic microwave background, Earth, Large Magellanic Cloud, local group, milky way, Orion Arm, Sagittarius A*, Small Megellanic Cloud, SMBH, supermassive black holes, Virgo Supercluster

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10 Interesting Facts About the Milky Way - Universe Today

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The Milky Way Galaxy is an immense and very interesting place. Not only does it measure some 120,000-180,000 light-years in diameter, it is home to planet Earth, the birthplace of humanity. Our Solar System resides roughly 27,000 light-years away from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas ...

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its longggggg

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Latest Secrets of The Solar System - Full BBC Documentary

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Uploaded by The Sun Documentary on 2015-08-31.

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BBC - Universe - Milky Way (pictures, video, facts & news)

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Watch videos from BBC programmes that are full of facts about the Milky Way galaxy and presented by easy-to-understand experts such as Patrick Moore.

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So just what are these constellations you keep hearing about? You may go outside some night and see all kinds of stars, and maybe you have even spotted the Big Dipper (northern hemisphere) or the Southern Cross (southern hemisphere), but what about Leo the Lion or Pisces the Fish? What are they?
Contents

Looking at constellations
Where did the constellations come from?

Looking at the Constellations
The first thing you need to know is that constellations are not real!
The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!). The real purpose for the constellations is to help us tell which stars are which, nothing more. On a really dark night, you can see about 1000 to 1500 stars. Trying to tell which is which is hard. The constellations help by breaking up the sky into more managable bits. They are used as mnemonics, or memory aids. For example, if you spot three bright stars in a row in the winter evening, you might realize, "Oh! That's part of Orion!" Suddenly, the rest of the constellation falls into place and you can declare: "There's Betelgeuse in Orion's left shoulder and Rigel is his foot." And once you recognize Orion, you can remember that Orion's Hunting Dogs are always nearby. Then you might recognize the two bright stars in the upper and lower left of the photograph as Procyon in Canis Minor and Sirius in Canis Major, respectively.

When you look in a sky atlas, you might see diagrams like this:

Obviously, this is very different from the photo above. This type of schematic draws the stars as different sizes to represent different brightnesses. In addition, there is a standard way to connect the stars that allow astronomers and others who use charts like this to quickly tell what they are looking at. In almost every star atlas, you will see Orion drawn with these same lines.

You might also notice that every star on the chart is labeled (sorry that it came out a little blurry). This chart is useful because it accurately shows the relative positions of the stars in this small region of the sky. In addition, other things besides stars are also labeled on the chart. For example, Barnard's Loop on the left and M42 in the bottom middle are pointed out. Barnard's Loop is a cloud of faintly glowing gas, which can't be seen without a telescope. M42 is the Great Orion Nebula and it is the red splotch in Orion's Sword in the photo above.

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Where did the constellations come from?
OK, so we know the constellations are helpful for remembering the stars, but why would people want to do that (besides astronomers, that is)? After all, I said at the beginning that farmers&nbsp invented the constellations. Why did they do that? Was it for some religious purpose?
Yes and no. Around the world, farmers know that for most crops, you plant in the spring and harvest in the fall. But in some regions, there is not much differentiation between the seasons. Since different constellations are visible at different times of the year, you can use them to tell what month it is. For example, Scorpius is only visible in the northern hemisphere's evening sky in the summer. Some historians suspect that many of the myths associated with the constellations were invented to help the farmers remember them. When they saw certain constellations, they would know it was time to begin the planting or the reaping.

This dependence on the sky became a strong part of many cultures. Perhaps there is something about the mystery of the night sky that makes people want to tell stories about the constellations. The picture at the left is an ornate star chart printed in 1835. Like the others, it shows the great hunter Orion. In this one, he is holding a lion's head instead of his traditional bow or shield. He has an eager look in his eye as he stalks Taurus, the Bull. Behind him, his faithful dog, Canis Major, is chasing Lepus, the Hare. Compare this picture to the photo near the top of the page. They are at about the same scale and they show the same stars.

The constellations have changed over time. In our modern world, many of the constellations have been redefined so now every star in the sky is in exactly one constellation. In 1929, the International Astronomical Union (IAU) adopted official constellation boundaries that defined the 88 official constellations that exist today.

The graphics are from "Constellations" by Motz and Nathanson and "Universe" by Kaufmann Back to Constellations Home Page

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So just what are these constellations you keep hearing about? You may go outside some night and see all kinds of stars, and maybe you have even spotted the Big Dipper (northern hemisphere) or the Southern Cross (southern hemisphere), but what about Leo the Lion or Pisces the Fish?

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Moons - Facts about the Moons of the Solar System

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There are at least 338 moons of the various planets, dwarf planets in the solar system. The planets Mercury and Venus do not have any

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When you look up at the night sky, assuming conditions are just right, you might just catch a glimpse of a faint, white band reaching across the heavens. This band, upon closer observation, looks speckled and dusty, filled with a million tiny points of light and halos of glowing matter. What you are seeing is the Milky Way, something that astronomers and stargazers alike have been staring up at since the beginning of time.

But just what is the Milky Way? Well, simply put, it is the name of the barred spiral galaxy in which our solar system is located. The Earth orbits the Sun in the Solar System, and the Solar System is embedded within this vast galaxy of stars. It is just one of hundreds of billions of galaxies in the Universe, and ours is called the Milky Way because the disk of the galaxy appears to be spanning the night sky like a hazy band of glowing white light.

Discovery and Naming:

Our galaxy was named because of the way the haze it casts in the night sky resembled spilled milk. This name is also quite ancient. It is translation from the Latin " Via Lactea", which in turn was translated from the Greek for Galaxias, referring to the pale band of light formed by stars in the galactic plane as seen from Earth.

Persian astronomer Nasir al-Din al-Tusi (1201-1274) even spelled it out in his book Tadhkira: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color."

Astronomers had long suspected the Milky Way was made up of stars, but it wasn't proven until 1610, when Galileo Galilei turned his rudimentary telescope towards the heavens and resolved individual stars in the band across the sky. With the help of telescopes, astronomers realized that there were many, many more stars in the sky, and that all of the ones that we can see are a part of the Milky Way.

In 1755, Immanuel Kant proposed that the Milky Way was a large collection of stars held together by mutual gravity. Just like the Solar System, this collection of stars would be rotating and flattened out as a disk, with the Solar System embedded within it. Astronomer William Herschel (who discovered Uranus) attempted to actually map out the shape of the Milky Way in 1785, but he didn't realize that large portions of the galaxy are obscured by gas and dust, which hide its true shape.

It wasn't until the 1920s, when Edwin Hubble provided conclusive evidence that the spiral nebulae in the sky were actually whole other galaxies, that the true shape of our galaxy was known. Thenceforth, astronomers came to understand that the Milky Way is a barred, spiral galaxy, and also came to appreciate how big the Universe truly is.

Structure and Composition:

The Milky Way looks brightest toward the galactic center, in the direction of Sagittarius. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the Solar System lies near the galactic plane. The Milky Way has a relatively low surface brightness due to the gases and dust that fills the galactic disk. That prevents us from seeing the bright galactic center or from observing clearly what is on the other side of it.

If you could travel outside the galaxy and look down on it from above, you'd see that the Milky Way is a barred spiral galaxy measuring about 120,000 light-years across and about 1,000 light-years thick. For the longest time, the Milky Way was thought to have 4 spiral arms, but newer surveys have determined that it actually seems to just have two spiral arms, called Scutum-Centaurus and Carina-Sagittarius.

The spiral arms are formed from density waves that orbit around the Milky Way. As these density waves move through an area, they compress the gas and dust, leading to a period of active star formation for the region. However, the existence of these arms has been determined from observing parts of the Milky Way - as well as other galaxies in our universe - and are not the result of seeing our galaxy as a whole.

In truth, all pictures that depict our galaxy are either artist's renditions or pictures of other spiral galaxies. Until recently, it was very difficult for scientists to gauge what the Milky Way looks like, mainly because we're embedded inside it. If you had never been outside of your own house, you wouldn't know what it looked like from outside. But you'd get a sense by looking at the interior and comparing it to other houses in the neighborhood.

From ongoing surveys of the night sky with ground-based telescopes, and more recent missions involving space telescopes, astronomers now estimate that there are between 100 and 400 billion stars in the Milky Way. They also think that each star has at least one planet, which means there are likely to be hundreds of billions of planets in the Milky Way - and at least 17 billion of those are believed to be the size and mass of the Earth.

The Milky Way, like all galaxies, is surrounded by a vast halo of dark matter, which accounts for some 90% of its mass. Nobody knows precisely what dark matter is, but its mass has been inferred by observations of how fast the galaxy rotates and other general behaviors. More importantly, it is believed that this mass helps keep the galaxy from tearing itself apart as it rotates.

Location of our Sun:

Our Sun is located in the Orion Arm, a region of space in between the two major arms of the Milky Way, and about 27,000 light years from the galactic core. At the heart of the Milky Way is a super-massive black hole, just like all of the other galaxies, known as Sagittarius A*. This monster is more than 4 million times

Our Sun takes about 240 million years to orbit the Milky Way once, in what is known as a galactic year (or cosmic year). Just imagine, the last time the Sun was at this region of the galaxy, dinosaurs roamed the Earth, and the Sun has only made an estimated 18-20 trips around in its entire life. By this reckoning, the birth of our Sun took place 18.4 galactic years ago, and the Universe itself was created approximately 61 galactic years ago.

Future of the Milky Way:

It is further believed that our galaxy formed through the collisions of smaller galaxies, early in the Universe. These mergers are still going on, and the Milky Way is expected to collide with the Andromeda galaxy in 3-4 billion years. The two galaxies will combine to form a giant elliptical galaxy, and their super-massive black holes might even merge.

The Milky Way and Andromeda are part of a larger collection of galaxies known as the Local Group. And these are contained within an even larger region called the Virgo Supercluster - a mass concentration of galaxies that contains at least 100 galaxy groups and clusters within its diameter of 33 megaparsecs (110 million light-years).

You might be amazed to know that dung beetles actually navigate at night using the Milky Way. If you've never seen the Milky Way with your own eyes, you should take the chance when you can. Go to a place with nice dark skies, free from light pollution, and look up and appreciate the Milky Way. And be sure to wave hello to all the neighboring stars who share our galaxy with us.

Those are just a few of the interesting facts, figures, and images that you will find among the links below.

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Andromeda Galaxy, milky way galaxy, Milky Way Galaxy Collision, Solar System, spiral galaxy, stars, the sun, Virgo Supercluster

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When you look up at the night sky, assuming conditions are just right, you might just catch a glimpse of a faint, white band reaching across the heavens. This band, upon closer observation, looks speckled and dusty, filled with a million tiny points of light and halos of glowing matter.

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A model of 'folded' space-time illustrates how a wormhole bridge might form with at least two mouths that are connected to a single throat or tube.

Credit: edobric | Shutterstock

A wormhole is a theoretical passage through space-time that could create shortcuts for long journeys across the universe. Wormholes are predicted by the theory of general relativity. But be wary: wormholes bring with them the dangers of sudden collapse, high radiation and dangerous contact with exotic matter.

Wormholes were first theorized in 1916, though that wasn't what they were called at the time. While reviewing another physicist's solution to the equations in Albert Einstein's theory of general relativity, Austrian physicist Ludwig Flamm realized another solution was possible. He described a "white hole," a theoretical time reversal of a black hole. Entrances to both black and white holes could be connected by a space-time conduit.

In 1935, Einstein and physicist Nathan Rosen used the theory of general relativity to elaborate on the idea, proposing the existence of "bridges" through space-time. These bridges connect two different points in space-time, theoretically creating a shortcut that could reduce travel time and distance. The shortcuts came to be called Einstein-Rosen bridges, or wormholes.

"The whole thing is very hypothetical at this point," said Stephen Hsu, a professor of theoretical physics at the University of Oregon, told our sister site, LiveScience. "No one thinks we're going to find a wormhole anytime soon."

Wormholes contain two mouths, with a throat connecting the two. The mouths would most likely be spheroidal. The throat might be a straight stretch, but it could also wind around, taking a longer path than a more conventional route might require.

Einstein's theory of general relativity mathematically predicts the existence of wormholes, but none have been discovered to date. A negative mass wormhole might be spotted by the way its gravity affects light that passes by.

Certain solutions of general relativity allow for the existence of wormholes where the mouth of each is a black hole. However, a naturally occurring black hole, formed by the collapse of a dying star, does not by itself create a wormhole.

Science fiction is filled with tales of traveling through wormholes. But the reality of such travel is more complicated, and not just because we've yet to spot one.

The first problem is size. Primordial wormholes are predicted to exist on microscopic levels, about 10 ⁻³³ centimeters. However, as the universe expands, it is possible that some may have been stretched to larger sizes.

Another problem comes from stability. The predicted Einstein-Rosen wormholes would be useless for travel because they collapse quickly.

"You would need some very exotic type of matter in order to stabilize a wormhole," said Hsu, "and it's not clear whether such matter exists in the universe."

But more recent research found that a wormhole containing "exotic" matter could stay open and unchanging for longer periods of time.

Exotic matter, which should not be confused with dark matter or antimatter, contains negative energy density and a large negative pressure. Such matter has only been seen in the behavior of certain vacuum states as part of quantum field theory.

If a wormhole contained sufficient exotic matter, whether naturally occurring or artificially added, it could theoretically be used as a method of sending information or travelers through space. Unfortunately, human journeys through the space tunnels may be challenging.

"The jury is not in, so we just don't know," physicist Kip Thorne, one of the world's leading authorities on relativity, black holes and wormholes, told Space.com. "But there are very strong indications that wormholes that a human could travel through are forbidden by the laws of physics. That's sad, that's unfortunate, but that's the direction in which things are pointing."

Wormholes may not only connect two separate regions within the universe, they could also connect two different universes. Similarly, some scientists have conjectured that if one mouth of a wormhole is moved in a specific manner, it could allow for time travel.

"You can go into the future or into the past using traversable wormholes," astrophysicist Eric Davis told LiveScience. But it won't be easy: "It would take a Herculean effort to turn a wormhole into a time machine. It's going to be tough enough to pull off a wormhole."

However, British cosmologist Stephen Hawking has argued that such use is not possible. [ Weird Science: Wormholes Make the Best Time Machines]

"A wormhole is not really a means of going back in time, it's a short cut, so that something that was far away is much closer," NASA's Eric Christian wrote.

Although adding exotic matter to a wormhole might stabilize it to the point that human passengers could travel safely through it, there is still the possibility that the addition of "regular" matter would be sufficient to destabilize the portal.

Today's technology is insufficient to enlarge or stabilize wormholes, even if they could be found. However, scientists continue to explore the concept as a method of space travel with the hope that technology will eventually be able to utilize them.

"You would need some of super-super-advanced technology," Hsu said. "Humans won't be doing this any time in the near future."

Follow Nola Taylor Redd on Twitter @NolaTRedd or Google+. Follow us at @Spacedotcom, Facebook or Google+.

Additional resources

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A wormhole is a theoretical passage through space-time that could create shortcuts for long journeys across the universe. Wormholes are predicted by the theory of general relativity. But be wary: wormholes bring with them the dangers of sudden collapse, high radiation and dangerous contact with exotic matter.

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The Solar System
For I dipped into the Future, far as human eye could see; saw the vision of the world, and all the wonder that would be. -Alfred Lord Tennyson, 1842

Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes: the satellites of the planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium. The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system. The Sun's nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away. The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy. It is a spiral galaxy like the Milky Way but is 4 times as massive and is 2 million light years away. Our galaxy, one of billions of galaxies known, is traveling through intergalactic space.

The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction, in nearly circular orbits. When looking down from above the Sun's north pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in or near the same plane, called the ecliptic. Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets. Because of this, for part of its orbit, Pluto is closer to the Sun than is Neptune. The axis of rotation for most of the planets is nearly perpendicular to the ecliptic. The exceptions are Uranus and Pluto, which are tipped on their sides.

The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. The following table is a list of the mass distribution within our Solar System.

Sun: 99.85%

Planets: 0.135%

Comets: 0.01% ?

Satellites: 0.00005%

Minor Planets: 0.0000002% ?

Meteoroids: 0.0000001% ?

Interplanetary Medium: 0.0000001% ?

Nearly all the solar system by volume appears to be an empty void. Far from being nothingness, this vacuum of "space" comprises the interplanetary medium. It includes various forms of energy and at least two material components: interplanetary dust and interplanetary gas. Interplanetary dust consists of microscopic solid particles. Interplanetary gas is a tenuous flow of gas and charged particles, mostly protons and electrons -- plasma -- which stream from the Sun, called the solar wind.

The solar wind can be measured by spacecraft, and it has a large effect on comet tails. It also has a measurable effect on the motion of spacecraft. The speed of the solar wind is about 400 kilometers (250 miles) per second in the vicinity of Earth's orbit. The point at which the solar wind meets the interstellar medium, which is the "solar" wind from other stars, is called the heliopause. It is a boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun's influence perhaps 100 AU from the Sun. The space within the boundary of the heliopause, containing the Sun and solar system, is referred to as the heliosphere.

The solar magnetic field extends outward into interplanetary space; it can be measured on Earth and by spacecraft. The solar magnetic field is the dominating magnetic field throughout the interplanetary regions of the solar system, except in the immediate environment of planets which have their own magnetic fields.

The terrestrial planets are the four innermost planets in the solar system, Mercury, Venus, Earth and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth's. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. The following diagram shows the approximate distance of the terrestrial planets to the Sun.

Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all gigantic compared with Earth, and they have a gaseous nature like Jupiter's. The Jovian planets are also referred to as the gas giants, although some or all of them might have small solid cores. The following diagram shows the approximate distance of the Jovian planets to the Sun.

Our Milkyway Galaxy
This image of our galaxy, the Milky Way, was taken with NASA's Cosmic Background Explorer's (COBE) Diffuse Infrared Background Experiment (DIRBE). This never-before-seen view shows the Milky Way from an edge-on perspective with the galactic north pole at the top, the south pole at the bottom and the galactic center at the center. The picture combines images obtained at several near-infrared wavelengths. Stars within our galaxy are the dominant source of light at these wavelengths. Even though our solar system is part of the Milky Way, the view looks distant because most of the light comes from the population of stars that are closer to the galactic center than our own Sun.(Courtesy NASA)

Our Milky Way Gets a Makeover
Like early explorers mapping the continents of our globe, astronomers are busy charting the spiral structure of our galaxy, the Milky Way. Using infrared images from NASA's Spitzer Space Telescope, scientists have discovered that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms.

This artist's concept illustrates the new view of the Milky Way, along with other findings presented at the 212th American Astronomical Society meeting in St. Louis, Mo. The galaxy's two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located between the major arms. The major arms consist of the highest densities of both young and old stars; the minor arms are primarily filled with gas and pockets of star-forming activity.

The artist's concept also includes a new spiral arm, called the "Far-3 kiloparsec arm," discovered via a radio-telescope survey of gas in the Milky Way. This arm is shorter than the two major arms and lies along the bar of the galaxy.

Our sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms. (Courtesy NASA/JPL-Caltech)

Spiral Galaxy, NGC 4414
The majestic galaxy, NGC 4414, is located 60 million light-years away. Like the Milky Way, NGC 4414 is a giant spiral-shaped disk of stars, with a bulbous central hub of older yellow and red stars. The outer spiral arms are considerably bluer due to ongoing formation of young, blue stars, the brightest of which can be seen individually at the high resolution provided by the Hubble camera. The arms are also very rich in clouds of interstellar dust, seen as dark patches and streaks silhouetted against the starlight.(Courtesy NASA/STSCI)

Obliquity of the Eight Planets
This illustration shows the obliquity of the eight planets. Obliquity is the angle between a planet's equatorial plane and its orbital plane. By International Astronomical Union (IAU) convention, a planet's north pole lies above the ecliptic plane. By this convention, Venus, Uranus, and Pluto have a retrograde rotation, or a rotation that is in the opposite direction from the other planets.(Copyright 2008 by Calvin J. Hamilton)

The Solar System
During the past three decades a myriad of space explorers have escaped the confines of planet Earth and have set out to discover our planetary neighbors. This picture shows the Sun and all nine planets of the solar system as seen by the space explorers. Starting at the top-left corner is the Sun followed by the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.(Copyright 1998 by Calvin J. Hamilton)

Sun and Planets
This image shows the Sun and nine planets approximately to scale. The order of these bodies are: Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.(Copyright Calvin J. Hamilton)

Jovian Planets
This image shows the Jovian planets Jupiter, Saturn, Uranus and Neptune approximately to scale. The Jovian planets are named because of their gigantic Jupiter-like appearance.(Copyright Calvin J. Hamilton)

The Largest Moons and Smallest Planets
This image shows the relative sizes of the largest moons and the smallest planets in the solarsystem. The largest satellites pictured in this image are: Ganymede (5262 km), Titan (5150 km), Callisto (4806 km), Io (3642 km), the Moon (3476 km), Europa (3138 km), Triton (2706 km), and Titania (1580 km). Both Ganymede and Titan are larger than planet Mercury followed by Io, the Moon, Europa, and Triton which are larger than the planet Pluto.(Copyright Calvin J. Hamilton)

Diagram of Portrait Frames
On February 14, 1990, the cameras of Voyager 1 pointed back toward the Sun and took a series of pictures of the Sun and the planets, making the first ever "portrait" of our solar system as seen from the outside. This image is a diagram of how the frames for the solar system portrait were taken.(Courtesy NASA/JPL)

All Frames from the Family Portrait
This image shows the series of pictures of the Sun and the planets taken on February 14, 1990, for the solar system family portrait as seen from the outside. In the course of taking this mosaic consisting of a total of 60 frames, Voyager 1 made several images of the inner solar system from a distance of approximately 6.4 billion kilometers (4 billion miles) and about 32° above the ecliptic plane. Thirty-nine wide angle frames link together six of the planets of our solar system in this mosaic. Outermost Neptune is 30 times further from the Sun than Earth. Our Sun is seen as the bright object in the center of the circle of frames. The insets show the planets magnified many times.(Courtesy NASA/JPL)

Portrait of the Solar System
These six narrow-angle color images were made from the first ever "portrait" of the solar system taken by Voyager 1, which was more than 6.4 billion kilometers (4 billion miles) from Earth and about 32° above the ecliptic. Mercury is too close to the Sun to be seen. Mars was not detectable by the Voyager cameras due to scattered sunlight in the optics, and Pluto was not included in the mosaic because of its small size and distance from the Sun. These blown-up images, left to right and top to bottom are Venus, Earth, Jupiter, Saturn, Uranus, and Neptune.(Courtesy NASA/JPL)

The following table lists statistical information for the Sun and planets:

* The Sun's period of rotation at the surface varies from approximately 25 days at the equator to 36 days at the poles. Deep down, below the convective zone, everything appears to rotate with a period of 27 days.

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The Solar System For I dipped into the Future, far as human eye could see; saw the vision of the world, and all the wonder that would be. -Alfred Lord Tennyson, 1842 Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

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This is a classic trick question. Ask a friend, "what is the closest star?" and then watch as they try to recall some nearby stars. Sirius maybe? Alpha something or other? Betelgeuse?

The answer, obviously, is the Sun; that massive ball of plasma located a mere 150 million km from Earth.

Let's be more precise; what's the closest star to the Sun?

Closest Star

You might have heard that it's Alpha Centauri, the third brightest star in the sky, just 4.37 light-years from Earth.

But Alpha Centauri isn't one star, it's a system of three stars. First, there's a binary pair, orbiting a common center of gravity every 80 years. Alpha Centauri A is just a little more massive and brighter than the Sun, and Alpha Centauri B is slightly less massive than the Sun. Then there's a third member of this system, the faint red dwarf star, Proxima Centauri.

It's the closest star to our Sun, located just a short 4.24 light-years away.

Alpha Centauri is located in the Centaurus constellation, which is only visible in the Southern Hemisphere. Unfortunately, even if you can see the system, you can't see Proxima Centauri. It's so dim, you need a need a reasonably powerful telescope to resolve it.

Let's get sense of scale for just how far away Proxima Centauri really is. Think about the distance from the Earth to Pluto. NASA's New Horizons spacecraft travels at nearly 60,000 km/h, the fastest a spacecraft has ever traveled in the Solar System. It will have taken more than nine years to make this journey when it arrives in 2015. Travelling at this speed, to get to Proxima Centauri, it would take New Horizons 78,000 years.

Proxima Centauri has been the nearest star for about 32,000 years, and it will hold this record for another 33,000 years. It will make its closest approach to the Sun in about 26,700 years, getting to within 3.11 light-years of Earth. After 33,000 years from now, the nearest star will be Ross 248.

What About the Northern Hemisphere?

For those of us in the Northern Hemisphere, the closest visible star is Barnard's Star, another red dwarf in the constellation Ophiuchus. Unfortunately, just like Proxima Centauri, it's too dim to see with the unaided eye.

The closest star that you can see with the naked eye in the Northern Hemisphere is Sirius, the Dog Star. Sirius, has twice the mass and is almost twice the size of the Sun, and it's the brightest star in the sky. Located 8.6 light-years away in the constellation Canis Major - it's very familiar as the bright star chasing Orion across the night sky in Winter.

How do Astronomers Measure the Distance to Stars?

They use a technique called parallax. Do a little experiment here. Hold one of your arms out at length and put your thumb up so that it's beside some distant reference object. Now take turns opening and closing each eye. Notice how your thumb seems to jump back and forth as you switch eyes? That's the parallax method.

To measure the distance to stars, you measure the angle to a star when the Earth is one side of its orbit; say in the summer. Then you wait 6 month, until the Earth has moved to the opposite side of its orbit, and then measure the angle to the star compared to some distant reference object. If the star is close, the angle will be measurable, and the distance can be calculated.

You can only really measure the distance to the nearest stars this way, since it only works to about 100 light-years.

The 20 Closest Stars

Here is a list of the 20 closest star systems and their distance in light-years. Some of these have multiple stars, but they're part of the same system.

1. Alpha Centauri - 4.2

2. Barnard's Star - 5.9

3. Wolf 359 - 7.8

4. Lalande 21185 - 8.3

5. Sirius - 8.6

6. Luyten 726-8 - 8.7

7. Ross 154 - 9.7

8. Ross 248 - 10.3

9. Epsilon Eridani - 10.5

10. Lacaille 9352 - 10.7

11. Ross 128 - 10.9

12. EZ Aquarii - 11.3

13. Procyon - 11.4

14. 61 Cygni - 11.4

15. Struve 2398 - 11.5

16. Groombridge 34 - 11.6

17. Epison Indi - 11.8

18. Dx Carncri - 11.8

19. Tau Ceti - 11.9

20. GJ 106 - 11.9

According to NASA data, there are 45 stars within 17 light years of the Sun. There are thought to be as many as 200 billion stars in our galaxy. Some are so faint that they are nearly impossible to detect. Maybe, with technological improvements, scientists will find even closer stars.

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Closest Star to the Sun - Universe Today

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This is a classic trick question. Ask a friend, "what is the closest star?" and then watch as they try to recall some nearby stars. Sirius maybe? Alpha something or other? Betelgeuse? The answer, obviously, is the Sun; that massive ball of plasma located a mere 150 million km from Earth.

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This view shows a close up of toward the south polar region of Saturn's largest moon, Titan, and show a depression within the moon's orange and blue haze layers near the south pole. NASA's Cassini spacecraft snapped the image on Sept. 11, 2011 and it was released on Dec. 22.

Credit: NASA/JPL-Caltech/Space Science Institute

Titan is Saturn's largest moon and the second largest in the solar system (after Ganymede of Jupiter). It is the only moon in the solar system with clouds and a dense, planet-like atmosphere.

Scientists think conditions on Titan are similar to Earth's early years (the main difference is that, because it is closer to the sun, Earth has always been warmer). According to NASA, "In many respects, Saturn's largest moon, Titan, is one of the most Earth-like worlds we have found to date."

An orange haze surrounding Titan kept its surface a mystery for Earth's scientists until the arrival of the Cassini mission in 2004. Titan's atmosphere extends about 370 miles high (about 600 kilometers), which makes it a lot higher than Earth's atmosphere. Because the atmosphere is so high, Titan was thought to be the largest moon in the solar system for a long time. It wasn't until 1980 that Voyager was close enough to discover it was actually smaller than Ganymede.

Titan's atmosphere is active and complex, and it is mainly composed of nitrogen (95 percent) and methane (5 percent). Titan also has a presence of organic molecules that contain carbon and hydrogen, and that often include oxygen and other elements similar to what is found in Earth's atmosphere and that are essential for life.

There is an unsolved mystery surrounding Titan's atmosphere: Because methane is broken down by sunlight, scientists believe there is another source that replenishes what is lost. One potential source of methane is volcanic activity, but this has yet to be confirmed.

Titan's atmosphere may escape to space in a similar way that Earth's atmosphere does. The Cassini spacecraft has detected polar winds that draw methane and nitrogen (charged with interactions with light) out along Saturn's magnetic field and out of the atmosphere. A similar process is believed to happen on Earth with our own magnetic field.

"At Saturn's largest moon, Titan, Cassini and Huygens showed us one of the most Earth-like worlds we've ever encountered, with weather, climate and geology that provide new ways to understand our home planet," says NASA's Jet Propulsion Laboratory website.

There is an abundance of methane lakes, which are mainly concentrated near its southern pole. In 2014, scientists found a transient feature they playfully referred to as " Magic Island." It's possible that nitrogen bubbles formed in Titan's oceans sit on the surface for a period of time, creating a temporary island that eventually dissipates.

"What I think is really special about Titan is that it has liquid methane and ethane lakes and seas, making it the only other world in the solar system that has stable liquids on its surfaces," Jason Hofgartner, a planetary scientist at Cornell University, told Space.com in 2014. "It not only has lakes and seas, but also rivers and even rain. It has what we call a hydrological cycle, and we can study it as an analog to Earth's hydrological cycle - and it's the only other place we know of where we can do that."

Large areas of Titan's surface are covered with sand dunes made of hydrocarbon. Dunes on Titan may resemble the Namibian desert in Africa.

Because methane exists as a liquid on Titan, it also evaporates and forms clouds, which occasionally causes methane rain. Clouds of methane ice and cyanide gas float over the moon's surface.

"Titan continues to amaze with natural processes similar to those on the Earth, yet involving materials different from our familiar water," Cassini deputy project scientist Scott Edgington, of NASA's Jet Propulsion Laboratory in Pasadena, California, said in a statement.

Sunlight is quite dim on Titan, and climate is driven mostly by changes in the amount of light that accompanies the seasons.

Data also suggests the presence of a liquid ocean beneath the surface, but it is still to be confirmed. [ VIDEO: Tour the Strangest Lakes of Saturn's Moon Titan]

As more planets have been found outside of the solar system, Titan has served as a model of cloudy bodies. Examining the atmosphere of the moon has helped scientists to understand the atmospheres of these distant systems.

"It turns out that there's a lot you can learn from looking at a sunset," said Tyler Robinson of NASA's Ames Research Center in a statement.

In 2017, the Cassini spacecraft wrapped up its two-decade-long mission to Saturn. Launched on October 15, 1997, the space craft arrived at Saturn on June 30, 2004. Upon its arrival, Cassini dropped the Huygens probe built by the European Space Agency. Huygens was equipped to study Titan by landing on the Saturn moon and achieved astounding results. For example, many mountains above 10,000 feet high have been identified on the moon. [ Video: Future Mission to Saturn's Moon Titan]

The Huygens probe landed via parachute on Jan. 14, 2005. Because of Huygens's observations, Titan became a top priority for scientists. The mission has achieved excellent results, such as taking the highest resolution images ever achieved of this moon´s surface.

During its primary and extended missions, Cassini was able to get fundamental data about Titan's structure and the complex organic chemistry of its atmosphere. It is because of Cassini's findings that scientists suspect the presence of an internal ocean composed of water and ammonia. The spacecraft has also spotted seasonal change, such as when an ice cloud formed in Titan's southern hemisphere in 2015 (suggesting that winter was going to be harsh in that zone).

The focus of the mission, as it relates to Titan, was to find signs of seasonal changes and volcanic activity.

Titan played a dominant role in Cassini's planned ending. The massive moon provided the gravitational boost the spacecraft needed to thread between Saturn's rings in its final months, exploring a never-before-seen region. Titan's boost, which came more than four months before Cassini's suicide plunge, was the point of no return, increasing the spacecraft's velocity by approximately 1,925 mph (3,098 km/h) with respect to Saturn.

"With this flyby we're committed to the Grand Finale," Earl Maize, Cassini project manager at JPL, said in a statement after the April 2017 flyby. "The spacecraft is now on a ballistic path, so that even if we were to forgo future small course adjustments using thrusters, we would still enter Saturn's atmosphere on Sept. 15 no matter what."

After this major boost, the spacecraft continued to use Titan to tweak its orbit, making its final close approach to Titan on September 12, 2017. Over its 13 years orbiting Saturn, Cassini made 127 encounters with the moon, some close and others more distant.

"Cassini has been in a long-term relationship with Titan, with a new rendezvous nearly every month for more than a decade," Maize said in a separate statement "This final encounter is something of a bittersweet goodbye, but as it has done throughout the mission, Titan's gravity is once again sending Cassini where we need it to go.

This false-color image from NASA's Cassini spacecraft shows Titan in ultraviolet and infrared wavelengths.

Credit: NASA/JPL/Space Science Institute

It is thought that conditions on Titan could make the moon more habitable in the far future. If the sun increases its temperature (6 billion years from now) and becomes a red giant star, Titan's temperature could increase enough for stable oceans to exist on the surface, according to some models. If this happens, conditions in Titan could be similar to Earth's, allowing conditions favorable for some forms of life. [ VIDEO: Life on Titan? Saturn's Cold Moon Fascinates Scientists]

Experiments on Earth suggest that Titan could be more habitable than previously thought. Complex organic chemicals once thought to hover high in the atmosphere may lie closer to the surface than estimated.

"Scientists previously thought that as we got closer to the surface of Titan, the moon's atmospheric chemistry was basically inert and dull," Murthy Gudipati, the paper's lead author at JPL, said in a statement. "Our experiment shows that's not true. The same kind of light that drives biological chemistry on Earth's surface could also drive chemistry on Titan, even though Titan receives far less light from the sun and is much colder. Titan is not a sleeping giant in the lower atmosphere, but at least half awake in its chemical activity."

NASA's Cassini spacecraft peers through the murk of Titan's thick atmosphere in this view, taken with Cassini's narrow-angle camera on Sept. 25, 2008.

Credit: NASA/JPL/Space Science Institute

Titan's name comes from Greek mythology. The Titans were elder gods who ruled the universe before the Olympians came to power, according to the Theoi Project website.

The moon was discovered by Dutch astronomer Christiaan Huygens in 1655. The Huygens lander probe sent to the moon aboard NASA's Cassini spacecraft by the European Space Agency is named in his honor. Huygens was the first human-built object to land on Titan's surface. [ Amazing Photos of Titan]

Titan's diameter is 50 percent larger than that of Earth's moon. Titan is larger than the planet Mercury but is half the mass of the planet.

Titan's mass is composed mainly of water in the form of ice and rocky material.

Titan has no magnetic field.

Diameter: 3,200 miles (5,150 kilometers), about half the size of Earth and almost as large as Mars

Surface temperature: minus 290 Fahrenheit (minus 179 degrees Celsius), which makes water as hard as rocks and allows methane to be found in its liquid form

Surface pressure: Slightly higher than Earth's pressure. Earth's pressure at sea level is 1 bar while Titan's is 1.6 bars.

Orbital period: Nearly 16 Earth-days. Titan is tidally locked in synchronous rotation with Saturn, keeping one face turned to the planet as it orbits.

Follow Nola Taylor Redd at @NolaTRedd, Facebook, or Google+. Follow us at @Spacedotcom, Facebook or Google+.

Additional reporting by contributor Elizabeth Howell.

Additional resources

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Titan: Facts About Saturn's Largest Moon

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Titan is Saturn's largest moon and the second largest in the solar system (after Ganymede of Jupiter). It is the only moon in the solar system with clouds and a dense, planet-like atmosphere. Scientists think conditions on Titan are similar to Earth's early years (the main difference is that, because it is closer to the sun, Earth has always been warmer).

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A white hole is a hypothetical feature of the universe. It is considered the opposite of a black hole. As black holes don't let anything escape from their surface, white holes are eruptions of matter and energy and nothing can get inside them.

White holes are a possible solution to the laws of general relativity. This law implies that if eternal black holes exist in the universe, then a white hole should also exist. It is a time-reversal of a black hole. They are expected to have gravity, so they attract objects, but anything on a collision course with a white hole would never reach it.

Theoretically, if you were to approach a white hole in a spacecraft, you would be inundated by a colossal amount of energy, which would most likely destroy your ship. Even if your spaceship could withstand gamma rays, light itself would start slowing you down like air resistance slowing down a moving vehicle on Earth.

And even if the spaceship is built to be unaffected by the energy emission, space-time would be weirdly warped around a white hole; approaching a white hole would be like going uphill. The acceleration required would get higher and higher while you move less and less. There isn't enough energy in the universe to get you inside.

Of course, this is fairly counterintuitive. How could energy in a white hole seemingly come from nowhere other than space-time itself? This is one reason why their existence is very unlikely. However, there are some theories in which white holes are possible, but perhaps not quite as described in general relativity.

As they are alleged counterparts of black holes, white holes too would be formed by a gravitational singularity. A singularity is a point-like feature in space-time where the gravitational field becomes infinite. Infinite values in physics are usually an indication of missing pieces in a theory, so it is not surprising that quantum mechanics and relativity struggle to explain the finer details of singularities.
Potential candidates
A lot of phenomena have been put forward as white holes. They are usually chosen because they are mysterious objects that we have not been able to explain in detail.

Gamma ray bursts, fast-spinning pulsars, and black holes reaching the end of their lives have all been considered. Even the Big Bang has been described as a white hole. But so far, no white holes have ever been directly observed, and even their theoretical existence raises some red flags. It seems like white holes are used as a placemark until more observations or a better theory come along.

The Big Bang as a white hole is a clear example of this trend. Until we were uncertain about the size of the universe, there was speculation that the cosmos was produced by a white hole larger than what we could see. We now know that the universe is most likely infinite, which makes the white hole explanation almost certainly wrong.
We know black holes exist - so should white holes exist, too? Vadim Sadovski/Shutterstock Theoretical constraints
A white hole is a particular kind of singularity: a naked singularity. Singularities like black holes cannot be directly observed, because the escape velocity (the speed you need to break free of its gravity) is greater than the speed of light, so nothing can escape from it. The singularity is "protected" by an event horizon, the surface that separates us from the black hole. Mathematically, when we have a singularity, space-time is broken. To avoid this issue, event horizons were introduced.

A naked singularity has no event horizon. According to the fundamental principles of general relativity, the universe doesn't allow naked singularities. The idea is aptly called the cosmic censorship hypothesis. Numerical simulations and the current theories of quantum gravity, however, hint at the possibility of naked singularities.

A curious phenomenon happens in describing a black hole's properties with a quantum mechanical approach, which doesn't include gravity. If you look at a black hole backward or forward in time, it behaves exactly in the same way and remains a black hole. This is not the most important clash between quantum theories and relativity, but it is significant nevertheless.

The most important constraint is entropy, the measure of the order of a system. According to the laws of thermodynamics, the net entropy of the universe is always increasing. Entropy could decrease locally; for example, a freezer decreases the entropy of water by turning it into ice, but the freezer engines emit a lot of heat, so the total entropy is still increasing.

White holes decrease entropy, which is a fundamental piece of evidence against them. In this universe, we obey the laws of thermodynamics. And so far, no confirmed violations have been observed, although we often hear claims of perpetual motion machines and unusual events.
The future of white holes
White holes fascinate a lot of people and they give us a sense of balance. People will and should continue to study them. Indeed, several features of general relativity, black holes for example, were at first considered a theoretical curiosity. There is no hard evidence proving that white holes exist, but maybe in our vast complicated universe, there's space even for them.

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A white hole is a hypothetical feature of the universe. It is considered the opposite of a black hole. As black holes don't let anything escape from their surface, white holes are eruptions of matter and energy and nothing can get inside them. White holes are a possible solution to the laws of general relativity.

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A light-year is how astronomers measure distance in space. It's defined by how far a beam of light travels in one year - a distance of six trillion miles. Think of it as the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong. If you like to keep up with what's going on in astronomy, it's worth spending a little bit of time understanding what the deal is with this funny unit of measurement.

Have you ever been on your way to meet some friends and, while en route, called to tell them you're running late? If you're like me, you might tell them "I'm about ten minutes away". You're using how long it will take you to get there as a substitute for how far away you are. Astronomers do the same thing when figuring out the distance to a star or galaxy, but instead of relying on how fast they could drive or walk, they use a beam of light as their reference.

Why do things this way? First, light is convenient. Throughout the universe, all light travels at exactly the same speed: about 670 million miles per hour. We don't usually think about light traveling anywhere because when we flick on a light switch - there it is! We don't have to wait for the room to light up. It happens instantaneously. Except that it's not instantaneous, just insanely fast. In fact, let's pause for a second and reflect on how ludicrously quick the speed of light is.

Traveling at that speed, you would encircle the globe of Earth almost eight times in one second.

Impressive! Certainly faster than traveling by airplane. So if you were to travel off the Earth in a straight line at light speed, you'd get pretty far in the same amount of time, right? Actually, since space is so vast, you wouldn't have even made it to the moon. Travel to the moon takes about a second-and-a-half, at light speed. Travel to the sun at light speed takes about eight minutes. Can you even imagine? Traveling at a speed where you cross nearly 200,000 miles every second for eight full minutes would only get you to the center of our solar system.

The fact that light takes time to get anywhere has an interesting side effect. If light from the sun takes eight minutes to get to us, then we're actually seeing the sun as it was eight minutes ago. We're looking into the past!

While this sounds a little crazy, it's actually something with which you're already familiar. If you've ever seen fireworks, for example, you know that you see the explosion and then a few seconds later you hear it. If you close your eyes during the fireworks show, you'd only have your ears to know when things were happening. Since it takes some time for the sound to get to you, you'd always be hearing things a few seconds after they happened. The same happens with light: we only see something once the light from that event actually gets to our eyes. When we're looking across a room, the time delay is only a few billionths of a second. But when we start looking across large enough distances, the light becomes noticeably delayed like the sounds are from exploding fireworks.

The main reason for using light years, however, is because the distances we deal with in space are immense. If we stick to miles or kilometers we quickly run into unwieldy numbers just measuring the distance to the nearest star: a dim red dwarf called Proxima Centauri that sits a mere 24,000,000,000,000 miles away! Using a longer yard stick, so to speak, helps keep the numbers at least manageable. To an astronomer, the distance to Proxima Centauri is only four light years. Put another way, it takes the light from our stellar next door neighbor four years to get to Earth.

The Milky Way galaxy in which our sun and all the stars we see at night reside spans 100,000 light-years from one end to the other. Putting that into perspective, the duration of recorded human history is roughly 5,000 years. So light from a star at one end of our galaxy takes 20 times longer than all of recorded history to get to the other end.

Moving beyond our galaxy, it's just over two million light years to our nearest galactic neighbor, the Andromeda galaxy. The light we currently see from that galaxy left there about the same time the ancestors of modern humans were first discovering stone tools.

Our last stop takes us to the edge of the visible universe. It's also here where the trickiness of measuring distance in an ever-expanding universe becomes apparent. The light we see coming from the farthest depths of the universe has been traveling across the cosmos for almost three times longer than our planet has existed: nearly 14 billion years! But there's a catch: we cannot say that the edge of the visible universe is 14 billion light-years away.

Why? Because the universe has grown larger in that time! A galaxy whose light took 14 billion years to reach our little planet has, in the intervening aeons, moved even further away. The current physical distance to that remote beacon, if we stopped the universe from expanding and stretched out a really long tape measure, is just over 46 billion light years! Even in light years, measuring distances across the universe becomes unwieldy. But measuring in something familiar, like miles, is truly humbling. From here to the edge of our vision spans a distance of approximately 276,000,000,000,000,000,000,000 miles.

And it's getting bigger every day.

Bottom line: How far is a light-year? It's the distance a beam of light travels in one year - a distance of six trillion miles.

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distance to sun and moon in light-minutes

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A light-year is how astronomers measure distance in space. It's defined by how far a beam of light travels in one year - a distance of six trillion miles. Think of it as the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong.

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Impact craters are geologic structures formed when a large meteoroid, asteroid or comet smashes into a planet or a satellite.

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NASA's Columbia orbiter launches skyward on April 12, 1981 on NASA's first-ever shuttle flight, STS-1. Commanding the 54-hour mission was astronaut veteran John Young with then-rookie flyer Robert Crippen as pilot.

Credit: NASA.

NASA's space shuttle was the world's first reusable spacecraft. It launched like a rocket and returned to Earth like a glider. It was designed to carry large payloads - such as satellites - into orbit and bring them back, if necessary, for repairs.

Following test flights using the shuttle Enterprise (which did not go into space), the first space shuttle mission, STS-1, launched on April 12, 1981, aboard the orbiter Columbia. The last shuttle to fly was Atlantis on the STS-135 mission in July 2011. The space shuttle suffered two major disasters - on Jan. 28, 1986 (Challenger) and Feb. 1, 2003 (Columbia); 14 astronauts died on the two missions.

The space shuttle's largest contribution was building the International Space Station, which remains in orbit today to conduct hundreds of science experiments annually on human health, engineering and other matters. The program is also remembered for launching and servicing the Hubble Space Telescope, visiting the Russian space station Mir, launching numerous satellites and probes, and performing thousands upon thousands of hours of basic science experiments.

The space shuttle, officially called the Space Transportation System, was made up of three main components [ Infographic: NASA's Space Shuttle - From Top to Bottom]:

Two solid rocket boosters, which provided most of the shuttle's thrust during launch

The huge rust-colored external tank, which fed fuel to the three main engines during launch

The orbiter, which contained the crew cabin, payload bay, and three main engines.

The solid rocket boosters (SRBs) operated for the first two minutes of flight to provide additional thrust needed to get the shuttle into orbit. About 24 miles (45 kilometers) up, the boosters separated from the external tank and descended on parachutes into the Atlantic Ocean. Ships recovered them, and they were refurbished for reuse.

Each booster contained a solid propellant motor - the largest ever developed for space flight. Each motor contained more than 1 million lbs. (450,000 kilograms) of a propellant, a solid mixture of ammonium perchlorate and aluminum, as well as an iron oxide catalyst to aid the burning reaction and a "binder" material to hold all the components together, according to the American Chemistry Council. The entire mixture has the consistency of a pencil eraser. Ammonium perchlorate is also used by the U.S. military in rockets, explosives, flares and ammunition..

The 15-story, rust-colored external tank was the only shuttle component that was not reused. It fed more than 500,000 gallons of fuel - liquid oxygen and liquid hydrogen - to the shuttle's main engines during launch. The tank was also the "backbone" of the space shuttle structure. It provided support for the rocket boosters and orbiter.

After the solid rocket boosters separated, the orbiter carried the external tank to about 70 miles (113 km) above the Earth. With its fuel spent, the tank separated and fell along a planned trajectory. Most of it burned up in the atmosphere, and the rest fell into the ocean.

The orbiter is the component most people think of as "the shuttle." It was the heart and brains of the system and the actual ship that took people to space and brought them back. The orbiter was about the same size as a DC-9 aircraft. It was 122 feet (37 meters) long and had a wingspan of 78 feet (23 m). The crew compartment, located in the forward fuselage, normally carried crews of seven astronauts, but occasionally carried fewer people. The largest crew size for a shuttle mission was eight astronauts.

The mid-fuselage housed a 60-foot (18-meter) payload bay and robotic arm. The bay could hold satellites, modules containing whole laboratories, and construction materials for the International Space Station. The aft fuselage held the orbital maneuvering system, main engines and vertical tail. Smaller thrusters located at the shuttle's nose and aft fuselage were used for small flight adjustments.

The space shuttle grew out of several efforts to develop reusable spacecraft. The X-15 program in the 1950s tested the idea of flying a space plane. The U.S. Air Force also conducted studies on semi-reusable spacecraft in the 1960s. NASA began work on an Integrated Launch and Re-entry Vehicle (ILRV) in 1968, and by 1969 the space shuttle's development received approval from then-President Richard Nixon.

The original vision of the space shuttle program was to develop a vehicle that would launch into space very frequently (several times a month) to deploy and repair satellites as required. The military was also an active participant in the development, and the shuttle's payload bay (which carried equipment into satellites into space) was enlarged in the design phase to accommodate larger military satellites.

Specifically, the National Reconnaissance Office asked that the payload bay be enlarged and that the shuttle eventually run polar missions, which are suitable for satellites to see the entire Earth's surface below. The Air Force constructed a launch pad in Vandenberg, Calif., for polar-orbiting missions, but the idea was abandoned after the Challenger disaster of 1986. Although several shuttle military missions ran in the 1980s, the practice dwindled down and ceased after Challenger's explosion. [ Related: Classified Shuttle Missions: Secrets in Space]

In the early days of the space shuttle program, mission tasks included bringing up civilians to manage experiments, and deploying satellites - both civilian and military. Astronauts tested daring equipment such as the Manned Maneuvering Unit, a sort of jet backpack that would allow crew members to leave the shuttle untethered and pick up satellites for repair. Other satellites were captured and/or deployed using the Canadarm, a robotic arm.

These activities greatly lessened (and then ceased) after the Challenger space shuttle explosion of 1986, when it became clear that space shuttles could only launch a few times a year instead of many. There was a concern that the astronauts were doing risky spacewalks. As well, military satellites were gradually moved to single-use rockets, providing more frequent launch opportunities at a lower cost.

What didn't change on space shuttle missions, however, was performing experiments. Over the 30 years of the space shuttle program, some 355 individual cosmonauts and astronauts used the space shuttle for launching, landing or both.. They collectively put in thousands upon thousands of hours of work in space investigating all sorts of science, ranging from human health to engineering to astronomy to animal studies. [ Related: NASA's Space Shuttle By the Numbers: 30 Years of a Spaceflight Icon]

The space shuttle flew 11 times to the Russian space station Mir between 1994 and 1998, with seven American astronauts doing extended stays on the space station. This was the first major in-space cooperation between Russia and the United States since the Apollo-Soyuz mission of 1975, when Americans and cosmonauts from the Soviet Union docked for a few brief days in space. After the Soviet Union collapse in 1991, NASA agreed to cooperation with the Russians that began with shuttle-Mir and continued with the International Space Station program.

Perhaps the most famous task that the shuttle undertook was bringing up astronauts, pieces and equipment to build the International Space Station. Completing the space station took 13 years and dozens of shuttle missions; the number of shuttle missions total that docked at the station was 37, or more than one-third of the shuttle's total count of 135 missions.

Besides hundreds of hours of astronaut spacewalks, some of the main components that the space shuttle itself contributed included the European Columbus laboratory, the Harmony node, the Tranquility node, the Japanese Kibo laboratory, solar panels, airlocks and the Canadarm2 robotic arm used for spacecraft berthing. The space shuttle also carried vital equipment needed for the interior, with some examples including exercise equipment, science racks, toilets and of course, fresh food.

The space shuttle is also well known for its repeated successful servicing of the Hubble Space Telescope. The telescope was deployed on April 25, 1990, during space shuttle Discovery mission STS-31. Unfortunately, a flaw in the telescope's mirror was discovered, which greatly compromised its ability to do astronomical observations. NASA deployed a follow-up servicing mission, STS-61, in December 1993. The astronauts installed several instruments - including a focus correction system - to complete success, garnering worldwide acclaim after the embarrassment of the initial Hubble launch.

NASA subsequently did servicing missions in 1997 (STS-82), 1999 (STS-103) and 2002 (STS-109). The last servicing mission was initially canceled in 2003 following concerns for astronaut safety after the Columbia disaster; since Hubble is on a different orbit from the International Space Station, the crew could not shelter at the station in case the shuttle was damaged.

Astronomers worried that there would be too big of a gap between the end of Hubble's lifetime and the beginning of the work of the James Webb Space Telescope, which initially was supposed to launch in 2010-11 but is expected to launch in 2020. After an outcry from scientists, the public and Congress, NASA administrator Sean O'Keefe said in 2004 that he would review the decision. A new administrator, Michael Griffin, arrived in 2005 and authorized a servicing mission for 2008. The servicing mission was delayed until 2009 (STS-125) to add a data-handling unit, replacing one that failed on orbit. The mission was executed to great success and international attention, and Hubble remains in good health as of 2018.

The shuttle program had five space shuttles, as wellas the test orbiter Enterprise. Here are some notable facts about each one.

While the shuttle program was mostly conducted in space, Enterprise was constructed for drop-and-landing tests in 1977, furthering previous NASA and military work on flying-wing space vehicles. Enterprise successfully did a series of rigorous tests, starting with taxi work and culminating in several free flights and touchdowns.. Enterprise was deployed on goodwill tours to several countries, and then became the property of the Smithsonian Institution. It temporarily was displayed at the Stephen F. Udvar-Hazy airport annex of the Smithsonian National Air and Space Museum in Fairfax, Va., and then moved permanently in 2012 to the Intrepid Sea, Air and Space Museum in New York City.

Columbia (1981-2003): This was the first space shuttle to fly in space. It conducted several test flights, which dealt with early problems with the heat shield and with the automatic landing systems. Operational missions began in 1982. Some of its major mission activities included Spacelab, deploying the Chandra X-Ray Observatory and servicing the Hubble Space Telescope. The last mission, STS-107, ended catastrophically on Feb. 1, 2003, during landing when the shuttle broke up in the atmosphere, killing its seven-person crew. The cause was traced to a foam piece falling off of the external tank during takeoff and smashing into the wing, causing a hole. Several design changes were made to the space shuttle program after Columbia's demise, and a new inspection procedures to look at tiles after launch (while the astronauts were in space) were devised.

Challenger (1983-1986): Challenger was originally constructed as a test vehicle and then was upgraded for spaceflight. Its major mission milestones included launching the first Tracking and Data Relay Satellite (which the shuttles used to stay in touch with Mission Control), flying the first American female astronaut (Sally Ride), flying the first African-American (Guion Bluford) and the first astronaut repair of a satellite (the Solar Maximum Mission satellite). The shuttle exploded on Jan. 28, 1986, during liftoff of mission STS-51L, killing all seven astronauts on board. The technical cause was traced to an external booster rocket seal failing, but managerial causes were also blamed - including extensive pressure on the shuttle program to launch frequently. The disaster prompted design changes, a safety review (and changes) of the shuttle program, and a permanent slowdown of shuttle launch frequency.

Discovery (1984-2011): Discovery's first mission, STS-41D in 1984, underwent a pad abort due to a fuel valve issue, but the mission launched safely a few weeks later and released three communications satellites. It flew 39 missions - the most of any shuttle - and some of the major payloads it released included the Hubble Space Telescope, the sun-bound Ulysses spacecraft and the Upper Atmosphere Research Satellite. It even flew Mercury astronaut John Glenn at age 77, making him the oldest person to fly in space to date. Discovery was also the first shuttle to fly "return to flight" missions after both the Challenger and Columbia disasters. It was decommissioned after its last flight in 2011 and is now displayed at the Stephen F. Udvar-Hazy airport annex of the Smithsonian National Air and Space Museum in Fairfax, Va.

Atlantis (1985-2011): The first mission of this shuttle was a secret military mission in 1985, for which few details are known even today. Some of its other activities included lobbing three communication satellites into orbit in one mission, launching the Magellan spacecraft toward Venus, launching the Galileo probe toward Jupiter, flying most of the missions of the shuttle-Mir program, and flying the last space shuttle mission (STS-135 in 2011). The shuttle was decommissioned and is currently on display at the Kennedy Space Center Visitor Complex near the Kennedy Space Center in Cape Canaveral, Fla.

Endeavour (1992-2011): Endeavour was constructed out of spare parts from other space shuttles, as a replacement for the Challenger space shuttle that exploded in 1986. During Endeavour's first mission, STS-49 in 1992, crewmembers performed the first three-person spacewalk to bring the hard-to-grasp Intelsat VI satellite into the payload bay for repairs. Endeavour performed several scientifically oriented missions and was the first shuttle to participate in assembly of the International Space Station. After its last mission in 2011, Endeavour was decommissioned and is now on display at the California Science Center.

Additional reporting by Elizabeth Howell, Space.com contributor.

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Space Shuttle: The First Reusable Spacecraft

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NASA's space shuttle was the world's first reusable spacecraft. It launched like a rocket and returned to Earth like a glider. It was designed to carry large payloads - such as satellites - into orbit and bring them back, if necessary, for repairs.

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This image was taken during the 1999 Leonid meteor storm as part of NASA's Leonid Multi-Instrument Aircraft Campaign (Leonid MAC). The image was captured with a 28 mm camera.

Credit: NASA

Meteor showers occur when dust or particles from asteroids or comets enter Earth's atmosphere at very high speed. When they hit the atmosphere, meteors rub against air particles and create friction, heating the meteors. The heat vaporizes most meteors, creating what we call shooting stars.

While there are stray bits of stuff hitting Earth from all directions, there also are regularly timed "meteor showers" when astronomers can make better predictions about how many meteors will hit the Earth, and from what direction. The key difference is that meteor showers occur when the Earth plows into the trail of particles left behind by a comet or asteroid. Depending on where the trail of particles falls in a particular year, meteor showers can be more or less intense.

Astronomers sometimes even find new meteor showers, such as the case of the Camelopardalids in 2014. Initial predictions put the shower at up to 200 meteors per hour, but in reality, it ended up being a quiet shower for amateur astronomers. The shower became active after the debris trail of Comet 209P/LINEAR intersected with Earth. (The debris trail of comets can shift because of the influence of Jupiter, or other reasons.)

Most meteors become visible at around 60 miles (96.5 kilometers) up. Some large meteors splatter, causing a brighter flash called a fireball, which can often be seen during the day and heard up to 30 miles (48 km) away. On average, meteors can speed through the atmosphere at about 30,000 mph (48,280 kph) and reach temperatures of about 3,000 degrees Fahrenheit (1,648 degrees Celsius).

Most meteors are very small, some as tiny as a grain of sand, so they disintegrate in the air. Larger ones that reach the Earth's surface are called meteorites and are rare.

Whether an object breaks apart depends on its composition, speed and angle of entry. A faster meteor at an oblique angle (slanting rather than straight-on) suffers greater stress. Meteors made of iron withstand the stress better than those of stone. Even an iron meteor will usually break up as the atmosphere becomes denser, around 5 to 7 miles up.

When meteorites do hit the ground, their speed is roughly half what it was upon entry, and they blast out craters 12 to 20 times their size. Craters on Earth form much as they would on the moon or any rocky planet. Smaller objects create bowl-shaped craters. Larger impacts cause a rebound that creates a central peak; slipping along the rim forms terraces. The largest impacts form basins in which multiple rebounds form several inner peaks.

Large meteors can explode above the surface, causing widespread damage from the blast and ensuing fire. This happened in 1908 over Siberia, in what's called the Tunguska event. On June 30, 1908, across hundreds of miles, witnesses saw a ball of fire streak through the sky, suggesting the meteor entered the atmosphere at an oblique angle. It exploded, sending out hot winds and loud noises and shook the ground enough to break windows in nearby villages. Small particles blown into the atmosphere lit the night sky for several days. No meteorite was ever found, and for years many scientists thought the devastation was caused by a comet. Now, the prevailing theory holds that a meteor exploded just above the surface.

A similar event occurred over Chelyabinsk, Russia, when a 17-meter rock exploded 12 to 15 miles above the Earth's surface on Feb. 15, 2013, damaging buildings and injuring more than 1,000 people. According to a statement by Peter Brown of the University of Western Ontario in Canada, "The energy of the resulting explosion exceeded 470 kilotons of TNT" - 30 to 40 times more powerful than the atomic bomb dropped on the Japanese city of Hiroshima during World War II.

Although the Russian event brought into focus the possible danger Earth could suffer from space rocks, most meteors don't cause nearly as much damage. Still, NASA and other entities keep careful track of all asteroids visible from Earth, and are actively engaged in discovering as many asteroids as possible - especially the ones that are larger and would pose more of a (theoretical) threat to Earth. Asteroid orbits are plotted and tracked to see if they will intersect with Earth in the future. While no imminently threatening object has been found, NASA continues the search and posts the results publicly on the Small Body Database Browser.

This view of comet Halley's nucleus was obtained by the Halley Multicolour Camera (HMC) on board the Giotto spacecraft, as it passed within 600 km of the comet nucleus on March 13, 1986.

Credit: ESA. Courtesy of MPAe, Lindau

In ancient times, objects in the night sky conjured superstition and were associated with gods and religion. But misunderstandings about meteors lasted longer than they did about most other celestial objects.

Meteorites (the pieces that make it to Earth) were long ago thought to be cast down as gifts from angels. Others thought the gods were displaying their anger. As late as the 17th century, many believed they fell from thunderstorms (they were nicknamed "thunderstones"). Many scientists were skeptical that stones could fall from the clouds or the heavens, and often they simply didn't believe the accounts of people who claimed to have seen such things.

In 1807, a fireball exploded over Connecticut, and several meteorites rained down. By then the first handful of asteroids had been discovered, and a new theory emerged suggesting meteorites were broken bits off asteroids or other planets. (A theory that still holds.)

The largest meteorite recovered in the United States fell in a wheat field in southern Nebraska in 1948. Witnesses saw a giant fireball in the afternoon that some said was brighter than the sun. The meteorite was found buried 10 feet deep in the ground. It weighed 2,360 pounds.

The most famous meteorite crater in the United States is misnamed Meteor Crater. It's in Arizona, and it's huge. The rim rises 150 feet from the surrounding plain, and the hole is 600 feet deep and nearly a mile wide. It was the first crater that was proven to be caused by a meteorite impact, which occurred between 20,000 and 50,000 years ago.

Meteors are often seen falling from the sky alone - one here, one there. But there are certain times in a year when dozens or even hundreds of meteors per hour will light up the sky, seemingly coming from one part of the sky, radiating in all directions, and falling toward Earth one after the other.

There are several periodic meteor showers that astronomers and amateur observers wait for every year. Meteor showers are named after the constellations from where the shower appears to be coming from. For example, the Orionids appear to originate from the mighty Orion constellation, while Perseid meteors seem to be coming from the Perseus constellation.

Leonids: The brightest and most impressive is the Leonid meteor shower, which can produce a meteor storm that showers the sky with thousands of meteors per minute at its peak. In fact, the term "meteor shower" was coined after astronomers' observed one of Leonids' most impressive displays in 1833. The Leonids occur every November, but the shower's most beautiful display happens at intervals of about 33 years, with the last one lighting Earth's sky in 2002; it is not expected to be repeated until 2028. [ Amazing Leonid Meteor Shower Photos]

Perseids: Another shower that is worth keeping awake for is the Perseid meteor shower, which is associated with the comet Swift-Tuttle, which takes 133 years to orbit the sun. Earth passes through the comet's orbit during the month of August every year. It is not as active as the Leonids, but it is the most widely watched meteor shower of the year, peaking on Aug. 12 with more than 60 meteors per minute.

Orionids: The Orionid meteor shower produces meteors from Halley's comet, which orbits the sun every 75 to 76 years. The Orionid shower happens every October and can last for a week, treating patient observers to a show of 50 to 70 shooting stars per hour at its peak. [ Orionid Meteor Shower Sparks Bright Fireballs (Video)]

Quadrantids: The Quadrantid meteor shower comes from the debris of an asteroid called 2003 EH1, which some astronomers think was part of a comet that broke apart centuries ago. The debris enters Earth's atmosphere in early January and offers astronomers and other observers a brief show. [ Spectacular Quadrantid Meteor Photos]

Richard Hay sends this image of a Quadrantid meteor and says: "My wife and I ventured out into the frigid 26-degree Florida night this morning to catch the shower with our own eyes, and [were] rewarded with about 20 or so bright Quadrantids over a 90 minute period."

Credit: Richard Hay

Geminids: Like the Quadrantids, the Geminid meteor shower also came from dust particles of an asteroid, this time a near-earth asteroid called 3200 Phaeton. Meteor showers are mostly from comets, so having an asteroid as parents make the Quadrantids and Geminids different from other meteor showers. The Geminids happen in December and spray up to 40 meteors per hour out of the Gemini constellation at its peak.

Other meteor showers to watch out for are the Eta Aquarids, also remnants of Halley's comet, in May; and the Lyrids, which have been chronicled for more than 2,000 years, in late April.

People living in the Northern Hemisphere are in the best position to observe the most beautiful meteor showers. For example, North America is right below the region of the sky where the January Quadrantids shower appears.

A bright moon can dim the prospect of seeing a good meteor shower, drowning out all but the brightest meteors. Local light pollution dampens prospects, too, so the best place to view a meteor shower is from out in a rural location.

Most meteor showers are best viewed in the pre-dawn hours, when the part of Earth you are standing on is facing the direction of Earth's orbit. It's like bugs hitting a car's windshield. In the late evening hours, on the other hand, the meteors are less frequent - loosely akin to bugs hitting a car's rear bumper.

Meteor showers can be seen at different times of the year depending on when Earth is going to pass through the comet's or asteroid's path. Some meteor showers happen annually; others only appear over a period of several years, while some of the best shows - meteor storms - happen just once or twice in a lifetime.

Weather can also hamper a good view of meteor showers. A clear sky is a gift to night gazers, which is why meteor showers during the summer are more anticipated than those that fall in the winter months.

Additional reporting by Nola Taylor Redd and Elizabeth Howell, Space.com contributors

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Meteor Showers and Shooting Stars: Formation, Facts and Discovery

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Meteor showers occur when dust or particles from asteroids or comets enter Earth's atmosphere at very high speed. When they hit the atmosphere, meteors rub against air particles and create friction, heating the meteors. The heat vaporizes most meteors, creating what we call shooting stars.

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Table of Moons in Solar System: Discovery, Size and Orbital Period - Windows to the Universe

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As of October 2008, there are known natural moons orbiting planets in our Solar System. moons orbit the "full-size" planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), while moons orbit the smaller " dwarf planets" ( Ceres, Pluto, Haumea, Makemake, and Eris).

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The vast majority of asteroids in the solar system are found in a region of the solar system out beyond Mars. They form the Asteroid Belt. Others orbit in near-Earth space and a few migrate or are thrown out to the outer solar system by gravitational interactions. The four largest asteroids in the belt are Ceres, Vesta, Pallas, and Hygiea. They contain half the mass of the entire belt. The rest of the mass is contained in countless smaller bodies. There was a theory once that if you combined all the asteroids they would make up the missing "Fifth" rocky planet. Planetary scientists estimate that if you could put all that material together that exists there today, it would make a tiny world smaller than Earth's moon.

Where is the asteroid belt located?

The Asteroid Belt is located in an area of space between the orbits of Mars and Jupiter. That places it between 2.2 and 3.2 astronomical units (AU) from the Sun. The belt is about 1 AU thick. The average distance between objects in the Asteroid Belt is quite large. If you could stand on an asteroid and look around, the next one would be too far away to see very well.

Asteroid Mining

The solar system contains many different types of asteroids, grouped by the minerals they contain. The abundances of precious metals such as nickel, iron, and titanium (to name a few), and water make asteroids an attractive target for mining operations when humans decide to expand their presence through interplanetary space. For example, water from asteroids could serve colonies in space, while the minerals and metals would be used to build habitats and grow food for future space colony inhabitants. Beginning 2013, companies interested in asteroid mining began announcing their plans for future operations on distant planetoids. In addition, NASA is looking into similar missions. The biggest obstacles to asteroid mining are the need to develop affordable spaceflight technology that would allow humans to get to the asteroids of interest.

Facts about the Asteroid Belt

What other fascinating things do we know about the Asteroid Belt?

Asteroid Belt objects are made of rock and stone. Some are solid objects, while others are orbiting "rubble piles".

The Asteroid Belt contains billions and billions of asteroids.

Some asteroids in the Belt are quite large, but most range in size down to pebbles.

The asteroid 1/Ceres is also designated as a dwarf planet, the largest one in the inner solar system.

We know of at least 7,000 asteroids.

The Asteroid Belt may contain many objects, but they are spread out over a huge area of space. This has allowed spacecraft to move through this region without hitting anything.

Asteroids get their names from suggestions by their discoverers and are also given a number.

The formation of Jupiter disrupted the formation of any worlds in the Asteroid Belt region by scattering asteroids away. This caused them to collide and break into smaller pieces.

Gravitational influences can move asteroids out of the Belt.

The Asteroid Belt is often referred to as the "Main Belt" to distinguish it from other groups of asteroids such as the Lagrangians and Centaurs.

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Asteroid Belt Facts - Interesting Facts about the Asteroid Belt

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Facts Asteroids What is the asteroid belt? The vast majority of asteroids in the solar system are found in a region of the solar system out beyond Mars. They form the Asteroid Belt. Others orbit in near-Earth space and a few migrate or are thrown out to the outer solar system by gravitational interactions.

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These close views of Pluto's four smaller moons were captured by NASA's New Horizons probe during its historic July 2015 flyby. Data from New Horizons suggests that at least two (and maybe even all four) were born from mergers of even smaller moons.

Credit: NASA/JHUAPL/SwRI

Pluto, the dwarf planet that was once considered the ninth planet, has a growing entourage of satellites. The tiny world has five moons of varying size in orbit around it that tumble and dance in a strange and chaotic pattern.

"The way I would describe this system is not just chaos, but pandemonium," Mark Showalter, a co-investigator on the New Horizons mission, said at a press conference last November.

To the surprise of many, NASA's New Horizons mission did not turn up any additional moons during its historical flyby.

Pluto has one very large moon that is almost half the planet's size. Discovered in 1978, it was named Charon after the demon who ferried souls to the underworld in Greek mythology. (Pluto is named for the god of the underworld.) The huge size of Charon (750 miles or 1,200 kilometers in diameter) sometimes leads scientists to refer to Pluto and Charon as a double dwarf planet or binary system. Pluto's diameter is 1,473 miles (2,370 km).

Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney, Australia. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation - a Pluto day - also takes 6.4 Earth days. This means Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking.

NASA's New Horizons spacecraft obtained this high-resolution enhanced color view of Pluto's moon Charon just before the closest approach on July 14, 2015.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

While Charon is mostly gray, its northern pole has a reddish tint. Most of the moon is covered with water-ice, but the red region is likely made up of shredded pieces of Pluto's atmosphere. The moon has a surprisingly young surface, suggestive that it once housed an ocean in its interior, one that would have long ago frozen.

Compared with most of solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun. Also, Pluto's rotation is retrograde compared to the other worlds - it spins backwards, from east to west.

In 2005, as scientists photographed Pluto with NASA's Hubble Space Telescope in preparation for the New Horizons mission - the first spacecraft to visit Pluto and the Kuiper Belt - they discovered two other tiny moons of Pluto, now dubbed Nix and Hydra. These are two to three times farther away from Pluto than Charon.

While NASA's New Horizons mission was able to capture significant details about Charon during its 2015 flyby, it was only able to view the other four moons from a distance. Still, it provided the best look at the satellites to date. The spacecraft revealed details as small as 2 miles (3 km) on Nix and 0.7 miles (1.2 km) on Hydra.

The mission also provided the best measurements for the tiny moons. Nix is estimated to be 26 miles (42 km) long and 22 miles (36 km) wide, while Hydra is 34 miles (55 km) in length.

Hydra has a shape resembling a mitten or a rubber duck with at least two large craters. The moon resembles Comet 67P/Churyumov-Gerasimenko, which Europe's Rosetta mission has been orbiting since August 2014. Scientists think that Comet 67P may have formed when two separate bodies merged, and the same process may have played a role in the formation of Hydra.

Hydra was the nine-headed mythological serpent that guarded Pluto's realm.

The irregularly shaped Nix looks much like a jellybean, with a surprisingly big impact crater. It is not as bright as Pluto, though brighter than Charon. In Greek mythology, Nyx was the goddess of the night and mother of Charon. The spelling was changed to Nix, the Egyptian goddess of night and darkness, to avoid confusion with two asteroids of the same name.

"Nix isn't very large, and there is a very fine line between an impact that will make a crater that big and one that will break Nix apart," New Horizons team member Simon Porter, of the Southwest Research Institute in Boulder, Colorado, wrote in a blog post.

"So, either Nix was very lucky in surviving that collision, or it's a fragment of an older moon that was somehow destroyed."

All four of the small moons appear to be coated by relatively pure water-ice.

Scientists using Hubble discovered a fourth moon, Kerberos, in 2011. New Horizons revealed that the moon has two lobes, one about 5 miles (8 km) wide and the other about 3 miles (5 km) in diameter. Like Hydra, it may have formed from the merger of two separate objects.

Kerberos orbits between Nix and Hydra. Despite its small size, Hubble observations revealed that it exerts a strong gravitational influence on the neighboring moons. Before New Horizons arrived, scientists thought the moon might be large but dark, keeping it obscured. Instead, the moon is small and bright. The moon was named for the three-headed dog in Greek mythology (Cerberus to the Romans).

The fifth moon, spotted in 2012, is named Styx after the river separating the living from the dead. It is estimated to be irregular in shape and 4.5 miles (7 km) wide. It is in a 59,030-mile (95,000-km) diameter circular orbit around Pluto that is assumed to lie in the same plane as Pluto's other known moons.

Scientists anticipated the discovery of more moons when New Horizons arrived at Pluto, but none were spotted.

Pluto's entire moon system is thought to have formed by a collision between Pluto and a similar-size body early in the history of the solar system. The collision threw out material that coalesced into the family of satellites orbiting Pluto.

New Horizons found that the four moons were surprisingly chaotic. Hydra is the fastest-rotating, spinning once every 10 hours during its 38-day orbit of Pluto, or 89 times every orbit.

"If Hydra were spinning much faster, material would fly off its surface due to the centrifugal force," Showalter said in a statement from NASA.

The other three moons rotate between 6 to 10 times during their orbit, which is still surprisingly fast. In addition to that, Nix is tilted on its axis by 132 degrees and rotating backward.

Such rapid rotations are surprising because the gravity of Pluto should slow down the spin of the smaller moons.

"The question is, why [don't they] slow down? It's not so much why [are they] so fast. ... There's clearly something fundamental about the dynamics of the system that we do not understand," Showalter said in the statement.

Additional reporting by Nola Taylor Redd, Space.com Contributor. Follow Nola on Twitter @NolaTRedd or Google+. Follow us at @Spacedotcom, Facebook or Google+. Originally published on Space.com.

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Pluto's Moons | Five Satellites of Pluto

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Pluto, the dwarf planet that was once considered the ninth planet, has a growing entourage of satellites. The tiny world has five moons of varying size in orbit around it that tumble and dance in a strange and chaotic pattern.

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Sen-Comets, asteroids and meteors are well used terms for objects travelling through the Solar System. The recent impact of an asteroid in Chelyabinsk and the anticipation of seeing comets PANSTARRS and ISON this year have raised interest in the different types of space rock that we can see flying around the Solar System or crashing down to Earth.

All of these objects are some form of space rock and most of the time they are harmless. However, as shown by the asteroids that hit Siberia in 1908 and Chelyabinsk in 2013, and the one believed to have caused the extinction of the dinosaurs, they can cause widespread devastation. In the same weekend that the asteroid injured Chelyabinsk, another asteroid was making a close flyby of Earth, 2012 DA14. The close passing of 2012 DA14 - a 50 metre wide asteroid - was unrelated to the Chelyabinsk one but happening at about the same time as the Russian impactor certainly raised awareness of the potential danger of space rocks and generated public interest in the topic!

Generally the Solar System and our corner of it is a pretty quiet place, with major impacts and collisions with space rocks being very rare. However, on a daily basis thousands of small objects hit Earth's atmosphere without us being aware of them, or they simply give the lucky observers a fantastic light show.

Nearly all of these objects are remnants of the creation of the Solar System and are part of it. Many space rocks were originally from areas such as the Asteroid belt, the Kuiper Belt and the Oort cloud, which extends almost half way to Alpha Centauri, the next closest star system.

The names or classification given to space rocks refer to their characteristics and behaviour, and can be summarised as follows:

Large rocky airless objects that resemble huge boulders, they range in size from a few meters to many tens or even hundreds of kilometres in diameter. The largest asteroid in the Solar System is Ceres with a diameter of 952km - it looks like a small moon and is classed as a dwarf planet! NASA's Dawn space probe is now making its way to study Ceres having recently left another massive asteroid, Vesta. The information gathered on Vesta has led scientists to conclude it was formed in the same way as the inner rocky planets.

Asteroids are usually found in the Asteroid belt between Mars and Jupiter, and the Kuiper belt beyond the most distant planet, Neptune. But there are many that drift through space orbiting the Sun. Some of these rogue asteroids pose the biggest danger to Earth as some pass quite close Asteroids have in the past collided with Earth and will inevitably (with much warning hopefully) collide with the Earth in the future. According to scientists it was a 10 - 15km wide asteroid that caused the extinction of the dinosaurs. Asteroids that pass close to Earth are called Near-Earth Objects (NEOs).

Large objects made up of rock, dust and ice, tens of kilometres in size that originate from the Oort cloud at the extremes of the solar system, they are remnants of its creation billions of years ago. Comets are often referred to as dirty snowballs that have been knocked out of the Oort cloud by a collision, or the gravity of a passing star sends them hurtling toward the Sun. They comprise of a nucleus - the head or rocky core which resembles an asteroid and a coma - a fuzzy atmosphere created by solar radiation heating the nucleus as it enters the inner solar system, causing water ice to vaporise along with other gasses. The coma can form a tail as it passes through the solar wind and some comets can be seen with the naked eye every few years on average and can linger in the sky for many weeks. Many comets are small and do not survive their encounter with the sun and are often swallowed up, but some continue to orbit for thousands of years. Comets can leave a legacy behind them with their trails of dust and small particles of rock known as meteoroids, which give rise to meteor showers.

Artist's Illustration of a comet in the night sky. Credit: Virtualastro

Meteoroids
A small particle of rock or dust left behind by an asteroid or comet and is significantly smaller in size, ranging from a grain of sand to roughly a meter wide.

The visible streak of light caused by an object such as a meteoroid, asteroid or other space debris as it enters Earth's atmosphere. Heat causes the object to glow as it heats up and the streak of light is also known as a shooting star.
A meteor that has impacted the ground, an object can only be classed as a meteorite after impact and can be identified by varying techniques including; testing the object to see if it is magnetic at the impact site or looking for elements such as iridium in a lab. Some meteorites can fetch big money if sold to collectors.

Artist illustration of a meteor. Credit: Virtualastro

Meteor Showers
Numerous meteors observed radiating from a fixed point in the sky, caused by meteoroids from the debris trails of comets. Meteor showers are annual events where the Earth encounters a particular comets debris trail in its orbit. Well known meteor showers include the Perseids in August and Geminids in December - both of these showers can produce up to 100 meteors per hour at their peak. Click here to read my article for Sen on the Geminids. One of the most famous meteor shower events was the Leonids in November 1966 where the shower became a storm with thousands of meteors falling like rain.

That's pretty much it; space rocks are classified and named as the above, and for much of the time they are something exciting and sometimes fun to look at - not to be afraid of. At present there are no large asteroids or comets known to come dangerously close or impact the Earth in the future; however there are some that may look spectacular in the night and even daytime skies.

Hopefully this article has helped you to be able to identify what the different types of space rocks are and how they behave. Good Luck

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Space Rocks - Comets, Asteroids, Meteorites and More - Sen.com

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Sen-Comets, asteroids and meteors are well used terms for objects travelling through the Solar System. The recent impact of an asteroid in Chelyabinsk and the anticipation of seeing comets PANSTARRS and ISON this year have raised interest in the different types of space rock that we can see flying around the Solar System or crashing down to Earth.

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Credit: ESO/L. Calçada

Dwarf planets are worlds that are too small to be considered full-fledged planets, but too large to fall into smaller categories.

In recent years, there's been a lot of hubbub about Pluto losing its status as one of the planets of the solar system. Pluto is no longer considered the ninth planet in the series of major planetary objects, but instead is now just one of the many so-called "dwarf planets." The debate started anew after the New Horizons mission passed by Pluto in 2015, revealing a world of surprising geological complexity. As of 2017, delegates from the mission are trying to get Pluto's planethood status back.

Astronomers estimate that there could be as many as 200 dwarf planets in the solar system and the Kuiper Belt. But the differences between planets and dwarf planets may not be obvious at first.

The International Astronomical Union defines a planet as being in orbit around the sun, has enough gravity to pull its mass into a rounded shape (hydrostatic equilibrium), and has cleared its orbit of other, smaller objects. This last criterion is the point at which planets and dwarf planets differ. A planet's gravity either attracts or pushes away the smaller bodies that would otherwise intersect its orbit; the gravity of a dwarf planet is not sufficient to make this happen. [ Meet the Dwarf Planets of the Solar System]

As of 2014, the IAU recognizes five named dwarf planets: Ceres, Pluto, Eris, Haumea, and Makemake. But those aren't the only ones. Other solar system bodies that are possibly dwarf planets include Sedna and Quaoar, small worlds far beyond Pluto's orbit, and 2012 VP113, an object that is thought to have one of the most distant orbits found beyond the known edge of our solar system. The object DeeDee could also be a dwarf planet, according to observations made in 2017. According to NASA, scientists think that there may be more than a hundred dwarf planets awaiting discovery.

However, the debate over the status of dwarf planets, particularly Pluto, remains a hot topic. The primary concern stems from the requirement for a planet to clear out its local neighborhood.

"In no other branch of science am I familiar with something that absurd," New Horizons principle investigator Alan Stern told Space.com in 2011. "A river is a river, independent of whether there are other rivers nearby. In science, we call things what they are based on their attributes, not what they're next to."

Is a dwarf planet a separate entity from a planet, or simply another classification? The question may not be settled in the near future.

The dwarf planet Ceres, also the largest asteroid in the solar system, is seen here in an amazing view from the Hubble Space Telescope. In March 2015, NASA's Dawn spacecraft is expected to enter orbit around Ceres to study the object like never before.

Credit: NASA, ESA, J. Parker (Southwest Research Institute), L. McFadden (University of Maryland)

Ceres is the earliest known and smallest of the current category of dwarf planets. Sicilian astronomer Giuseppe Piazzi discovered Ceres in 1801 based on the prediction that the gap between Mars and Jupiter contained a missing planet. It is only 590 miles (950 km) in diameter and has a mass of just 0.015 percent that of Earth.

In fact, Ceres is so small that it is classified as both a dwarf planet and an asteroid, and is often named in scientific literature as one of the largest asteroids in the solar system. Although it makes up approximately a fourth of the mass of the asteroid belt, it is still 14 less massive than Pluto.

Unlike its asteroid neighbors, Ceres has a nearly round body. The rocky dwarf planet may have water ice beneath its crust. In 2014, the European Space Agency's Herschel Space Observatory detected water vapor spewing from two regions on Ceres.

NASA's robotic Dawn mission arrived at Ceres in 2015. The mission has shown many interesting features on its surface, ranging from various bright spots to a 4-mile-high (6.5-kilometer-high) mountain. (Another mission, the European Space Agency's Herschel Space Observatory, spotted evidence of water vapor in 2014.)

Pluto is the most well known of the dwarf planets. Since its discovery in 1930 and until 2006, it had been classified as the ninth planet from the sun. Pluto's orbit was so erratic, however, that at times it was closer to the sun than the eighth planet, Neptune. In 2006, with the discovery of several other rocky bodies similar in size or larger than Pluto, the IAU decided to re-classify Pluto as a dwarf planet.

This is the most detailed view to date of the entire surface of the dwarf planet Pluto, as constructed from multiple NASA Hubble Space Telescope photographs taken from 2002 to 2003.

Credit: NASA, ESA, and M. Buie (Southwest Research Institute)

Despite its small size - 0.2 percent the mass of Earth and only 10 percent the mass of Earth's moon - Pluto's gravity is enough to capture five moons of its own. The pairing between Pluto and its largest moon, Charon, is known as a binary system, because both objects are orbiting around a central point that is not within the mass of Pluto.

NASA's New Horizons mission flew by Pluto in 2015 and revealed a wealth of surprises. This included zones that are bereft of craters (indicating the surface is relatively young), mountains that are likely as high as 11,000 feet (3,500 meters), and even haze above the dwarf planet's surface.

When it was first discovered, Eris was thought to be the largest of the dwarf planets, with a mass 27 percent larger than that of Pluto and a diameter of approximately 1,400 to 1,500 miles (2,300 to 2,400 km). It was the discovery of Eris that prompted the IAU to reconsider the definition of a planet. Further observation went on to suggest that the dwarf planet is slightly smaller than Pluto.

The orbit of Eris is very erratic, crossing that of Pluto and nearly intersecting the orbit of Neptune, but is still more than three times larger than Pluto's orbit. It takes 557 years for Eris to orbit the sun. At its farthest point from the sun, a point that is also called its aphelion, Eris and its satellite Dysmonia travel far beyond the Kuiper Belt. The surface of Eris is likely nitrogen and methane-rich, but in a thin (1 millimeter) layer across the surface. Some scientists suggest the surface is the condensed atmosphere of Eris, which expands into gas when the dwarf planet is closer to the sun.

An early artist's interpretation of the dwarf planet Makemake beyond Pluto.

Credit: NASA

Haumea and Makemake are the most recently named dwarf planets in the solar system.

Haumea is unique because of its ellipsoid shape, only just meeting the hydrostatic equilibrium criteria for dwarf planet status. The elongated shape of the dwarf planet is due to its rapid rotational spin, not a lack of mass, which is about one-third that of Pluto. The cigar-shaped dwarf planet rotates on its axis every four hours, likely a result of a collision. The odd object also hosts a red spot and a layer of crystalline ice. Finally, Haumea is the only object in the Kuiper belt other than Pluto known to host more than one moon.

A moon was discovered around Makemake in 2016, more than a decade after the dwarf planet itself was found. Its diameter is known to be about two-thirds that of Pluto, and the newly found moon will allow for measurements of its mass. Makemake is also of value to the astronomical community, as it is another reason for the reconsideration of the definition of a planet. Its comparable mass and diameter to Pluto would grant it planet status if Pluto wasn't also stripped of that title.

Pluto, Eris, Haumea and Makemake are all known as " plutoids," unlike the asteroidal dwarf planetoid Ceres. A plutoid is a dwarf planet with an orbit outside that of Neptune. Plutoids are sometimes also referred to as " ice dwarfs" due to their diminutive size and cold surface temperatures.

The outer planets show evidence of interaction with plutoids. Triton, the largest moon of Neptune, is likely a captured plutoid, and it is even possible that the odd tilt of Uranus on its axis is due to a collision with a plutoid. Similarly to dwarf planets, there are potentially hundreds of plutoid objects in the solar system that have yet to be given official status.

Additional reporting by Elizabeth Howell and Nola Taylor Redd, Space.com contributors

Additional resources

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Dwarf Planets: Science & Facts About the Solar System's Smaller Worlds

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Dwarf planets are worlds that are too small to be considered full-fledged planets, but too large to fall into smaller categories. In recent years, there's been a lot of hubbub about Pluto losing its status as one of the planets of the solar system.

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by Gemma Lavender, 5 September 2013

What do we know about these rather intense gravitational regions?
Asked by Sebastian Paul
The simple answer is that we don't know. A black hole is defined as a region of spacetime from which extremely strong gravity prevents anything, including light, from escaping.

We know that matter falling into black holes is no different from the matter which can be found lurking around the rest of the Universe. However, the closer we get to the centre of a black hole, the faster our understanding of physics breaks down. Thanks to General Relativity, we think we understand what happens in this extreme gravity and with the help of Quantum Mechanics, we can make an intelligent estimate as to what happens at smaller, microscopic scales. But if the two theories are combined - like they would be at the centre of a black hole - they break down, leaving us with no idea as to what's going on!

To get around the problem, astrophysicists need a theory of gravity that is compatible with Quantum Mechanics that might just describe the physics inside a black hole. At the moment though, no such model exists but physicists are working on it.
Image credit: Alain r

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What is a black hole made of? | Space Facts - Astronomy, the Solar System & Outer Space | All About Space Magazine

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by Gemma Lavender, 5 September 2013 What do we know about these rather intense gravitational regions? Asked by Sebastian Paul The simple answer is that we don't know. A black hole is defined as a region of spacetime from which extremely strong gravity prevents anything, including light, from escaping.

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As a black hole sucks material from nearby objects (like this illustration showing the beast pulling gas from a companion star), its event horizon gets bigger.

Credit: NASA E/PO, Sonoma State University, Aurore Simonnet

If you believe the creations of science fiction, black holes serve as gateways to other worlds, either distant parts of this universe or other universes entirely. But the reality might be more complicated than that. And outside of the sci-fi realm, dropping into a black hole is a bad idea.

Even so, it turns out that people who enter a black hole would have at least a slight chance of escaping, either back into their own world or to some exotic place. This is because black holes actually bend space itself, and so could bring points that are ordinarily distant from each other much closer together.

An oft-used analogy is the bending of a piece of paper. If you draw a line on the paper, it follows the paper's shape and the line's length is unchanged by bending the paper. But if you go through the paper, the end points of the line are much closer to one another. Understanding this requires diving into Einstein's theory of relativity as applied to gravity. [ 5 Reasons We May Live in a Multiverse]

Escaping a black hole's grip

It's important to understand that a black hole is not empty space, but rather a place where an enormous amount of matter is shoved into a teensy, tiny area, called a singularity. In fact, the singularity is infinitely small and dense. (There's actually some debate among scientists on this point, but more on that in a minute.)

As one moves closer to the black hole, the escape velocity - the speed needed to escape the black hole's gravity - goes up. At a certain point, escape velocity is greater than the speed of light, or 186,282 miles/second (299,792 kilometers/second). For comparison, the Earth's escape velocity is about 25,000 mph (40,270 km/h) at the surface.

Since nothing can go faster than light, that means nothing can escape a black hole. But there's a loophole: A black hole doesn't suck up everything around it, like a vacuum cleaner or a bathtub drain. Its power extends only as far as the black hole's event horizon, whose radius is the distance from the center of a black hole beyond which nothing can get out. That radius gets bigger as more matter falls into the dense beast. Perhaps it's better to think of a back hole as a ball whose surface allows matter to pass inside, but never the other way.

This illustration shows a black hole named Cygnus X-1, which is sucking the life out of a blue star beside it.

Credit: NASA/CXC/M.Weiss

What's inside that surface is one of the biggest mysteries in astrophysics. Remember that most scientists think a black hole is a singularity. All the matter from whatever originally supplied the black hole's mass (a star, for example) gets crushed into a point that has infinite density. If you were to fall into a black hole, the usual description of such an event says that you would first get stretched into spaghetti by tidal forces, then crushed into nothingness. Your matter would then add to the radius of the black hole's event horizon.

Eventually you'd be emitted as Hawking radiation. Physicist Stephen Hawking's calculations showed that black holes give off photons. In doing so, the black holes lose mass, because according to Einstein's famous E = mc^2 equation, energy and mass are equivalent. Black holes eventually evaporate, but you would be waiting around a long time for that to happen. [ 8 Ways You Can See Einstein's Theory of Relativity in Real Life]

A black hole with the mass of the sun - by cosmic standards that's a small one - takes on the order of 10^87 years to evaporate and turn into a burst of gamma-rays. The universe is about 14 billion years old, or 1.4 x 10^9 years. There's some debate in the scientific community about how long it takes for a black hole to evaporate, because the Hawking radiation doesn't preserve any information about the stuff that fell into the black hole in the first place; but the fact remains that being emitted as radiation is still not good.

What about wormholes?

There might be a better way out of a black hole, though: Gravity bends space. (Think of a sumo wrestler rolling on a mat, indenting the mat with his weight.) Any object creates a local "gravity well." That well gets deeper toward the center of the object. A planet, for example, has a gravity well, but as you go toward the center of a planetary sphere, the well flattens out. Using the mat analogy, any normal object would have a well shaped like a depression with a finite depth.

Black holes don't behave like normal objects ... perhaps fortunate for the trapped individual. The curvature of space just keeps going up and up until you reach the singularity at the center of the black hole, where that curvature is infinite. Instead of a depression, you just have a hole whose sides get steeper as you go toward the center, until they are basically vertical and space is shaped like an infinitely stretched dimple.

And that's why it's a mystery. Scientists use Einstein's theory of relativity to describe the curving of space, but Einstein's equations start to break down in the singularities of black holes. These singularities are also very small, and at that point, one should see quantum mechanical effects. However, nobody has worked out a way to make quantum mechanical theory work with gravity, to figure out what a singularity might look like.

It gets even weirder when you realize that black holes aren't static. Realistically, any object in space tends to rotate. That means the singularity could, if it rotates fast enough, becomes a ring, rather than a point. A ring singularity could provide a gateway to other universes (as in the 1994 sci-fi novel "Ring," by Stephen Baxter, published by HarperCollins). So a black hole could be a wormhole, a gateway through space and time.

The idea is so intriguing because when you have a point singularity, no matter how you travel, the singularity is always in your future if you are inside the event horizon. But a ring singularity can behave differently; the part that crushed you into nothing doesn't always have to be in your future, because of the weird ways a ring singularity would bend and twist space and time.

However, the concept of a ring singularity as a gateway is far from a sure thing. First, nobody knows how a ring singularity would come into existence. The other problem is that whenever people have tried to work out the mathematics of a black-hole-made wormhole, they run into problems of keeping the gateway stable. "In any realistic construction, they are still considered wildly unstable to anything that we'd consider regular matter," said Robert McNees, an associate professor of physics at Loyola University Chicago. Previous work by other theorists seems to show that the only potential way to make wormholes is to be with what's called "exotic matter," matter with negative mass. But there's no clear idea what that would mean.

Which brings up the fundamental problem: While most scientists say black holes can be wormholes, "Without a theory of quantum gravity, such questions are hard to answer conclusively," McNees said.

The other issue is that nobody has observed stuff coming out of nowhere, as one would expect if black holes could be gateways to other universes. After all, something would get through, even if by accident. One set of theories even proposes that black holes start whole other universes, causing other "Big Bangs" - and our own universe was one - but that idea is still controversial.

And last, one implication of black holes as gateways is time travel. Because of relativity, there's no such thing as "now" that applies everywhere in the universe. "Instant" travel from point A to point B anywhere in the universe would also involve time travel, and you could end up arriving somewhere before you left. Physicist Stephen Hawking noted that since no one sees time travelers today (at least that's been reported) it seems unlikely that time travel is even possible in our universe; that would point to black holes being less useful as wormhole generators.

So while it's possible black holes could be gateways, it's probably a good bet that they aren't.

Update: This story was updated to correct units for the speed of light.

Follow Life's Little Mysteries on Twitter @llmysteries. We're also on Facebook & Google+.

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Can Black Holes Transport You to Other Worlds?

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If you believe the creations of science fiction, black holes serve as gateways to other worlds, either distant parts of this universe or other universes entirely. But the reality might be more complicated than that. And outside of the sci-fi realm, dropping into a black hole is a bad idea.

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It could happen to anyone. Maybe you're out trying to find a new habitable planet for the human race, or maybe you're just on a long walk and you slip. Whatever the circumstances, at some point we all find ourselves confronted with the age-old question: what happens when you fall into a black hole?

You might expect to get crushed, or maybe torn to pieces. But the reality is stranger than that.

The instant you entered the black hole, reality would split in two. In one, you would be instantly incinerated, and in the other you would plunge on into the black hole utterly unharmed.

A black hole is a place where the laws of physics as we know them break down. Einstein taught us that gravity warps space itself, causing it to curve. So given a dense enough object, space-time can become so warped that it twists in on itself, burrowing a hole through the very fabric of reality.

A massive star that has run out of fuel can produce the kind of extreme density needed to create such a mangled bit of world. As it buckles under its own weight and collapses inward, space-time caves in with it. The gravitational field becomes so strong that not even light can escape, rendering the region where the star used to be profoundly dark: a black hole.

As you go deeper into the black hole, space becomes ever more curvy

The outermost boundary of the hole is its event horizon, the point at which the gravitational force precisely counteracts the light's efforts to escape it. Go closer than this, and there's no escape.

The event horizon is ablaze with energy. Quantum effects at the edge create streams of hot particles that radiate back out into the universe. This is called Hawking radiation, after the physicist Stephen Hawking, who predicted it. Given enough time, the black hole will radiate away its mass, and vanish.

As you go deeper into the black hole, space becomes ever more curvy until, at the centre, it becomes infinitely curved. This is the singularity. Space and time cease to be meaningful ideas, and the laws of physics as we know them - all of which require space and time - no longer apply.

What happens here, no one knows. Another universe? Oblivion? The back of a bookcase? It's a mystery.

So what happens if you accidentally fall into one of these cosmic aberrations? Let's start by asking your space companion - we'll call her Anne - who watches in horror as you plunge toward the black hole, while she remains safely outside. From where she's floating, things are about to get weird.

As you accelerate toward the event horizon, Anne sees you stretch and contort, as if she were viewing you through a giant magnifying glass. What's more, the closer you get to the horizon the more you appear to move in slow motion.

Before you ever cross over into the black hole's darkness, you're reduced to ash

You can't shout to her, as there's no air in space, but you might try flashing her a Morse message with the light on your iPhone (there's an app for that). However, your words reach her ever more slowly, the light waves stretching to increasingly lower and redder frequencies: "Alright, a l r i g h t, a l r i..."

When you reach the horizon, Anne sees you freeze, like someone has hit the pause button. You remain plastered there, motionless, stretched across the surface of the horizon as a growing heat begins to engulf you.

According to Anne, you are slowly obliterated by the stretching of space, the stopping of time and the fires of Hawking radiation. Before you ever cross over into the black hole's darkness, you're reduced to ash.

But before we plan your funeral, let's forget about Anne and view this gruesome scene from your point of view. Now, something even stranger happens: nothing.

You sail straight into nature's most ominous destination without so much as a bump or a jiggle - and certainly no stretching, slowing or scalding radiation. That's because you're in freefall, and therefore you feel no gravity: something Einstein called his "happiest thought".

In a big enough black hole, you could live out the rest of your life pretty normally

After all, the event horizon is not like a brick wall floating in space. It's an artefact of perspective. An observer who remains outside the black hole can't see through it, but that's not your problem. As far as you're concerned there is no horizon.

Sure, if the black hole were smaller you'd have a problem. The force of gravity would be much stronger at your feet than at your head, stretching you out like a piece of spaghetti. But lucky for you this is a big one, millions of times more massive than our Sun, so the forces that might spaghettify you are feeble enough to be ignored.

In fact, in a big enough black hole, you could live out the rest of your life pretty normally before dying at the singularity.

How normal could it really be, you might wonder, given that you're being sucked toward a rupture in the space-time continuum, pulled along against your will, unable to head back the other way?

You can't turn around and escape the black hole

But when you think about it, we all know that feeling, not from our experience with space but with time. Time only goes forwards, never backwards, and it pulls us along against our will, preventing us from turning around.

This isn't just an analogy. Black holes warp space and time to such an extreme that inside the black hole's horizon, space and time actually swap roles. In a sense, it really is time that pulls you in toward the singularity. You can't turn around and escape the black hole, any more than you can turn around and travel back to the past.

At this point you might want to stop and ask yourself a pressing question: What the hell is wrong with Anne? If you're chilling inside the black hole, surrounded by nothing weirder than empty space, why is she insisting that you've been burned to a crisp by radiation outside the horizon? Is she hallucinating?

Actually, Anne is being perfectly reasonable. From her point of view, you really have been burned to a crisp at the horizon. It's not an illusion. She could even collect your ashes and send them back to your loved ones.

In fact, the laws of nature require that you remain outside the black hole as seen from Anne's perspective. That's because quantum physics demands that information can never be lost. Every bit of information that accounts for your existence has to stay on the outside of the horizon, lest Anne's laws of physics be broken.

You have to be in two places, but there can only be one copy of you

On the other hand, the laws of physics also require that you sail through the horizon without encountering hot particles or anything out of the ordinary. Otherwise you'd be in violation of Einstein's happiest thought, and his theory of general relativity.

So the laws of physics require that you be both outside the black hole in a pile of ashes and inside the black hole alive and well. Last but not least, there's a third law of physics that says information can't be cloned. You have to be in two places, but there can only be one copy of you.

Somehow, the laws of physics point us towards a conclusion that seems rather nonsensical. Physicists call this infuriating conundrum the black hole information paradox. Luckily, in the 1990s they found a way to resolve it.

Leonard Susskind realized that there is no paradox, because no one person ever sees your clone. Anne only sees one copy of you. You only see one copy of you. You and Anne can never compare notes. And there's no third observer who can see both inside and outside a black hole simultaneously. So, no laws of physics are broken.

Reality depends on whom you ask

Unless, that is, you demand to know which story is really true. Are you really dead or are you really alive?

The great secret that black holes have revealed to us is that there is no really. Reality depends on whom you ask. There is Anne's reality and there is your reality. End of story.

Well, almost. In the summer of 2012, the physicists Ahmed Almheiri, Donald Marolf, Joe Polchinski and James Sully, collectively known as AMPS, devised a thought experiment that threatened to upend everything we thought we knew about black holes.

They realized that Susskind's solution hinged on the fact that any disagreement between you and Anne is mediated by the event horizon. It didn't matter if Anne saw the unlucky version of you scattered amongst the Hawking radiation, because the horizon prevented her from seeing the other version of you floating along inside the black hole.

Anne might sneak a peek behind the horizon

But what if there was a way for her to find out what was on the other side of the horizon, without actually crossing it?

Ordinary relativity would say that's a no-no, but quantum mechanics makes the rules a little fuzzier. Anne might sneak a peek behind the horizon, using a little trick that Einstein called "spooky action-at-a-distance".

This happens when two sets of particles that are separated in space are mysteriously "entangled". They are part of a single, indivisible whole, so that the information needed to describe them can't be found in either set alone, but in the spooky links between them.

The AMPS idea went something like this. Let's say Anne grabs hold of a bit of information near the horizon - call it A.

Each bit of information can only be entangled once

If her story is right, and you are a goner, scrambled amongst the Hawking radiation outside the black hole, then A must be entangled with another bit of information, B, which is also part of the hot cloud of radiation.

On the other hand, if your story is the true one, and you're alive and well on the other side of the event horizon, then A must be entangled with a different bit of information, C, which is somewhere inside the black hole.

Here's the kicker: each bit of information can only be entangled once. That means A can only be entangled with B or with C, not with both.

So Anne takes her bit, A, and puts it through her handy entanglement-decoding machine, which spits out an answer: either B or C.

Do you glide right through and live a normal life?

If the answer turns out to be C, then your story wins, but the laws of quantum mechanics are broken. If A is entangled with C, which is deep inside the black hole, then that piece of information is lost to Anne forever. That breaks the quantum law that information can never be lost.

That leaves B. If Anne's decoding machine finds that A is entangled with B, then Anne wins, and general relativity loses. If A is entangled with B, then Anne's story is the one true story, which means you really were burned to a crisp. Instead of sailing straight through the horizon, as relativity says you should, you hit a burning firewall.

So we're back where we started: what happens when you fall into a black hole? Do you glide right through and live a normal life, thanks to a reality that's strangely observer-dependent? Or do you approach the black hole's horizon only to collide with a deadly firewall?

No one knows the answer, and it's become one of the most contentious questions in fundamental physics.

It would take Anne an extraordinarily long time to decode the entanglement

Physicists have spent more than a century trying to reconcile general relativity with quantum mechanics, knowing that eventually one or the other was going to have to give. The solution to the firewall paradox should tell us which, and point the way to an even deeper theory of the universe.

One clue might lie in Anne's decoding machine. Figuring out which other bit of information A is entangled with is an extraordinarily complicated problem. So physicists Daniel Harlow of Princeton University in New Jersey and Patrick Hayden, now at Stanford University in California, wondered how long it would take.

In 2013 they calculated that, even given the fastest computer that the laws of physics would allow, it would take Anne an extraordinarily long time to decode the entanglement. By the time she had an answer, the black hole would have long evaporated, disappearing from the universe and taking with it the threat of a deadly firewall.

If that's the case, the sheer complexity of the problem could prevent Anne from ever figuring out which story is the real one. That would leave both stories simultaneously true, reality intriguingly observer-dependent, all the laws of physics intact, and no one in danger of running into an inexplicable wall of fire.

If the true nature of reality lies hidden somewhere, the best place to look is a black hole

It also gives physicists something new to think about: the tantalizing connections between complex calculations (like the one Anne apparently can't do) and space-time. This may open the door to something deeper still.

That's the thing about black holes. They're not just annoying obstacles for space travellers. They're also theoretical laboratories that take the subtlest quirks in the laws of physics, then amplify them to such proportions that they can't be ignored.

If the true nature of reality lies hidden somewhere, the best place to look is a black hole. It's probably best to look from the outside, though: at least until they figure out this whole firewall thing. Or send Anne in. It's her turn already.

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The strange fate of a person falling into a black hole

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It could happen to anyone. Maybe you're out trying to find a new habitable planet for the human race, or maybe you're just on a long walk and you slip. Whatever the circumstances, at some point we all find ourselves confronted with the age-old question: what happens when you fall into a black hole?

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National Geographic stories take you on a journey that's always enlightening, often surprising, and unfailingly fascinating. This month-The Truth About Black Holes

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https://exoplanets.nasa.gov/

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Most of the exoplanets discovered so far are in a relatively small region of our galaxy, the Milky Way. (“Small” meaning within thousands of light-years of

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Space and astronomy news from Fraser Cain, publisher of Universe Today and co-host of Astronomy Cast. If you're a fan of space, sci-fi and pop culture, you'll love our Guide to Space. These short videos come out every Monday and Thursday and answer a burning question that astronomy fans want to know.

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Jupiter is the 5th planet from the Sun, and the largest planet in the Solar System. How much bigger is Jupiter than Earth? Just to give you a sense of scale, Jupiter is 2.5 times more massive than all the rest of the planets in the Solar System combined.

Jupiter's diameter is 11.2 times larger than Earth. In other words, you could put 11.2 Earths side-by-side to match the diameter of Jupiter.

And Jupiter's volume is even bigger. It would take 1321.3 Earths to fill up the volume of Jupiter. In terms of surface area, Jupiter is 121.9 times bigger than the Earth. That's how many Earths could be flattened out to cover the surface of Jupiter.

Jupiter has 317.8 times the mass of the Earth.

Even though Jupiter is an enormous, massive planet, it's much smaller than the Sun. The Sun accounts for 99.86% of the mass of the Solar System. You could fit 109 Earths side by side to match the diameter of the Sun, and it would take 1.3 million planets the size of the Earth to fill it up.

We've written many articles about Jupiter for Universe Today. Here's an article about pictures of Jupiter, and here's an article about missions to Jupiter.

If you'd like more information on Jupiter, check out Hubblesite's News Releases about Jupiter, and here's a link to NASA's Solar System Exploration Guide to Jupiter.

We've also recorded an episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

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How Much Bigger is Jupiter Than Earth? - Universe Today

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caption] Jupiter is the 5th planet from the Sun, and the largest planet in the Solar System. How much bigger is Jupiter than Earth? Just to give you a sense of scale, Jupiter is 2.5 times more massive than all the rest of the planets in the Solar System combined.

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This artist's conception shows the giant impact that created Earth's moon. New research suggests the impact was powerful enough to vaporize a large portion of the young Earth.

Credit: Dana Berry/SwRI

The massive collision that created the moon may have vaporized most of the early Earth, according to a new analysis of samples collected during the Apollo moon missions.

In the early days of planet formation, a grazing collision between the newborn Earth and a Mars-size rock named Theia (named after the mother of the moon in Greek myth) may have led to the birth of the moon, according to a prevailing hypothesis. Debris from the impact later coalesced into the moon.

This " giant-impact hypothesis" seemed to explain many details about Earth and the moon, such as the large size of the moon compared with Earth and the rotation rates of the two bodies. But in the last 15 years, evidence has arisen that has challenged scientists to alter the details of this hypothesis. [ How the Moon Evolved: A Timeline in Images]

In 2001, scientists began discovering that terrestrial and lunar rocks had a lot in common: the two bodies possess many of the same chemical isotopes. (Isotopes of an element have different numbers of neutrons from each other. These subvarieties are identified by different numbers; for example, potassium-39 or potassium-40). Isotopes can act as geologic fingerprints, because prior work has suggested that planetary bodies that formed in different parts of the solar system generally have different isotopic compositions.

These discoveries threw the giant-impact hypothesis into crisis because previous computer simulations of the collision predicted that 60 to 80 percent of the material that coalesced into the moon came from Theia rather than Earth. The likelihood that Theia happened to have virtually the same isotopic composition as Earth seemed extremely unlikely.

At first, scientists thought more precise isotopic analyses might help resolve this "isotopic crisis." However, more accurate measurements of oxygen isotopes reported in 2016 only helped confirm this problem, said study lead author Kun Wang, a geochemist now at Washington University in St. Louis.

"Now we need to rethink the ideas that we had about the giant impact," Wang told Space.com.

New models of the giant impact seek to explain how the moon could have formed from mostly the same material that makes up the Earth, rather than mostly from Theia.

A diagram showing two recent models for how the moon formed from a collision between the early Earth and another massive body. One model (top) allows for an exchange of material between the Earth and the moon through a silicate atmosphere, while the other model creates a more thoroughly mixed sphere of a supercritical fluid (bottom). Each of these models leads to a different prediction for potassium isotope ratios in lunar and terrestrial rocks (right).

Credit: Kun Wang

"There are many new models - everyone is trying to come up with one - but two have been very influential," Wang said in a statement. [ How the Moon Formed: 5 Wild Lunar Theories]

The original giant-impact model suggested that a relatively low-energy collision melted part of Earth and the whole of Theia, flinging some of the molten debris outward. One relatively new model, proposed in 2007, starts with a low-energy impact just like the original model, but adds an atmosphere of silicate vapor around Earth and the disk of debris that ends up forming the moon. This model suggests that this vapor shroud helps Earth and the disk exchange material before the moon emerges from the debris.

One drawback of this low-energy impact model is that it would take a long time to exchange material through an atmosphere, Wang said. This scenario would make it difficult to achieve the mix of material seen in terrestrial and lunar rocks, he said.

Another model, proposed in 2015, suggests that a high-energy impact created the moon, one so violent that it vaporized Theia as well as most of Earth, including the young planet's mantle region (the layer just above the core). This dense vapor then formed an atmosphere that filled a space more than 500 times bigger than today's Earth. Much of this material would fall back onto the Earth as it cooled, but in addition, some of the debris formed the moon.

In this high-energy model, the atmosphere would behave like a "supercritical fluid," without a distinct separation between liquids and gases. Material could mix thoroughly in such an atmosphere, which could help explain the identical isotopic compositions of Earth and the moon, Wang said.

To see which model might best explain how the moon formed, Wang and his colleague Stein Jacobsen at Harvard University focused on potassium isotope data from terrestrial rocks and lunar samples gathered during the Apollo missions. Potassium is volatile, or easy to evaporate, and previous research suggested that analyzing potassium isotopes could shed light on the conditions during the event that formed the moon.

The scientists analyzed seven moon rocks collected during the Apollo 11, 12, 14 and 16 missions. They compared their potassium isotope ratios with those of eight rocks representative of Earth's mantle.

The researchers developed a method to analyze potassium isotopes with a level of precision 10 times better than the best previous technique. Potassium has three stable isotopes, but only two of them, potassium-39 and potassium-41, are abundant enough to be measured with sufficient precision for this research.

The scientists discovered that lunar rocks were richer by about 0.4 parts per thousand compared with Earth rocks when it came to potassium-41, the heavier stable isotope of potassium.

These findings support the high-energy impact model, which predicted that lunar rocks would possess more of the heavier isotope than terrestrial rocks. In contrast, the low-energy impact model suggested that lunar rocks would contain less of the heavier isotope.

The best explanation for how the heavier isotope came to dominate was that the moon condensed in a cloud with a pressure of more than 10 bar, or about 10 times the atmospheric pressure on Earth at sea level.

"I'm kind of surprised that the new model fits the data the best," Wang said. Still, "we had no expectation which model we were going to support," he said.

Future research should conduct follow-up studies to test these new findings. "We're definitely hoping more people will follow up and try to confirm our results," Wang said.

Wang and Jacobsen detailed their findings online Sept. 12 in the journal Nature.

Follow Charles Q. Choi on Twitter @cqchoi. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

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Moon's Birth May Have Vaporized Most of Earth, Study Shows

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The massive collision that created the moon may have vaporized most of the early Earth, according to a new analysis of samples collected during the Apollo moon missions. In the early days of planet formation, a grazing collision between the newborn Earth and a Mars-size rock named Theia (named after the mother of the moon in Greek myth) may have led to the birth of the moon, according to a prevailing hypothesis.

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First, for additional references, there is the original press release. Also, a similar report from a different black hole is here. It seems the Chandra people like this sort of thing. It is also worth noting that as far as I can tell, there are only press releases and no published scientific articles on this phenomenon.

Now to address the questions.

Black holes are so massive that light, which is faster than sound, can't escape.

Well, light from inside the black hole cannot escape. But active black holes create violent neighborhoods around them. In general, there will be an accretion disk - a relatively flat disk of material slowly spiraling into the black hole. Friction due to differential rotation in this disk can make it glowing hot, especially close to the black hole. Furthermore, angular momentum and magnetic fields conspire to make jets shooting out of the "poles" of the black hole (pretty much all black holes in astronomy are expected to be rotating significantly). Again, the matter never actually got into the event horizon, since by definition we would not see it again.

So what is this sound? As best I can tell from the press releases, these particular black holes cycle through periods of low accretion and high accretion. There may be a lot of material falling in for a long time, powering jets that pump energy out beyond even the galaxy in which the black hole resides. ¹ Then there will be a few million years of almost nothing falling into the black hole, during which the jets are little more than trickles. This cycle repeats semi-regularly, causing periodic bursts of energy to be sent out.

Sound can't travel in space (space has too much, well, space).

Well, yes and no. It is true there is some material even in the space between galaxies. On the other hand, it is extremely diffuse. In fact, it has been said (I cannot remember the source) that the densest clouds of material in interstellar space are more diffuse than the best vacuums we can make in laboratories. So you might imagine there can be some propagation, but not in the traditional sense.

Really, when the Chandra people say there is "sound," what they mean is this. Thus pulses of energy sent out form shock waves ² that propagate through the intergalactic medium. If you look far enough, you will see these periodic dense regions in a pattern not unlike the ripples in a pond. Since "sound" in the normal sense consists of overdensities traveling through the air (albeit without shocks in most cases), and since these are periodic overdensities in the diffuse gas between galaxies, we may as well make a connection between them in terminology.

Take this with a large grain of salt. I doubt the duty cycle of the black hole is so regular as to produce a monochromatic "pitch." This is more of a whimsical calculation. Nathaniel in a comment did the reverse calculation - going from "note" to frequency. To see how the scientists changed a frequency $f$ (basically the reciprocal of the time between active periods of the black hole) to a named note, see this Wikipedia entry. In short, the number of the pitch is $$ p = 69 + 12 \log_2\left(\frac{f}{440~\mathrm{Hz}}\right). $$ They plugged in an $f$ and must have gotten a $p$ around $-614$ or $-626$ (the wording is ambiguous). The B-flat above middle C has $p = 70$, the one below middle C has $p = 58$, the next one down is $p = 46$, etc.

Addendum: Why is this interesting? Beyond the whimsy, there is scientific value to this. Believe it or not, most of the mass (or at least, most of the mass of normal, non-dark matter) of a cluster of galaxies is found outside the galaxies themselves, in this intergalactic (aka intracluster) medium. It makes up a significant portion of the universe despite its low density, due entirely to the large volume it occupies. More interestingly, this gas is very hot - millions of degrees, despite the "background temperature" of the universe arguably being a few degrees above absolute zero. It is so hot that it glows in X-rays, which is why Chandra - a space-based X-ray telescope - studies it. These sound waves might explain a significant source of heating and injection of turbulence into this matter, which can have broad implications for extragalactic cosmology and galaxy formation. This all falls under the key term "AGN feedback," which is quite a hot topic in astrophysics these days.

¹ Our own Milky Way's supermassive black hole probably had an active phase at some point. Astronomers recently detected the so-called Fermi bubbles that indicate two jets were shooting out of the center of our galaxy millions of years ago.

² "Shock waves" have a technical meaning in fluid dynamics. Basically, there is a discontinuity in the density - ahead of the shock, everything is normal and diffuse, but as it passes there is almost instantaneous compression and heating. Think of the blast wave from an explosion.

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First, for additional references, there is the original press release. Also, a similar report from a different black hole is here. It seems the Chandra people like this sort of thing. It is also worth noting that as far as I can tell, there are only press releases and no published scientific articles on this phenomenon.

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Credit: NASA

The sun lies at the heart of the solar system, where it is by far the largest object. It holds 99.8 percent of the solar system's mass and is roughly 109 times the diameter of the Earth - about one million Earths could fit inside the sun.

The visible part of the sun is about 10,000 degrees Fahrenheit (5,500 degrees Celsius), while temperatures in the core reach more than 27 million F (15 million C), driven by nuclear reactions. One would need to explode 100 billion tons of dynamite every second to match the energy produced by the sun, according to NASA.

The sun is one of more than 100 billion stars in the Milky Way. It orbits some 25,000 light-years from the galactic core, completing a revolution once every 250 million years or so. The sun is relatively young, part of a generation of stars known as Population I, which are relatively rich in elements heavier than helium. An older generation of stars is called Population II, and an earlier generation of Population III may have existed, although no members of this generation are known yet.

The sun was born about 4.6 billion years ago. Many scientists think the sun and the rest of the solar system formed from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun.

The sun has enough nuclear fuel to stay much as it is now for another 5 billion years. After that, it will swell to become a red giant. Eventually, it will shed its outer layers, and the remaining core will collapse to become a white dwarf. Slowly, this will fade, to enter its final phase as a dim, cool theoretical object sometimes known as a black dwarf.

A huge solar filament snakes around the southwestern horizon of the sun in this full disk photo taken by NASA's Solar Dynamics Observatory on Nov. 17, 2010.

Credit: NASA

The sun and its atmosphere are divided into several zones and layers. The solar interior, from the inside out, is made up of the core, radiative zone and the convective zone. The solar atmosphere above that consists of the photosphere, chromosphere, a transition region and the corona. Beyond that is the solar wind, an outflow of gas from the corona.

The core extends from the sun's center to about a quarter of the way to its surface. Although it only makes up roughly 2 percent of the sun's volume, it is almost 15 times the density of lead and holds nearly half of the sun's mass. Next is the radiative zone, which extends from the core to 70 percent of the way to the sun's surface, making up 32 percent of the sun's volume and 48 percent of its mass. Light from the core gets scattered in this zone, so that a single photon often may take a million years to pass through.

The convection zone reaches up to the sun's surface, and makes up 66 percent of the sun's volume but only a little more than 2 percent of its mass. Roiling "convection cells" of gas dominate this zone. Two main kinds of solar convection cells exist - granulation cells about 600 miles (1,000 kilometers) wide and supergranulation cells about 20,000 miles (30,000 km) in diameter.

The photosphere is the lowest layer of the sun's atmosphere, and emits the light we see. It is about 300 miles (500 km) thick, although most of the light comes from its lowest third. Temperatures in the photosphere range from 11,000 F (6,125 C) at bottom to 7,460 F (4,125 C) at top. Next up is the chromosphere, which is hotter, up to 35,500 F (19,725 C), and is apparently made up entirely of spiky structures known as spicules typically some 600 miles (1,000 km) across and up to 6,000 miles (10,000 km) high.

After that is the transition region a few hundred to a few thousand miles thick, which is heated by the corona above it and sheds most of its light as ultraviolet rays. At the top is the super-hot corona, which is made of structures such as loops and streams of ionized gas. The corona generally ranges from 900,000 F (500,000 C) to 10.8 million F (6 million C) and can even reach tens of millions of degrees when a solar flare occurs. Matter from the corona is blown off as the solar wind.

The strength of the sun's magnetic field is typically only about twice as strong as Earth's field. However, it becomes highly concentrated in small areas, reaching up to 3,000 times stronger than usual. These kinks and twists in the magnetic field develop because the sun spins more rapidly at the equator than at the higher latitudes and because the inner parts of the sun rotate more quickly than the surface. These distortions create features ranging from sunspots to spectacular eruptions known as flares and coronal mass ejections. Flares are the most violent eruptions in the solar system, while coronal mass ejections are less violent but involve extraordinary amounts of matter - a single ejection can spout roughly 20 billion tons (18 billion metric tons) of matter into space.

Just like most other stars, the sun is made up mostly of hydrogen, followed by helium. Nearly all the remaining matter consists of seven other elements - oxygen, carbon, neon, nitrogen, magnesium, iron and silicon. For every 1 million atoms of hydrogen in the sun, there are 98,000 of helium, 850 of oxygen, 360 of carbon, 120 of neon, 110 of nitrogen, 40 of magnesium, 35 of iron and 35 of silicon. Still, hydrogen is the lightest of all elements, so it only accounts for roughly 72 percent of the sun's mass, while helium makes up about 26 percent.

Sunspots are relatively cool, dark features on the sun's surface that are often roughly circular. They emerge where dense bundles of magnetic field lines from the sun's interior break through the surface. [ Related: Largest Sunspot in 24 Years Wows Scientists, But Also Mystifies]

The number of sunspots varies as solar magnetic activity does - the change in this number, from a minimum of none to a maximum of roughly 250 sunspots or clusters of sunspots and then back to a minimum, is known as the solar cycle, and averages about 11 years long. At the end of a cycle, the magnetic field rapidly reverses its polarity.

Ancient cultures often modified natural rock formations or built stone monuments to mark the motions of the sun and moon, charting the seasons, creating calendars and monitoring eclipses. Many believed the sun revolved around the Earth, with ancient Greek scholar Ptolemy formalizing this "geocentric" model in 150 B.C. Then, in 1543, Nicolaus Copernicus described a heliocentric, sun-centered model of the solar system, and in 1610, Galileo Galilei's discovery of Jupiter's moons revealed that not all heavenly bodies circled the Earth.

To learn more about how the sun and other stars work, after early observations using rockets, scientists began studying the sun from Earth orbit. NASA launched a series of eight orbiting observatories known as the Orbiting Solar Observatory between 1962 and 1971. Seven of them were successful, and analyzed the sun at ultraviolet and X-ray wavelengths and photographed the super-hot corona, among other achievements.

In 1990, NASA and the European Space Agency launched the Ulysses probe to make the first observations of its polar regions. In 2004, NASA's Genesis spacecraft returned samples of the solar wind to Earth for study. In 2007, NASA's double-spacecraft Solar Terrestrial Relations Observatory (STEREO) mission returned the first three-dimensional images of the sun. NASA lost contact with STEREO-B in 2014, which is remained out of contact except for a brief period in 2016. STEREO-A remains fully functional.

One of the most important solar missions to date has been the Solar and Heliospheric Observatory (SOHO), which was designed to study the solar wind, as well as the sun's outer layers and interior structure. It has imaged the structure of sunspots below the surface, measured the acceleration of the solar wind, discovered coronal waves and solar tornadoes, found more than 1,000 comets, and revolutionized our ability to forecast space weather. Recently, NASA's Solar Dynamics Observatory (SDO), the most advanced spacecraft yet designed to study the sun, has returned never-before-seen details of material streaming outward and away from sunspots, as well as extreme close-ups of activity on the sun's surface and the first high-resolution measurements of solar flares in a broad range of extreme ultraviolet wavelengths.

There are other missions planned to observe the sun in the next few years. The European Space Agency's Solar Orbiter will launch in 2018, and by 2021 will be in operational orbit around the sun. Its closest approach to the sun will be 26 million miles (43 million km) - about 25 percent closer than Mercury. Solar Orbiter will look at particles, plasma and other items in an environment relatively close to the sun, before these things are modified by being transported across the solar system. The goal is to better understand the solar surface and the solar wind.

The Parker Solar Probe will launch in 2018 to make an extremely close approach to the sun, getting as near as 4 million miles (6.5 million km). The spacecraft will look at the corona - the superheated outer atmosphere of the sun - to learn more about how energy flows through the sun, the structure of the solar wind, and how energetic particles are accelerated and transported.

Additional reporting by Elizabeth Howell and Nola Taylor Redd, Space.com Contributors

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Earth's Sun: Facts About the Sun's Age, Size and History

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The sun lies at the heart of the solar system, where it is by far the largest object. It holds 99.8 percent of the solar system's mass and is roughly 109 times the diameter of the Earth - about one million Earths could fit inside the sun.

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(Image credit: solarseven via Getty Images)

Black holes are some of the strangest and most fascinating objects in space. They're extremely dense, with such strong gravitational attraction that not even light can escape their grasp.

The Milky Way could contain over 100 million black holes, though detecting these gluttonous beasts is very difficult. At the heart of the Milky Way lies a supermassive black hole - Sagittarius A*. The colossal structure is about 4 million times the mass of the sun and lies approximately 26,000 light-years away from , according to a statement from NASA.

The first image of a black hole was captured in 2019 by the Event Horizon Telescope (EHT) collaboration. The striking photo of the black hole at the center of the M87 galaxy 55 million light-years from Earth thrilled scientists around the world.

Related: White holes: What we know about black holes' neglected twins

Black hole FAQs answered by an expert

We asked theoretical astrophysicist Priyamvada Natarajan a few commonly asked questions about black holes.

Theoretical Astrophysicist

First black hole discovered

Albert Einstein first predicted the existence of black holes in 1916, with his general theory of relativity. The term "black hole" was coined many years later in 1967 by American astronomer John Wheeler. After decades of black holes being known only as theoretical objects.

The first black hole ever discovered was Cygnus X-1, located within the Milky Way in the constellation of Cygnus, the Swan. Astronomers saw the first signs of the black hole in 1964 when a sounding rocket detected celestial sources of X-rays according to NASA. In 1971, astronomers determined that the X-rays were coming from a bright blue star orbiting a strange dark object. It was suggested that the detected X-rays were a result of stellar material being stripped away from the bright star and "gobbled" up by the dark object - an all-consuming black hole.

How many black holes are there?

According to the Space Telescope Science Institute (STScI) approximately one out of every thousand stars is massive enough to become a black hole. Since the Milky Way contains over 100 billion stats, our home galaxy must harbor some 100 million black holes.

Though detecting black holes is a difficult task and estimates from NASA suggest there could be as many as 10 million to a billion stellar black holes in the Milky Way.

The closest black hole to Earth is dubbed " The Unicorn" and is situated approximately 1,500 light-years away. The nickname has a double meaning. Not only does the black hole candidate reside in the constellation Monoceros ("the unicorn"), its incredibly low mass - about three times that of the sun - makes it nearly one of a kind.
Related: How many black holes are there in the universe?
Black hole images

In 2019 the Event Horizon Telescope (EHT) collaboration released the first image ever recorded of a black hole. The EHT saw the black hole in the center of galaxy M87 while the telescope was examining the event horizon or the area past which nothing can escape from a black hole. The image maps the sudden loss of photons (particles of light). It also opens up a whole new area of research in black holes, now that astronomers know what a black hole looks like.

In 2021, astronomers revealed a new view of the giant black hole at the center of M87, showing what the colossal structure looks like in polarized light. As polarized light waves have a different orientation and brightness compared to unpolarized light, the new image shows the black hole in even more detail. Polarization is a signature of magnetic fields and the image makes it clear that the black hole's ring is magnetized.

In May 2022, scientists revealed the historic first image of the supermassive black hole at the center of our galaxy - Sagitarrius A*.

Related: The 1st photo of the Milky Way's monster black hole explained in images

What do black holes look like?

Black holes have three "layers": the outer and inner event horizon, and the singularity.

The event horizon of a black hole is the boundary around the mouth of the black hole, past which light cannot escape. Once a particle crosses the event horizon, it cannot leave. Gravity is constant across the event horizon.

The inner region of a black hole, where the object's mass lies, is known as its singularity, the single point in space-time where the mass of the black hole is concentrated.

Scientists can't see black holes the way they can see stars and other objects in space. Instead, astronomers must rely on detecting the radiation black holes emit as dust and gas are drawn into the dense creatures. But supermassive black holes, lying in the center of a galaxy, may become shrouded by the thick dust and gas around them, which can block the telltale emissions.

Sometimes, as matter is drawn toward a black hole, it ricochets off the event horizon and is hurled outward, rather than being tugged into the maw. Bright jets of material traveling at near-relativistic speeds are created. Although the black hole remains unseen, these powerful jets can be viewed from great distances.

The EHT's image of a black hole in M87 (released in 2019) was an extraordinary effort, requiring two years of research even after the images were taken. That's because the collaboration of telescopes, which stretches across many observatories worldwide, produces an astounding amount of data that is too large to transfer via the internet.

With time, researchers expect to image other black holes and build up a repository of what the objects look like. The next target is likely Sagittarius A*, which is the black hole in the center of our own Milky Way galaxy. Sagittarius A* is intriguing because it is quieter than expected, which may be due to magnetic fields smothering its activity, a 2019 study reported. Another study that year showed that a cool gas halo surrounds Sagittarius A*, which gives unprecedented insight into what the environment around a black hole looks like.

Types of black holes

So far, astronomers have identified three types of black holes: stellar black holes, supermassive black holes and intermediate black holes.

Stellar black holes - small but deadly

When a star burns through the last of its fuel, the object may collapse, or fall into itself. For smaller stars (those up to about three times 's mass), the new core will become a neutron star or a white dwarf. But when a larger star collapses, it continues to compress and creates a stellar black hole.

Black holes formed by the collapse of individual stars are relatively small but incredibly dense. One of these objects packs more than three times the mass of the sun into the diameter of a city. This leads to a crazy amount of gravitational force pulling on objects around the object. Stellar black holes then consume the dust and gas from their surrounding galaxies, which keeps them growing in size.

Supermassive black holes - the birth of giants

Small black holes populate the universe, but their cousins, supermassive black holes, dominate. These enormous black holes are millions or even billions of times as massive as the sun but are about the same size in diameter. Such black holes are thought to lie at the center of pretty much every galaxy, including the Milky Way.

Scientists aren't certain how such large black holes spawn. Once these giants have formed, they gather mass from the dust and gas around them, material that is plentiful in the center of galaxies, allowing them to grow to even more enormous sizes.

Supermassive black holes may be the result of hundreds or thousands of tiny black holes that merge. Large gas clouds could also be responsible, collapsing together and rapidly accreting mass. A third option is the collapse of a stellar cluster, a group of stars all falling together. Fourth, supermassive black holes could arise from large clusters of dark matter. This is a substance that we can observe through its gravitational effect on other objects; however, we don't know what dark matter is composed of because it does not emit light and cannot be directly observed.

Scientists once thought that black holes came in only small and large sizes, but research has revealed the possibility that midsize, or intermediate, black holes (IMBHs) could exist. Such bodies could form when stars in a cluster collide in a chain reaction. Several of these IMBHs forming in the same region could then eventually fall together in the center of a galaxy and create a supermassive black hole.

In 2014, astronomers found what appeared to be an intermediate-mass black hole in the arm of a spiral galaxy. And in 2021 astronomers took advantage of an ancient gamma-ray burst to detect one.

"Astronomers have been looking very hard for these medium-sized black holes," study co-author Tim Roberts, of the University of Durham in the United Kingdom, said in a statement. "There have been hints that they exist, but IMBHs have been acting like a long-lost relative that isn't interested in being found."

Research, from 2018, suggested that these IMBHs may exist in the heart of dwarf galaxies (or very small galaxies). Observations of 10 such galaxies (five of which were previously unknown to science before this latest survey) revealed X-ray activity - common in black holes - suggesting the presence of black holes of from 36,000 to 316,000 solar masses. The information came from the Sloan Digital Sky Survey, which examines about 1 million galaxies and can detect the kind of light often observed coming from black holes that are picking up nearby debris.

Binary black holes: double trouble

In 2015, astronomers using the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from merging stellar black holes.

"We have further confirmation of the existence of stellar-mass black holes that are larger than 20 solar masses - these are objects we didn't know existed before LIGO detected them," David Shoemaker, the spokesperson for the LIGO Scientific Collaboration (LSC), said in a statement. LIGO's observations also provide insights into the direction a black hole spins. As two black holes spiral around one another, they can spin in the same direction or the opposite direction.

There are two theories on how binary black holes form. The first suggests that the two black holes in a binary form at about the same time, from two stars that were born together and died explosively at about the same time. The companion stars would have had the same spin orientation as one another, so the two black holes left behind would as well.

Under the second model, black holes in a stellar cluster sink to the center of the cluster and pair up. These companions would have random spin orientations compared to one another according to LIGO Scientific Collaboration. LIGO's observations of companion black holes with different spin orientations provide stronger evidence for this formation theory.

"We're starting to gather real statistics on binary black hole systems," said LIGO scientist Keita Kawabe of Caltech, who is based at the LIGO Hanford Observatory. "That's interesting because some models of black hole binary formation are somewhat favored over the others even now, and in the future, we can further narrow this down."

If you fell into a black hole, theory has long suggested that gravity would stretch you out like spaghetti, though your death would come before you reached the singularity. But a 2012 study published in the journal Nature suggested that quantum effects would cause the event horizon to act much like a wall of fire, which would instantly burn you to death.

Black holes don't suck. Suction is caused by pulling something into a vacuum, which the massive black hole definitely is not. Instead, objects fall into them just as they fall toward anything that exerts gravity, like the Earth.

The first object considered to be a black hole is Cygnus X-1. Cygnus X-1 was the subject of a 1974 friendly wager between Stephen Hawking and fellow physicist Kip Thorne, with Hawking betting that the source was not a black hole. In 1990, Hawking conceded defeat.

Miniature black holes may have formed immediately after the Big Bang. Rapidly expanding space may have squeezed some regions into tiny, dense black holes less massive than the sun.

If a star passes too close to a black hole, the star can be torn apart.

Astronomers estimate that the Milky Way has anywhere from 10 million to 1 billion stellar black holes, with masses roughly three times that of the sun.

Black holes remain terrific fodder for science fiction books and movies. Check out the movie "Interstellar," which relied heavily on Thorne to incorporate science. Thorne's work with the movie's special effects team led to scientists' improved understanding of how distant stars might appear when seen near a fast-spinning black hole.

Black hole facts

Additional resources

Dive deeper into the mystery of black holes with NASA Science. Watch videos and read more about black holes from NASA's Hubblesite. Discover more about black holes with the National Science Foundation.

Bibliography

Hubblesite: Black holes: Gravity's relentless pull interactive: Encyclopedia. STScI Home. Retrieved May 6, 2022.

NASA. Imagine the universe! NASA. Retrieved May 6, 2022.

Boen, B. (2013, August 29). Supermassive black hole Sagittarius A*. NASA. Retrieved May 6, 2022.

NASA's Chandra Finds Intriguing Member of Black Hole Family Tree. Chandra X-ray Observatory. (2015, February 25). Retrieved May 6, 2022.

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Black holes: Everything you need to know

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Black holes are real, they're some of the strangest objects in space. Discover what black holes are and how they form with our informative black hole guide.

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Jupiter is the fifth planet from our Sun and is, by far, the largest planet in the solar system - more than twice as massive as all the other planets combined. Jupiter's stripes and swirls are actually cold, windy clouds of ammonia and water, floating in an atmosphere of hydrogen and helium. Jupiter's iconic Great Red Spot is a giant storm bigger than Earth that has raged for hundreds of years.

Jupiter is surrounded by more than 65 known moons. Scientists are most interested in the Galilean satellites - the four largest moons discovered by Galileo Galilei in 1610: Io, Europa, Ganymede and Callisto. Jupiter also has several rings, but unlike the famous rings of Saturn, Jupiter's rings are very faint and made of dust, not ice.

Jupiter is named for the king of the ancient Roman gods.​

Size and Distance

With a radius of 43,440.7 miles (69,911 kilometers), Jupiter is 11 times wider than Earth. If Earth were the size of a nickel, Jupiter would be about as big as a basketball.

From an average distance of 484 million miles (778 million kilometers), Jupiter is 5.2 astronomical units away from the Sun. One astronomical unit (abbreviated as AU), is the distance from the Sun to Earth. From this distance, it takes Sunlight 43 minutes to travel from the Sun to Jupiter.

Orbit and Rotation

Jupiter has the shortest day in the solar system. One day on Jupiter takes only about 10 hours (the time it takes for Jupiter to rotate or spin around once), and Jupiter makes a complete orbit around the Sun (a year in Jovian time) in about 12 Earth years (4,333 Earth days).

Its equator is tilted with respect to its orbital path around the Sun by just 3 degrees. This means Jupiter spins nearly upright and does not have seasons as extreme as other planets do.

Jupiter took shape when the rest of the solar system formed about 4.5 billion years ago, when gravity pulled swirling gas and dust in to become this gas giant. Jupiter took most of the mass left over after the formation of the Sun, ending up with more than twice the combined material of the other bodies in the solar system. In fact, Jupiter has the same ingredients as a star, but it did not grow massive enough to ignite.

About 4 billion years ago, Jupiter settled into its current position in the outer solar system, where it is the fifth planet from the Sun.

The composition of Jupiter is similar to that of the Sun-mostly hydrogen and helium. Deep in the atmosphere, pressure and temperature increase, compressing the hydrogen gas into a liquid. This gives Jupiter the largest ocean in the solar system-an ocean made of hydrogen instead of water. Scientists think that, at depths perhaps halfway to the planet's center, the pressure becomes so great that electrons are squeezed off the hydrogen atoms, making the liquid electrically conducting like metal. Jupiter's fast rotation is thought to drive electrical currents in this region, generating the planet's powerful magnetic field. It is still unclear if, deeper down, Jupiter has a central core of solid material or if it may be a thick, super-hot and dense soup. It could be up to 90,032 degrees Fahrenheit (50,000 degrees Celsius) down there, made mostly of iron and silicate minerals (similar to quartz).

As a gas giant, Jupiter doesn't have a true surface. The planet is mostly swirling gases and liquids. While a spacecraft would have nowhere to land on Jupiter, it wouldn't be able to fly through unscathed either. The extreme pressures and temperatures deep inside the planet crush, melt and vaporize spacecraft trying to fly into the planet.

Atmosphere

Jupiter's appearance is a tapestry of colorful cloud bands and spots. The gas planet likely has three distinct cloud layers in its "skies" that, taken together, span about 44 miles (71 kilometers). The top cloud is probably made of ammonia ice, while the middle layer is likely made of ammonium hydrosulfide crystals. The innermost layer may be made of water ice and vapor.

The vivid colors you see in thick bands across Jupiter may be plumes of sulfur and phosphorus-containing gases rising from the planet's warmer interior. Jupiter's fast rotation-spinning once every 10 hours-creates strong jet streams, separating its clouds into dark belts and bright zones across long stretches.

With no solid surface to slow them down, Jupiter's spots can persist for many years. Stormy Jupiter is swept by over a dozen prevailing winds, some reaching up to 335 miles per hour (539 kilometers per hour) at the equator. The Great Red Spot, a swirling oval of clouds twice as wide as Earth, has been observed on the giant planet for more than 300 years. More recently, three smaller ovals merged to form the Little Red Spot, about half the size of its larger cousin. Scientists do not yet know if these ovals and planet-circling bands are shallow or deeply rooted to the interior.

Potential for Life

Jupiter's environment is probably not conducive to life as we know it. The temperatures, pressures and materials that characterize this planet are most likely too extreme and volatile for organisms to adapt to.

While planet Jupiter is an unlikely place for living things to take hold, the same is not true of some of its many moons. Europa is one of the likeliest places to find life elsewhere in our solar system. There is evidence of a vast ocean just beneath its icy crust, where life could possibly be supported.

With four large moons and many smaller moons, Jupiter forms a kind of miniature solar system. Jupiter has 53 confirmed moons, as well as 14 provisional ones. These newly discovered, provisional moons are reported by astronomers and acknowledged with a temporary designation by the International Astronomical Union. Once their orbits are confirmed, they are included in Jupiter's confirmed moon count.

Jupiter's four largest moons-Io, Europa, Ganymede and Callisto-were first observed by the astronomer Galileo Galilei in 1610 using an early version of the telescope. These four moons are known today as the Galilean satellites, and they're some of the most fascinating destinations in our solar system. Io is the most volcanically active body in the solar system. Ganymede is the largest moon in the solar system (even bigger than the planet Mercury). Callisto's very few small craters indicate a small degree of current surface activity. A liquid-water ocean with the ingredients for life may lie beneath the frozen crust of Europa, making it a tempting place to explore.

Discovered in 1979 by NASA's Voyager 1 spacecraft, Jupiter's rings were a surprise, as they are composed of small, dark particles and are difficult to see except when backlit by the Sun. Data from the Galileo spacecraft indicate that Jupiter's ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet's small innermost moons.

Magnetosphere

The Jovian magnetosphere is the region of space influenced by Jupiter's powerful magnetic field. It balloons 600,000 to 2 million miles (1 to 3 million kilometers) toward the Sun (seven to 21 times the diameter or Jupiter itself) and tapers into a tadpole-shaped tail extending more than 600 million miles (1 billion kilometers) behind Jupiter, as far as Saturn's orbit. Jupiter's enormous magnetic field is 16 to 54 times as powerful as that of the Earth. It rotates with the planet and sweeps up particles that have an electric charge. Near the planet, the magnetic field traps swarms of charged particles and accelerates them to very high energies, creating intense radiation that bombards the innermost moons and can damage spacecraft.

Jupiter's magnetic field also causes some of the solar system's most spectacular aurorae at the planet's poles

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In Depth | Jupiter - Solar System Exploration: NASA Science

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Jupiter is the fifth planet from our Sun and is, by far, the largest planet in the solar system - more than twice as massive as all the other planets combined. Jupiter's stripes and swirls are actually cold, windy clouds of ammonia and water, floating in an atmosphere of hydrogen and helium.

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It's finally here. The super moon is set to shine down on us on Monday night, and is not expected to roll around again until 2034.

So with the rare astronomical event about to take place, here's a quick guide to making the most of the biggest and brightest moon since 1948.
What's the big fuss about?
A super moon happens when a full moon passes really close to Earth in its monthly orbit. That's because of the elliptical orbit of the moon, which causes the earth and the moon to get extra near once in a while.

Tonight's super moon is extra special because it will be "the closest full moon to date in the 21st century," according to NASA.

What is a #Supermoon and why should you step outside to see it overnight tonight? That and more, just watch here: https://t.co/2XF3HTSg64 pic.twitter.com/AWCRVy8tXI
- NASA (@NASA) November 13, 2016
What's it going to look like?
Basically, tonight's moon is going to look bigger and brighter than a regular full moon.

According to space.com, in addition to the effect of the moon's orbit, there's also the impact of the so-called "moon illusion", that will make the moon appear extra large as it rises from the horizon.

Looking at the #Supermoon tonight? Check our new interactive map to see if @Space_Station is flying overhead, too: https://t.co/XHjJhtkLGX pic.twitter.com/ozacS0BMFX
- NASA (@NASA) November 13, 2016

Experts say the moon could appear 14 percent larger and 30 percent brighter than when it is farthest from Earth. However, there's also talk that the difference between the super moon and regular moon can be quite subtle.

Taking a cue from social media, tonight's moon could look pretty cool.
Best time and place to check it out
Aussie stargazers are going to stay up pretty late to get the best look at the rare phenomenon. The very peak of the effect is actually scheduled for early Tuesday morning, 12:52am (AEDT) to be exact.

The good news is that you won't have to stay up until then to get a good look.

Look at the sky tonight, and you may be able to witness a historic sight: the biggest supermoon since 1948 https://t.co/wNrGPVw6s9
- National Geographic (@NatGeo) November 13, 2016

"I'd be looking at the full moon rising on the night of the 14th. It'll be almost full, so close to full that you'll never notice. And I think really you'll get the maximum impression when you look at the moon low on the horizon," the University of Sydney's John O'Byrne recently told HuffPost Australia.

Moonrise tonight in Sydney is 7:07pm, Melbourne 7:40pm and Brisbane 5:51pm. In Perth the moon with rise at 6.33pm and in Adelaide at 7.32pm. There could be bad news for Sydneysiders, with the Bureau of Meteorology forecasting a cloudy night that could obscure the once-in-a-generation spectacle.

The outlook is also not looking great for Melburnians keen to see the rare event.
How to photograph the super moon
Your best shot will likely be when the huge moon is low in the sky.

"If you were very careful about it, you take the same photograph in a month's time or six months' time and you measured it very carefully you would find that it's up to 14 percent bigger in diameter, between biggest and smallest," Associate Professor O'Byrne said.

The #supermoon is looking spectacular tonight. Here is a useful Q&A: https://t.co/wvvp4lEof9 pic.twitter.com/qo9U99WHpu
- The Telegraph (@Telegraph) November 13, 2016

For photography enthusiasts, try some of these tips to get the best shot.

However, your best bet could be just to soak up the phenomenon without any tech.

"Enjoy it -- you don't need a telescope or binoculars, although they would make it look nice," Australasian Science magazine David Reneke's told the ABC recently.

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Super Moon: What You Need To Know About Tonight's Giant Moon

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It's finally here. The super moon is set to shine down on us on Monday night, and is not expected to roll around again until 2034. So with the rare astronomical event about to take place, here's a quick guide to making the most of the biggest and brightest moon since 1948.

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NASA brings you images, videos and features from the unique perspective of America's space agency. Get updates on missions, watch NASA TV, read blogs, view the latest discoveries, and more.

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Camelopardalis, or U Cam for short, is a star nearing the end of its life. As stars run low on fuel, they become unstable. Every few thousand years, U Cam coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse.

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Did you know that Venus is so hot that the first space probes that landed there melted within 2 hours

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Most likely not. Black holes, whether they have the mass of one star or a million stars, have a very strong gravitational pull that increases enormously the closer you get to them.

This pull would create powerful tidal forces that would stretch you until they pulled you apart in a messy process called spaghettification. However, the more massive the black hole, the weaker its gravity - so for the biggest black holes in the universe the tidal forces might not be so great as to rip you apart. That is, at least not until you reach the singularity at the heart of one of these objects.

However, time will appear to slow down for you as seen from observers on the outside and once you are past the event horizon - the point of no return - there is no way that anything, not even light, can escape the black hole.
Got a question for us? Send it into questions@spaceanswers.com and you could see it featured in All About Space - available every month for just £3.99. Alternatively you can subscribe here for a fraction of the price!

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Is it possible to go inside a black hole and survive? | Space Facts - Astronomy, the Solar System & Outer Space | All About Space Magazine

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Most likely not. Black holes, whether they have the mass of one star or a million stars, have a very strong gravitational pull that increases enormously the closer you get to them. This pull would create powerful tidal forces that would stretch you until they pulled you apart in a messy process called spaghettification.

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NASA brings you images, videos and features from the unique perspective of America's space agency. Get updates on missions, watch NASA TV, read blogs, view the latest discoveries, and more.

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Wanna learn about stars well here's some information

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Stars are the most widely recognized astronomical objects, and represent the most fundamental building blocks of galaxies. The age, distribution, and composition of the stars in a galaxy trace the history, dynamics, and evolution of that galaxy. Moreover, stars are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen, and oxygen, [...]

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If a white dwarf is a "dead" star, why is it so hot? (Intermediate)
Is a white dwarf a "dead" star? Is a dead star defined as one that has burned through all its fuel? If a white dwarf is a dead star, why is it located on the hotter side of the HR diagram in my textbook? Hasn't it burned through all its fuel?
A white dwarf is a stellar remnant and can be considered as a dead star in this regard. A "dead" star is one that has no more nuclear fusion going in it. When a star dies, it leaves some remnant behind. Depending on the mass of the star, the remnant can be a white dwarf, neutron star or a black hole.

White dwarfs are what was once the core of a star. During a star's life, nuclear fusion goes on in the core (and not in the entire star). The temperatures are extremely high in the core (15 million degrees Kelvin for main sequence stars burning hydrogen, and 100 million degrees for stars burning helium). As a result, when a low mass star dies by shedding its envelope leaving behind the core as a white dwarf, it is very hot at around 100 million degrees. In addition, the core has become very compact (size of the Earth). This is the reason why the white dwarf is located on the bottom left part of the HR diagram: left because it is very hot, and bottom because its luminosity is very small (compared to normal stars) due to its small size.

Left to itself, the white dwarf slowly cools since there is no longer any energy production taking place in it. So, after a few million years, the white dwarf which was very hot when it was born in a planetary nebula will become quite cool.
This page was last updated June 27, 2015.
About the Author

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.

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If a white dwarf is a "dead" star, why is it so hot? (Intermediate)

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Is a white dwarf a "dead" star? Is a dead star defined as one that has burned through all its fuel? If a white dwarf is a dead star, why is it located on the hotter side of the HR diagram in my textbook? Hasn't it burned through all its fuel?

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HubbleSite: Black Holes: Gravity's Relentless Pull

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Black Holes: Gravity's Relentless Pull. Information, virtual journeys, and simulations about black holes from the Space Telescope Science Institute