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The solar system consists of the Sun; the eight official planets, at least three "dwarf planets", more than 130 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are probably also many more planetary satellites that have not yet been discovered.)

Orbits

The solar system consists of the Sun; the eight official planets, at least three "dwarf planets", more than 130 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are probably also many more planetary satellites that have not yet been discovered.)

The inner solar system contains the Sun, Mercury, Venus, Earth and Mars:

The main asteroid belt (not shown) lies between the orbits of Mars and Jupiter. The planets of the outer solar system are Jupiter, Saturn, Uranus, and Neptune (Pluto is now classified as a dwarf planet):

The first thing to notice is that the solar system is mostly empty space. The planets are very small compared to the space between them. Even the dots on the diagrams above are too big to be in proper scale with respect to the sizes of the orbits.

The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. The above diagrams show the relative sizes of the orbits of the eight planets (plus Pluto) from a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense.

(The above diagrams show correct positions for October 1996 as generated by the excellent planetarium program Starry Night; there are also many other similar programs available, some free. You can also use Emerald Chronometer on your iPhone or Emerald Observatory on your iPad to find the current positions. This information is also useful for designing a solar panel system.)

Sizes

The above composite shows the eight planets and Pluto with approximately correct relative sizes (see another similar composite and a comparison of the terrestrial planets or Appendix 2 for more).

One way to help visualize the relative sizes in the solar system is to imagine a model in which everything is reduced in size by a factor of a billion. Then the model Earth would be about 1.3 cm in diameter (the size of a grape). The Moon would be about 30 cm (about a foot) from the Earth. The Sun would be 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 20 and 30 blocks away. A human on this scale would be the size of an atom but the nearest star would be over 40000 km away.

Not shown in the above illustrations are the numerous smaller bodies that inhabit the solar system: the satellites of the planets; the large number of asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also elsewhere; the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic; and the many small icy bodies beyond Neptune in the Kuiper Belt. With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids. The classification of these objects is a matter of minor controversy. Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites (a.k.a. moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits). Unfortunately, the solar system has been found to be more complicated than this would suggest:

there are several moons larger than Pluto and two larger than Mercury;

there are many small moons that are probably started out as asteroids and were only later captured by a planet;

comets sometimes fizzle out and become indistinguishable from asteroids;

the Kuiper Belt objects (including Pluto) and others like Chiron don't fit this scheme well

The Earth/Moon and Pluto/Charon systems are sometimes considered "double planets".

Other classifications based on chemical composition and/or point of origin can be proposed which attempt to be more physically valid. But they usually end up with either too many classes or too many exceptions. The bottom line is that many of the bodies are unique; the actual situation is too complicated for simple categorization. In the pages that follow, I will use the conventional categorizations.

The eight bodies officially categorized as planets are often further classified in several ways:

by composition:

terrestrial or rocky planets: Mercury, Venus, Earth, and Mars:
The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.

jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune:
The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites.

by size:

small planets: Mercury, Venus, Earth, Mars.
The small planets have diameters less than 13000 km.

giant planets: Jupiter, Saturn, Uranus and Neptune.
The giant planets have diameters greater than 48000 km.

The giant planets are sometimes also referred to as gas giants.

by position relative to the Sun:

inner planets: Mercury, Venus, Earth and Mars.

outer planets: Jupiter, Saturn, Uranus, Neptune.

The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system.

by position relative to Earth:

inferior planets: Mercury and Venus.

closer to the Sun than Earth.

The inferior planets show phases like the Moon's when viewed from Earth.

Earth.

superior planets: Mars thru Neptune.

farther from the Sun than Earth.

The superior planets always appear full or nearly so.

by history:

classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.

known since prehistorical times

visible to the unaided eye

in ancient times this term also refered to the Sun and the Moon; the order was usually specificied as: Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon, based on the time for them to go "all the way round" the sphere of the "fixed" stars).

modern planets: Uranus, Neptune.

discovered in modern times

visible only with optical aid

Earth.

The IAU decided that "classical" should refer to all eight planets (Mercury thru Neptune, including Earth but not Pluto). This is contrary to historical usage but makes some sense from a 21st century perspective.

PicturesNote: most of the images in The Nine Planets are not true color. Most of them were created by combining several black and white images taken thru various color filters. Though the colors may look "right" chances are they aren't exactly what your eye would see.

The Nine Planets montage (larger version of the above) 36k jpg

Another relative size comparison (from LANL) 93k gif

Sun and large planet comparison (from Extrema) 15k jpg

Earth and small body comparison (from Extrema) 13k jpg

Voyager 1 mosaic of the solar system from 4 billion miles out 36k jpg; html (caption)

Voyager 1 images of 6 planets from 4 billion miles out 123k jpg; html

Pale Blue Dot, reflections on the above image by Carl Sagan.

More General Overview

The largest, smallest, brightest, etc bodies

The history of solar system discovery

Solar System Introduction from LANL

Solar System Family Portrait from NSSDC

Solar System Live, the interactive Orrery of the Web.

notes about the most distant object in the solar system and the surface temperatures of the planets from RGO

scale models of the solar system

Walk the Solar System, a nice size comparison calculator from the Exploratorium
Meta Calc, a simple online calculator

Your Weight on Other Worlds, another neat calculator from the Exploratorium

ConverTable Planets, a nice little Macintosh app to calculate your weight on other planets

Galileo SSI Education Module on Planetary Surfaces

a good bibliography of print material about the solar system

The Big Questions
What is the origin of the solar system? It is generally agreed that it condensed from a nebula of dust and gas. But the details are far from clear.

How common are planetary systems around other stars? (Updated June 2014)
The number of planets around other stars has increased dramatically since the first discoveries of HD 1144762 b in 1989 and gamma Cephei b in 1988 (confirmed in 2003). exoplanet.eu lists 1,811 planets up to 25 July 2014, including over 400 multiple planet systems. Plus there are over 3,000 additional potential planets indicated by the Kepler spacecraft according to the NASA Exoplanet Archive. The reader will note there may be differences in the reported numbers between the two sites referenced.

What conditions allow the formation of terrestrial planets? It seems unlikely that the Earth is totally unique but we still have no direct evidence one way or the other.

Is there life elsewhere in the solar system? If not, why is Earth special? (Updated June 2014)
We do not yet know of life elsewhere. One of the things that makes Earth special of particular interest to the exoplanet search is our location with respect to our Sun - the habitable or so-called "goldilocks zone". The "goldilocks zone" the area around a star where water would be a liquid on the surface of a planet. The location and extent of which would depend on a number of criteria such as the parent star size and temperature. Once planets in these habitable zones are found the size of the planet is taken into account. The size is what may enable a suitable atmosphere for our familiar life forms. The Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo maintains the Habitable Exoplanet Catalog

Is there life beyond the solar system? Intelligent life?

Is life a rare and unusual or even unique event in the evolution of the universe or is it adaptable, widespread and common?

Answers to these questions, even partial ones, would be of enormous value. Answers to the lesser questions on the pages that follow may help answer some of these big ones.

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Solar System Facts | Information, Size, History and Definition

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Te solar system consists of the Sun; the eight official planets, at least three "dwarf planets", 130+ satellites and a large number of small bodies

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The Solar System is made up of the Sun and all of the smaller objects that move around it. Apart from the Sun, the largest members of the Solar System are the eight major planets. Nearest the Sun are four fairly small, rocky planets - Mercury, Venus, Earth and Mars.

Beyond Mars is the asteroid belt - a region populated by millions of rocky objects. These are left-overs from the formation of the planets, 4.5 billion years ago.

On the far side of the asteroid belt are the four gas giants - Jupiter, Saturn, Uranus and Neptune. These planets are much bigger than Earth, but very lightweight for their size. They are mostly made of hydrogen and helium.

Until recently, the furthest known planet was an icy world called Pluto. However, Pluto is dwarfed by Earth's Moon and many astronomers think it is too small to be called a true planet.

An object named Eris, which is at least as big as Pluto, was discovered very far from the Sun in 2005. More than 1,000 icy worlds such as Eris have been discovered beyond Pluto in recent years. These are called Kuiper Belt Objects. In 2006, the International Astronomical Union decided that Pluto and Eris must be classed as "dwarf planets".

Even further out are the comets of the Oort Cloud. These are so far away that they are invisible in even the largest telescopes. Every so often one of these comets is disturbed and heads towards the Sun. It then becomes visible in the night sky.

Last modified 07 August 2014

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The Solar System is made up of the Sun and all of the smaller objects that move around it. Apart from the Sun, the largest members of the Solar System are the eight major planets. Nearest the Sun are four fairly small, rocky planets - Mercury, Venus, Earth and Mars.

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Solar System Facts for Kids | Planets for Kids | Geography | Quiz

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The European Space Agency (ESA) said on November 18, 2019, that its scientists have recently been investigating the process of placing astronauts into hibernation to cross the vastness of space. These scientists met at ESA's Concurrent Design Facility to assess the advantages of human hibernation for a trip to a neighboring planet, such as Mars. They took as their reference an existing study that described sending six humans to Mars and back on a five-year timescale. They studied how crew hibernation would impact space mission design, and put some numbers to known advantages to human hibernation for space travel, for example, that a smaller space capsule could be used if the crew were hibernating, rather than awake, for the months-long journey to Mars.

Jennifer Ngo-Anh, a team leader in ESA's Science in Space Environment ( SciSpacE) program, commented:

For a while now hibernation has been proposed as a game-changing tool for human space travel.

If we were able to reduce an astronaut's basic metabolic rate by 75% - similar to what we can observe in nature with large hibernating animals such as certain bears - we could end up with substantial mass and cost savings, making long-duration exploration missions more feasible.

EarthSky 2020 lunar calendars are available! They make great gifts. Order now. Going fast!
Why do we need to put astronauts into hibernation at all? The reason is that space is vast. Even our own neighborhood of space - our solar system - is subject to a space-is-vast issue that factors heavily into our missions to explore the other planets near us in space. Consider that the New Horizons mission to Pluto, for example - launched in 2006 - needed nine years to get to its flyby.

To get an idea of the distance scale of our solar system, visit "If the moon were only 1 pixel" showing the relative distances of the planets to scale on a single extra-wide page. Then try - if you can - to extend what you've learned to the billions of other likely solar systems in our galaxy alone.

As Joe Hansen - host of the PBS series "It's Okay to be Smart" - says in the video below:

The human brain just can't fathom how big things like the solar system are.

Robin Biesbroek of ESA - who has worked in the past on the removal of space debris from low-Earth orbit - was part of the recent ESA study on human hibernation. He commented:

We worked on adjusting the architecture of the spacecraft, its logistics, protection against radiation, power consumption and overall mission design.

We looked at how an astronaut team could be best put into hibernation, what to do in case of emergencies, how to handle human safety and even what impact hibernation would have on the psychology of the team.

Finally we created an initial sketch of the habitat architecture and created a roadmap to achieve a validated approach to hibernate humans to Mars within 20 years.

The scientists found that the mass of a spacecraft for human hibernation could be reduced by a third.

What would it be like for the astronauts? ESA explained:

The assumption was that a drug would be administered to induce 'torpor' - the term for the hibernating state. Like hibernating animals, the astronauts would be expected to acquire extra body fat in advance of torpor. Their soft-shell pods would be darkened and their temperature greatly reduced to cool their occupants during their projected 180-day Earth-Mars cruise.

ESA said the hibernating cruise phase would end with a 21-day recuperation period. It said that - based on the experience of animal hibernation - the crew would likely not experience bone or muscle wastage. ESA also explained:

Radiation exposure from high-energy particles is a key hazard of deep space travel, but because the hibernating crew will be spending so much time in their hibernation pods, then shielding - such as water containers - could be concentrated around them.

And ESA also spoke of the "largely autonomous operations, with optimum use of artificial intelligence" and "fault detection, isolation and recovery" needed on a spaceship where most humans are hibernating.

Sound a bit creepy or lonely? Maybe. But Ngo-Anh commented:

... the basic idea of putting astronauts into long-duration hibernation is actually not so crazy: a broadly comparable method has been tested and applied as therapy in critical care trauma patients and those due to undergo major surgeries for more than two decades. Most major medical centres have protocols for inducing hypothermia in patients to reduce their metabolism to basically gain time, keeping patients in a better shape than they otherwise would be.

We aim to build on this in future, by researching the brain pathways that are activated or blocked during initiation of hibernation, starting with animals and proceeding to people.

By the way, if you're interested in reading a wonderful recent science fiction series depicting deep-space travel via human hibernation - two of the best sci-fi books I've ever read (and I've read a bunch) - try " Children of Time" and " Children of Ruin " by Adrian Tchaikovksy. Both have all the things I love in science fiction: travel over millenia among the stars, how the hibernating travelers perceive time passing, strange planets, weird aliens, a human love story. Human hibernation plays a big role in these awesome books!

Bottom line: The European Space Agency has been studying how real-life human hibernation would impact space mission design.

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esa study human hibernation trip to mars

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ESA studies human hibernation for space travel | EarthSky.org

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The European Space Agency (ESA) said on November 18, 2019, that its scientists have recently been investigating the process of placing astronauts into hibernation to cross the vastness of space. These scientists met at ESA's Concurrent Design Facility to assess the advantages of human hibernation for a trip to a neighboring planet, such as Mars.

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Earlier this week, Richard Branson's Virgin Galactic made waves when it became the first commercial human spaceflight company to trade on a stock exchange. With space tourism long considered a futuristic pipe dream, this move signals new confidence among investors in the industry's long term viability. Shortly after the IPO, NASA made announcements regarding Boeing's and SpaceX's upcoming Commercial Crew capsule abort tests, which represent important technical milestones towards achieving commercial orbital spaceflight. All the while, the International Space Station (ISS) has continued with business as usual, with the first all-female space walk occurring just 2 weeks ago.

With so many options for human spaceflight now on the table, how should an aspiring astronaut plan on getting to space in 2019? The answer to this question is less straightforward than it was several years ago, and is influenced by many factors including the aspirant's age, health, work experience, affluence and personal goals. Below is a breakdown of the currently available options.
1. Astronaut Corps
The most storied path to space has traditionally been through the government. NASA and Russia's ROSCOSMOS have both been sending humans to orbit since 1961, and in recent decades many other countries have started their own space agencies and astronaut programs. All national space agencies provide a comprehensive training program to those lucky enough to be selected as astronaut candidates. The astronaut candidates (nicknamed "ASCANs" by NASA) are hired into the government as civil servants for the duration of their astronaut careers, and each of them must pass their space agency's official training program to be eligible for flight.

The ultimate destination of a government astronaut varies according to both their agency's capabilities and their country's national priorities: NASA astronauts during the Apollo program of the 1960s and 1970s traveled to the moon, whereas astronauts in 2019 travel exclusively to the ISS. Astronauts are hired not only to be passengers onboard a spacecraft, but to execute missions that advance human knowledge of the universe. In contrast with space tourists, who visit destinations that have been frequented by dozens of humans before them, members of the astronaut corps sign on to the possibility of being the first person to visit an asteroid, moon or planet, or to perform a critical spacewalk. With this possibility comes the inherent risk of exploring the unknown, but most astronauts will agree that the risk is worth it to them for the opportunity to influence future generations.

So how does one become a government astronaut candidate? For NASA's astronaut corps, the recent 0.065% acceptance rate speaks for itself - the selection process is highly competitive. At this level of competition, it is impossible to guarantee anyone a spot regardless of their qualifications, but a look into the data from recent classes provides some insight into characteristics commonly sought by the selection board.
I. Education
Officially, the minimum educational requirement to become a NASA astronaut is a bachelor's degree in a science, technology, engineering or mathematics (STEM) discipline. However, the last time an astronaut was selected for training without a graduate degree was in 2000. The last 5 classes (2000, 2004, 2009, 2013 and 2017) have included a total of 56 astronaut candidates, and among the last 4 classes, every selected astronaut candidate has held at least a master's degree. Many hold doctorate degrees (Ph.D., M.D. or D.V.M.); the majority of civilian candidates (68%) are doctors whereas a relatively small percentage military candidates (18%) hold a doctorate. Of the 7 selected civilians who did not hold a doctorate, 2 were teachers selected as part of the Educator Mission Specialist program in 2004.

Astronaut Candidate Data, 2000-2017: Highest level of education obtained before selection.
Chart created with Vennage

II. Professional Experience
All astronaut candidates selected since 2000 have had at least 1 year of professional experience before their selection to the corps. (Officially, NASA requires at least 3 years of professional experience or 1,000 hours of pilot-in-command time on a jet aircraft in order to apply, but graduate degrees can substitute for up to 2 years of experience.) By far, the most common pre-astronaut career was military officer, with academia and civilian NASA experience representing the most common careers for non-military candidates.

Astronaut Candidate data, 2000-2017: Professional Experience
Chart created with Vennage.

As today's astronauts spend the majority of their time in space performing experiments on the ISS, scientific research skills have become a commonly sought trait in the selection process. All civilian candidates selected during the last 5 classes had STEM research experience obtained either through an M.D. or Ph.D. degree or through a research and development position in industry.
III. Operational Experience
Current astronauts note that while it is not an officially stated requirement, most new candidates have obtained significant operational experience prior to selection. Since being an astronaut may require responding quickly to on-orbit emergencies or surviving in isolation under extreme conditions after an off-nominal landing, any evidence of analogous skills is an advantage in the selection process.

While many candidates obtain operational experience through the military, others have demonstrated their ability to survive in extreme environments through field research or remote expeditions. A few recent examples include drilling ice in Antarctica, participating in NASA's underwater space analog project at Pavilion Lake, and managing remote scientific bases for the National Oceanic and Atmospheric Association (NOAA).
IV. Physical and Mental Health
In addition to the above qualifications, astronauts must exhibit a high level of both physical and mental fitness. As an official requirement for selection, NASA's long duration astronaut physical mandates that applicants demonstrate visual acuity that is correctable to 20/20 in each eye (use of glasses is acceptable). Once an astronaut candidate is selected for training, NASA's qualification program includes rigorous physical conditioning requirements that must be met to gain eligibility for flight assignments. Tasks that must be completed on a regular basis include treading water for 10 minutes while wearing a 280 pound flight suit.

To gauge mental fitness, the astronaut selection process includes evaluation by medical professionals for suitability to work in confined spaces or isolated environments for extended periods of time. Applicants who are offered onsite interviews with NASA are also evaluated on their ability to work in a team, as being an astronaut requires extensive cooperation both with other astronauts and with operators supporting the mission from the ground.
V. Other Insights
Some folklore suggests that due to the nature of the job, competitive applicants to the astronaut corps must speak Russian and hold a pilot's license prior to selection. On the contrary, astronauts have attested that neither skill is required at the time of application. Any necessary skills training is provided to selected candidates during their first few weeks on the job.

The astronaut corps does not have any official requirements regarding age. However, the requirements for the position make it a natural target for early to mid career professionals. The age range of recent classes has been 29 to 43 (at time of selection).

While this analysis focuses on the NASA selection process, the qualifications required by other space agencies are similar and the odds of being chosen are comparably low. To be selected as a government-affiliated astronaut, one must be both highly qualified and extremely lucky. Those interested in applying should look out for their space agency's next announcement on astronaut recruitment. For the past decade, NASA has inducted a new astronaut class every 4 years (2009, 2013 and 2017). This timeline would suggest another selection in approximately 2021, though the exact date is dependent both on funding and on demand for new astronauts.

Out of pocket cost: None - selected astronaut candidates are hired as civil servants and paid a salary according to the government's pay scale

Time in space: Average mission duration of 6 months
2. Orbital Space Tourism
Though the odds of being selected for the astronaut corps may seem daunting, aspiring spacefarers need not despair - the ISS has been open to tourists since 2001. Since then, 7 private passengers have flown to Station through trips facilitated by Space Adventures. All previous tourists completed their flight training in Russia's Star City and flew to the ISS aboard Russian Soyuz vehicles. Flights have been priced between $20 and $25 million, plus an additional $15 million for those participating in a space walk. Space Adventures remains open to reservations, but opportunities to reserve seats have been limited in recent years, as the Soyuz has been the only vehicle servicing ISS since retirement of the Space Shuttle in 2011.

Fortunately, the rise of the commercial space industry has opened up additional pathways for human space exploration. Earlier this year, NASA announced that starting in 2020 the U.S. segment will host private passengers for stays of up to 30 days on the ISS for a fee of $35,000 per night. Of note, this cost does not include transportation to and from the ISS: These rides will be provided by commercial providers, including SpaceX and Boeing, who are currently developing orbital human spaceflight capsules under NASA's Commercial Crew Program. This move to open up the ISS to tourism is part of a larger effort by NASA to privatize the Station, which the U.S. government is hoping to defund within the decade.

Commercial passengers who wish to visit the ISS need not fulfill the education and professional experience requirements of the astronaut corps, though they must still complete a comprehensive training program and provide evidence of physical and mental fitness for flight. Additionally, the trip will cost passengers a pretty penny: In June, CNBC pegged the price for a private ISS trip at $52 million USD.

Other commercial vendors such as Bigelow Aerospace hope to construct new venues on orbit to provide tourists with additional options. In 2016, Bigelow launched its Bigelow Expandable Activity Module (BEAM) to the ISS, where it remains to date as a technology demonstration for Bigelow's inflatable habitat concept. By scaling this technology, Bigelow plans to launch and activate its B330 space station by as early as 2021. The B330 space station is intended to serve as a commercial alternative to the ISS, and will host both scientific missions and private tourists. Bigelow has stated that the fee for a stay on B330 will likely be in the low eight figures.

Out of pocket cost: Upwards of $50 million USD

Time in space: Up to 30 days, plus flight time (additional 4 to 6 days)
3. Suborbital Space Tourism
For tourists seeking a lower price point, several companies have set out to offer shorter duration "hops" into space. Built with commercial passengers in mind, Virgin Galactic's SpaceShipTwo and Blue Origin's New Shepard are slated to begin suborbital tourist flights starting as early as 2020. These flights do not achieve orbital velocity (suborbital flights achieve a maximum speed of around 3,500 kph, whereas orbital insertion requires 27,000 kph) and thus cannot carry passengers to orbital destinations, but still provide passengers with several minutes of microgravity during flight. Flights on these vehicles will last approximately 2.5 hours, and will be both launched and recovered from the vehicle's spaceport in North America. Both companies have said that their flights will cary a price tag of $250,000 USD.

Of the 2.5 hour flight time, how many minutes are spent in space? It turns out, the answer to this is not straightforward, as it depends on the definition of "space." The World Air Sports Federation (FAI) defines the Karman line, which represents the boundary between Earth and space, as 100 kilometers above the Earth's mean sea level. While the FAI is the body responsible for setting international standards in aeronautics and astronautics, the U.S. government defines space according a different standard, with NASA and the Federal Aviation Administration (FAA) setting the boundary at 50 miles (80 km). Earlier this year, Jeff Bezos noted that this distinction may be important to potential customers who wish to obtain astronaut wings: SpaceShipTwo has yet to cross the 100 km mark with cargo or humans on board, whereas New Shepard has consistently achieved this altitude during its uncrewed cargo flights. Both vehicles have consistently achieved the U.S. government's standard of 80 km.

Passengers wishing to fly on either vehicle may pre-order tickets directly through the operating company. While each company maintains medical requirements for its passengers for safety reasons, these requirements are likely to be less stringent than those required for ISS tourism as the forces experienced during a suborbital launch and landing are much more benign than those seen during orbital flight. A 2014 medical study commissioned by Virgin Galactic concluded that suborbital human spaceflight was safe for the vast majority of people, even if they suffer from chronic medical conditions.

Out of pocket cost: $250,000 USD

Time in space: 0 to 7 minutes
4. Parabolic Flight
Technically this option won't get you into space, since parabolic flights are conducted using commercial aircraft that never actually leave the Earth's atmosphere. However, parabolic flight participants do experience up to 25 seconds at a time of weightlessness, which provides a small taste of what it's like to leave the planet.

To create a reduced gravity environment, the aircraft used for these flights follow a parabolic flight path relative to the center of the Earth. At the top of the parabola, the plane and its passengers are in free fall, which simulates the sensation of weightlessness. These flights have been used by NASA since 1957 as part of the official astronaut training program. Though the physical requirements for participation are no different from the requirements for boarding a commercial airplane, the aircraft used for parabolic flights have been nicknamed the "vomit comet" due to their long history of inducing nausea in passengers and should be approached with caution by anyone with a propensity for motion sickness.

Stages of parabolic flight.
5W Infographics (Source: Laboratory for Space and Microgravity Research)

Parabolic flights are available to scientific researchers whose experiments have been accepted into government-funded grant programs such as NASA's Flight Opportunities Program or the European Space Agency's annual parabolic flight campaign. Paying customers also have the option to purchase flights from commercial providers such as Zero-G Corporation. Tourists can expect to pay at least $5,000 USD per seat on a shared flight, or up to $180,000 to rent out an entire aircraft for a private experience.

Out of pocket cost: $0 to $180,000 USD

Time in space: None, but up to 6 minutes total of weightlessness per flight

In conclusion: If one hopes to make a giant leap for (wo)mankind, joining the astronaut corps is probably still their best bet. But for those seeking a slightly less risky, albeit pricey opportunity to explore the cosmos, there are plenty of other options available.

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Earlier this week, Richard Branson's Virgin Galactic made waves when it became the first commercial human spaceflight company to trade on a stock exchange. With space tourism long considered a futuristic pipe dream, this move signals new confidence among investors in the industry's long term viability.

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Interstellar space travel. Fantasy of every five-year-old kid within us. Staple of science fiction serials. Boldly going where nobody has gone before in a really fantastic way. As we grow ever more advanced with our rockets and space probes, the question arises: could we ever hope to colonize the stars? Or, barring that far-flung dream, can we at least send space probes to alien planets, letting them tell us what they see?

The truth is that interstellar travel and exploration is technically possible. There's no law of physics that outright forbids it. But that doesn't necessarily make it easy, and it certainly doesn't mean we'll achieve it in our lifetimes, let alone this century. Interstellar space travel is a real pain in the neck.
Related: Gallery: Visions of Interstellar Starship Travel
Voyage outward

If you're sufficiently patient, then we've already achieved interstellar exploration status. We have several spacecraft on escape trajectories, meaning they're leaving the solar system and they are never coming back. NASA's Pioneer missions, the Voyager missions, and most recently New Horizons have all started their long outward journeys. The Voyagers especially are now considered outside the solar system, as defined as the region where the solar wind emanating from the sun gives way to general galactic background particles and dust.

So, great; we have interstellar space probes currently in operation. Except the problem is that they're going nowhere really fast. Each one of these intrepid interstellar explorers is traveling at tens of thousands of miles per hour, which sounds pretty fast. They're not headed in the direction of any particular star, because their missions were designed to explore planets inside the solar system. But if any of these spacecraft were headed to our nearest neighbor, Proxima Centauri, just barely 4 light-years away, they would reach it in about 80,000 years.

I don't know about you, but I don't think NASA budgets for those kinds of timelines. Also, by the time these probes reach anywhere halfway interesting, their nuclear batteries will be long dead, and just be useless hunks of metal hurtling through the void. Which is a sort of success, if you think about it: It's not like our ancestors were able to accomplish such feats as tossing random junk between the stars, but it's probably also not exactly what you imagined interstellar space travel to be like.

Related: Superfast Spacecraft Propulsion Concepts (Images)

Speed racer

To make interstellar spaceflight more reasonable, a probe has to go really fast. On the order of at least one-tenth the speed of light. At that speed, spacecraft could reach Proxima Centauri in a handful of decades, and send back pictures a few years later, well within a human lifetime. Is it really so unreasonable to ask that the same person who starts the mission gets to finish it?

Going these speeds requires a tremendous amount of energy. One option is to contain that energy onboard the spacecraft as fuel. But if that's the case, the extra fuel adds mass, which makes it even harder to propel it up to those speeds. There are designs and sketches for nuclear-powered spacecraft that try to accomplish just this, but unless we want to start building thousands upon thousands of nuclear bombs just to put inside a rocket, we need to come up with other ideas.

Perhaps one of the most promising ideas is to keep the energy source of the spacecraft fixed and somehow transport that energy to the spacecraft as it travels. One way to do this is with lasers. Radiation is good at transporting energy from one place to another, especially over the vast distances of space. The spacecraft can then capture this energy and propel itself forward.

This is the basic idea behind the Breakthrough Starshot project, which aims to design a spacecraft capable of reaching the nearest stars in a matter of decades. In the simplest outline of this project, a giant laser on the order of 100 gigawatts shoots at an Earth-orbiting spacecraft. That spacecraft has a large solar sail that is incredibly reflective. The laser bounces off of that sail, giving momentum to the spacecraft. The thing is, a 100-gigawatt laser only has the force of a heavy backpack. You didn't read that incorrectly. If we were to shoot this laser at the spacecraft for about 10 minutes, in order to reach one-tenth the speed of light, the spacecraft can weigh no more than a gram.
Related: Breakthrough Starshot in Pictures: Laser-Sailing Nanocraft to Study Alien Planets
That's the mass of a paper clip.

A spaceship for ants

This is where the rubber meets the interstellar road when it comes to making spacecraft travel the required speeds. The laser itself, at 100 gigawatts, is more powerful than any laser we've ever designed by many orders of magnitude. To give you a sense of scale, 100 gigawatts is the entire capacity of every single nuclear power plant operating in the United States combined.

And the spacecraft, which has to have a mass no more than a paper clip, must include a camera, computer, power source, circuitry, a shell, an antenna for communicating back home and the entire lightsail itself.

That must be almost perfectly reflective. If it absorbs even a tiny fraction of that incoming laser radiation it will convert that energy to heat instead of momentum. At 100 gigawatts, that means straight-up melting, which is generally considered not good for spacecraft.

Once accelerated to one-tenth the speed of light, the real journey begins. For 40 years, this little spacecraft will have to withstand the trials and travails of interstellar space. It will be impacted by dust grains at that enormous velocity. And while the dust is very tiny, at those speeds motes can do incredible damage. Cosmic rays, which are high-energy particles emitted by everything from the sun to distant supernova, can mess with the delicate circuitry inside. The spacecraft will be bombarded by these cosmic rays non-stop as soon as the journey begins.

Is Breakthrough Starshot possible? In principle, yes. Like I said above, there's no law of physics that prevents any of this from becoming reality. But that doesn't make it easy or even probable or plausible or even feasible using our current levels of technology (or reasonable projections into the near future of our technology). Can we really make a spacecraft that small and light? Can we really make a laser that powerful? Can a mission like this actually survive the challenges of deep space?

The answer isn't yes or no. The real question is this: are we willing to spend enough money to find out if it's possible?

Learn more by listening to the episode "Is interstellar travel possible?" on the Ask A Spaceman podcast, available on and on the Web at http://www.askaspaceman.com. Thanks to @infirmus, Amber D., neo, and Alex V. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul and facebook.com/PaulMattSutter.

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There's no law of physics that outright forbids interstellar travel. But that doesn't necessarily make it easy, and it certainly doesn't mean we'll achieve it in our lifetimes, let alone this century.

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Independent publication covering space travel, rocket technology, human spaceflight, space medicine, and the engineering driving the future of exploration.

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It's a dark, clear, moonless night. You look up into the sky. You see thousands of stars arranged in patterns or constellations. The light from these stars has traveled great distances to reach Earth. But what are stars? How far away are they? Are they all the same? Are there other planets around them?

In this article, we will look at the fascinating world of stars. We will examine the nature of stars, types of stars, how stars form and how stars die. If you have read How the Sun Works, you already know a lot about the nature of Earth's nearest star. As you read the following pages, you'll find out even more about what you can see in the night sky.

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It's a dark, clear, moonless night. You look up into the sky. You see thousands of stars arranged in patterns or constellations. The light from these stars has traveled great distances to reach Earth. But what are stars? How far away are they? Are they all the same?

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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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A star develops from a giant, slowly rotating cloud that is made up entirely or almost entirely of hydrogen and helium. Due to its own gravitational pull, the cloud begins to collapse inward, and as it shrinks, it spins more and more quickly, with the outer parts becoming a disk while the innermost parts become a roughly spherical clump.

According to NASA, this collapsing material grows hotter and denser, forming a ball-shaped protostar. When the heat in the protostar reaches about 1.8 million degrees Fahrenheit (1 million degrees Celsius), atomic nuclei that normally repel each other start fusing together, and the star ignites. Nuclear fusion converts a small amount of the mass of these atoms into extraordinary amounts of energy - for instance, 1 gram of mass converted entirely to energy would be equal to an explosion of roughly 22,000 tons of TNT.

Stellar evolution

The greater the mass of such a star, the more quickly it will use its hydrogen fuel and the shorter it stays on the main sequence. After all the hydrogen in the core is fused into helium, the star changes rapidly - without nuclear radiation to resist it, gravity immediately crushes matter down into the star's core, quickly heating the star. This causes the star's outer layers to expand enormously and to cool and glow red as they do so, rendering the star a red giant.

A high-mass star forms and dies quickly. These stars form from protostars in just 10,000 to 100,000 years. While on the main sequence, they are hot and blue, some 1,000 to 1 million times as luminous as the sun and are roughly 10 times wider. When they leave the main sequence, they become a bright red supergiant and eventually become hot enough to fuse carbon into heavier elements. After some 10,000 years of such fusion, the result is an iron core roughly 3,800 miles (6,000 km) wide, and since any more fusion would consume energy instead of liberating it, the star is doomed, as its nuclear radiation can no longer resist the force of gravity.

A low-mass star uses hydrogen fuel so sluggishly that they can shine as main-sequence stars for 100 billion to 1 trillion years - since the universe is only about 13.7 billion years old, according to NASA, this means no low-mass star has ever died. Still, astronomers calculate these stars, known as red dwarfs, will never fuse anything but hydrogen, which means they will never become red giants. Instead, they should eventually just cool to become white dwarfs and then black dwarves.

History of star observations

Since the dawn of recorded civilization, stars played a key role in religion and proved vital to navigation, according to the International Astronomical Union. Astronomy, the study of the heavens, may be the most ancient of the sciences. The invention of the telescope and the discovery of the laws of motion and gravity in the 17th century prompted the realization that stars were just like the sun, all obeying the same laws of physics. In the 19th century, photography and spectroscopy - the study of the wavelengths of light that objects emit - made it possible to investigate the compositions and motions of stars from afar, leading to the development of astrophysics.

In 1937, the first radio telescope was built, enabling astronomers to detect otherwise invisible radiation from stars. The first gamma-ray telescope launched in 1961, pioneering the study of star explosions ( supernovae). Also in the 1960s, astronomers commenced infrared observations using balloon-borne telescopes, gathering information about stars and other objects based on their heat emissions; the first infrared telescope (the Infrared Astronomical Satellite) was launched in 1983.

Microwave emissions were first studied from space in 1992, with NASA's Cosmic Microwave Background Explorer (COBE) satellite. (Microwave emissions are generally used to probe the young universe's origins, but they are occasionally used to study stars.) In 1990, the first space-based optical telescope, the Hubble Space Telescope, was launched, providing the deepest, most detailed visible-light view of the universe.

There have been, of course, more advanced observatories (in all wavelengths) over the years, and even more powerful ones are planned. A couple of examples are the Extremely Large Telescope (ELT), which is planned to start observations in 2024 in infrared and optical wavelengths. Also, NASA's James Webb Space Telescope - billed as a successor to Hubble - will launch in 2018 to probe stars in infrared wavelengths.

How are stars named?

Ancient cultures saw patterns in the heavens that resembled people, animals or common objects - constellations that came to represent figures from myth, such as Orion the Hunter, a hero in Greek mythology.

Astronomers now often use constellations in the naming of stars. The International Astronomical Union, the world authority for assigning names to celestial objects, officially recognizes 88 constellations. Usually, the brightest star in a constellation has "alpha," the first letter of the Greek alphabet, as part of its scientific name. The second brightest star in a constellation is typically designated "beta," the third brightest "gamma," and so on until all the Greek letters are used, after which numerical designations follow.

Since there are so many stars in the universe, the IAU uses a different system for newfound stars. Most consist of an abbreviation that stands for either the type of star or a catalog that lists information about the star, followed by a group of symbols. For instance, PSR J1302-6350 is a pulsar, thus the PSR. The J reveals that a coordinate system known as J2000 is being used, while the 1302 and 6350 are coordinates similar to the latitude and longitude codes used on Earth.

In recent years, the IAU formalized several names for stars amid calls from the astronomical community to include the public in their naming process. The IAU formalized 14 star names in the 2015 "Name ExoWorlds" contest, taking suggestions from science and astronomy clubs around the world.

Binary stars and other multiples

(Image credit: NASA/ Chandra X-ray Observatory)

Binary stars develop when two protostars form near each other. One member of this pair can influence its companion if they are close enough together, stripping away matter in a process called mass transfer. If one of the members is a giant star that leaves behind a neutron star or a black hole, an X-ray binary can form, where matter pulled from the stellar remnant's companion can get extremely hot - more than 1 million F (555,500 C) and emit X-rays.

If a binary includes a white dwarf, gas pulled from a companion onto the white dwarf's surface can fuse violently in a flash called a nova. At times, enough gas builds up for the dwarf to collapse, leading its carbon to fuse nearly instantly and the dwarf to explode in a Type I supernova, which can outshine a galaxy for a few months.

Key characteristics of stars
Brightness
Astronomers describe star brightness in terms of magnitude and luminosity.

The magnitude of a star is based on a scale more than 2,000 years old, devised by Greek astronomer Hipparchus around 125 BC, according to NASA's Ask and Astrophysicist. He numbered groups of stars based on their brightness as seen from Earth - the brightest stars were called first magnitude stars, the next brightest were the second magnitude, and so on up to sixth magnitude, the faintest visible ones.

Nowadays astronomers refer to a star's brightness as viewed from Earth as its apparent magnitude, but since the distance between Earth and the star can affect the light one sees from it, they now also describe the actual brightness of a star using the term absolute magnitude, which is defined by what its apparent magnitude would be if it were 10 parsecs or 32.6 light-years from Earth. The magnitude scale now runs to more than six and less than one, even descending into negative numbers - the brightest star in the night sky is Sirius, with an apparent magnitude of -1.46.

Luminosity is the power of a star - the rate at which it emits energy. Although power is generally measured in watts - for instance, the sun's luminosity is 400 trillion trillion watts - the luminosity of a star is usually measured in terms of the luminosity of the sun. For example, Alpha Centauri A is about 1.3 times as luminous as the sun. To figure out luminosity from absolute magnitude, one must calculate that a difference of five on the absolute magnitude scale is equivalent to a factor of 100 on the luminosity scale - for instance, a star with an absolute magnitude of 1 is 100 times as luminous as a star with an absolute magnitude of 6.

The brightness of a star depends on its surface temperature and size.
Color
Stars come in a range of colors, from reddish to yellowish to blue. The color of a star depends on surface temperature.

A star might appear to have a single color, but actually emits a broad spectrum of colors, potentially including everything from radio waves and infrared rays to ultraviolet beams and gamma rays. Different elements or compounds absorb and emit different colors or wavelengths of light, and by studying a star's spectrum, one can divine what its composition might be.
Surface temperature
Astronomers measure star temperatures in a unit known as the kelvin, with a temperature of zero K ("absolute zero") equaling minus 273.15 degrees C, or minus 459.67 degrees F. A dark red star has a surface temperature of about 2,500 K (2,225 C and 4,040 F); a bright red star, about 3,500 K (3,225 C and 5,840 F); the sun and other yellow stars, about 5,500 K (5,225 C and 9,440 F); a blue star, about 10,000 K (9,725 C and 17,540 F) to 50,000 K (49,725 C and 89,540 F).

(Image credit: Getty Images)

The surface temperature of a star depends in part on its mass and affects its brightness and color. Specifically, the luminosity of a star is proportional to temperature to the fourth power. For instance, if two stars are the same size but one is twice as hot as the other in kelvin, the former would be 16 times as luminous as the latter.

Related: How hot is the sun?
Size
Astronomers generally measure the size of stars in terms of the radius of our sun. For instance, Alpha Centauri A has a radius of 1.05 solar radii (the plural of radius). Stars range in size from neutron stars, which can be only 12 miles (20 kilometers) wide, to supergiants roughly 1,000 times the diameter of the sun.

The size of a star affects its brightness. Specifically, luminosity is proportional to radius squared. For instance, if two stars had the same temperature, if one star was twice as wide as the other one, the former would be four times as bright as the latter.
Mass
Astronomers represent the mass of a star in terms of the solar mass, the mass of our sun. For instance, Alpha Centauri A is 1.08 solar masses.

Stars with similar masses might not be similar in size because they have different densities. For instance, Sirius B is roughly the same mass as the sun but is 90,000 times as dense, and so is only a fiftieth its diameter.

The mass of a star affects surface temperature.
Magnetic field
Stars are spinning balls of roiling, electrically charged gas, and thus typically generate magnetic fields. When it comes to the sun, researchers have discovered its magnetic field can become highly concentrated in small areas, creating features ranging from sunspots to spectacular eruptions known as flares and coronal mass ejections. A survey at the Harvard-Smithsonian Center for Astrophysics found that the average stellar magnetic field increases with the star's rate of rotation and decreases as the star ages.
Metallicity
The metallicity of a star measures the amount of "metals" it has - that is, any element heavier than helium.

Three generations of stars may exist based on metallicity. Astronomers have not yet discovered any of what should be the oldest generation, Population III stars born in a universe without "metals." When these stars died, they released heavy elements into the cosmos, which Population II stars incorporated relatively small amounts of. When a number of these died, they released more heavy elements, and the youngest Population I stars like our sun contain the largest amounts of heavy elements.

Star classification

Stars are typically classified by their spectrum in what is known as the Morgan-Keenan or MK system, according to the European Southern Observatory. There are eight spectral classes, each analogous to a range of surface temperatures - from the hottest to the coldest, these are O, B, A, F, G, K, M and L. Each spectral class also consists of 10 spectral types, ranging from the numeral 0 for the hottest to the numeral 9 for the coldest.

Stars are also classified by their luminosity under the Morgan-Keenan system. The largest and brightest classes of stars have the lowest numbers, given in Roman numerals - Ia is a bright supergiant; Ib, a supergiant; II, a bright giant; III, a giant; IV, a subgiant; and V, a main sequence or dwarf.

A complete MK designation includes both spectral type and luminosity class - for instance, the sun is a G2V.

Stellar structure

The structure of a star can often be thought of as a series of thin nested shells, somewhat like an onion.

A star during most of its life is a main-sequence star, which consists of a core, radiative and convective zones, a photosphere, a chromosphere and a corona. The core is where all the nuclear fusion takes place to power a star.

In the radiative zone, energy from these reactions is transported outward by radiation, like heat from a light bulb, while in the convective zone, energy is transported by the roiling hot gases, like hot air from a hairdryer. Massive stars that are more than several times the mass of the sun are convective in their cores and radiative in their outer layers, while stars comparable to the sun or less in mass are radiative in their cores and convective in their outer layers. Intermediate-mass stars of spectral type A may be radiative throughout.

After those zones come the part of the star that radiates visible light, the photosphere, which is often referred to as the surface of the star. After that is the chromosphere, a layer that looks reddish because of all the hydrogen found there. Finally, the outermost part of a star's atmosphere is the corona, which if super-hot might be linked with convection in the outer layers.

Additional resources

To explore the stars of our universe for yourself, you can use NASA's Skymap tool. Additionally, to see images of stars taken by the Hubble Space Telescope, browse the European Space Agency's (ESA) image archive.

Bibliography

Salaris, M., & Cassisi, S. (2005). "Evolution of stars and stellar populations". John Wiley & Sons. https://books.google.co.uk

"Spectral Classification". Annual Review of Astronomy and Astrophysics (1973). https://www.annualreviews.org/doi/abs/10.1146/annurev.aa.11.090173.000333?journalCode=astro

"The Gas-Star Formation Cycle in Nearby Star-forming Galaxies. I. Assessment of Multi-scale Variations". The Astrophysical Journal (2019). https://iopscience.iop.org/article/10.3847/1538-4357/ab50c2/meta

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Star Facts: The Basics of Star Names and Stellar Evolution

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How are stars named? And what happens when they die? These star facts explain the science of the night sky.

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#EZScience Episode 3: Our Favorite Star — The Sun

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Sun Facts: Interesting Facts about Sun (or Sol)

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How does the Sun work? How old is it? What missions are there going to the Sun? Explore some incredible Sun Facts here in our guide

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In astronomy, a satellite is an object that orbits (goes around) a planet. There are several hundred natural satellites, or moons, in our Solar System. Thousands of artificial (human-made) satellites have also been launched since 1957. These have many different uses, including taking pictures of the Sun, Earth, and other planets, and looking deep into space at black holes, and distant stars and galaxies. There are also communications satellites, weather satellites, and the International Space Station.

The first artificial satellite, Sputnik 1, was launched in 1957. It was very simple. A small aluminium ball, about the size of a beach ball, with four long antennas, and was powered by batteries. Inside Sputnik were radio transmitters that sent out a distinctive beep, beep sound which was heard all over the world. The launch of this small and simple satellite marked the beginning of the space age.

Modern satellites are much more complicated. Most satellites are designed to be as strong and light as possible. They are built using the same basic model. A platform called a bus contains all the main systems, including the batteries, computer and thrusters. Attached to the bus are antennas, solar arrays and payload instruments (such as cameras, telescopes and communications equipment).

Satellites have to power themselves. This is usually done by using large solar arrays (wings) covered with light-sensitive solar cells. The arrays are many metres long and have to be folded during launch. The solar cells provide several kilowatts of power, although they become less efficient as they get older. Most arrays can be turned so that they collect as much sunlight as possible. When the satellite goes into shadow, it gets its power from rechargeable batteries.

The side of a satellite facing the Sun gets very hot, while the shaded side becomes very cold. This is a problem because most satellite equipment is sensitive to extreme heat or cold. Ways of protecting the instruments include using layered blankets that look like aluminium foil to keep heat in, and adding radiators to release heat from electrical equipment.

Last modified 19 December 2018

Useful space

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In astronomy, a satellite is an object that orbits (goes around) a planet. There are several hundred natural satellites, or moons, in our Solar System. Thousands of artificial (human-made) satellites have also been launched since 1957. These have many different uses, including taking pictures of the Sun, Earth, and other planets, and looking deep into space at black holes, and distant stars and galaxies.

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A satellite is an object that orbits another object. In space, satellites may be made by man, or they may be natural. The moon is a natural satellite that orbits the Earth. Most man-made satellites also orbit the Earth, but some orbit other planets, such as Saturn, Venus or Mars, or the moon.

Satellites are used for many purposes. There are weather satellites, communications satellites, reconnaissance satellites, astronomy satellites and many other kinds.

Herman Potočnik proposed and Arthur C. Clarke popularized the idea of the communications satellite.

The world's first artificial satellite, the Sputnik 1, was launched by the Soviet Union on October 4, 1957. This surprised the world, and the United States quickly worked to launch their own satellite, starting the space race. Sputnik 2 was launched on November 3, 1957 and carried the first living passenger into orbit, a dog named Laika. The United States launched their first satellite, called Explorer 1 on January 31, 1958. The UK launched its first satellite in 1962.

Since then, thousands of satellites have been launched into orbit around the Earth. Some satellites, notably space stations, have been launched in parts and assembled in orbit.

Satellites orbiting now

Artificial satellites come from more than 50 countries and have used the satellite launching capabilities of ten nations. A few hundred satellites are currently working, but thousands of unused satellites and satellite fragments orbit the Earth as space debris. The largest satellite is the International Space Station, which was put together by several different countries (including the organizations of NASA, ESA, JAXA and RKA). It usually has a crew of six astronauts or cosmonauts living on board. It is permanently occupied, but the crew changes. The Hubble Space Telescope has been repaired and updated by astronauts in space several times.

There are also man-made satellites orbiting something other than the Earth. The Mars Reconnaissance Orbiter is one of those orbiting Mars. Cassini-Huygens is orbiting Saturn. Venus Express, run by the ESA, is orbiting Venus. Two GRAIL satellites orbited the moon until December 2012. Several satellites have orbited the Sun for years and one is to be added in 2017.

Man-made satellites have several main uses:

Most of the man-made satellites are in a low Earth orbit (LEO) or a geostationary orbit. To stay in orbit, the satellite's sideways speed must balance the force of gravity. Satellites in low orbit are often less than one thousand kilometers above the ground. Close to the Earth, in LEO, the satellites must move faster to stay in orbit. Low orbits work well for satellites that take pictures of the Earth. Many do jobs that call for high orbital inclination (they swing above and below the equator), so they can communicate, or look at other areas. It is easier to put a satellite in low Earth orbit, but the satellite appears to move when viewed from Earth. This means a satellite dish (a type of antenna) must be always moving in order to send or receive communications with that satellite.

Medium orbit works well for GPS satellites - receivers on Earth use the satellite's changing position and precise time (and a type of antenna that does not have to be pointed) to find where on Earth the receiver is. But constantly changing positions does not work for satellite TV and other types of satellites that send and receive a lot of information. Those need to be in geostationary orbit.

A satellite in a geostationary orbit moves around the Earth as fast as the Earth spins, so from the ground it looks like it is stationary (not moving). To move this way, the satellite must be straight above the equator, and 35,786 kilometers (22,236 miles) above the ground.

Related pages

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A satellite is an object that orbits another object. In space, satellites may be made by man, or they may be natural. The moon is a natural satellite that orbits the Earth. Most man-made satellites also orbit the Earth, but some orbit other planets, such as Saturn, Venus or Mars, or the moon.

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What is a Satellite?

Imagine laying out in the grass on a summer night and looking up into the big, dark sky. You see some twinkling stars, but what really catches your eye is the bright, full moon. It's so bright that it seems to light up your back yard like a night light. And while you're looking at that bright white ball in the sky, you might not have thought about the fact that it revolves around the Earth and is a satellite!

A satellite is anything that orbits, or revolves, around a bigger object.

When you think about satellites, you might think of a science fiction movie where scientists are launching a top secret, man-made satellite into space, but there are natural satellites, too!

Examples of Natural Satellites

The moon is an example of a satellite that you probably see on most nights. It orbits around the Earth, like a hula-hoop spins around your waist.

And you might not even realize you live on a satellite. The Earth orbits the Sun, which means your home, toys, computer, and favorite sneakers are all sitting on a satellite!

But moons and planets aren't the only natural satellites. Asteroids and comets are satellites, too. Halley's Comet is a satellite of the Sun, even though it can take as many as 76 years for one orbit.

Man-Made Satellites and What They Do

There are also thousands of man-made satellites, which are machines that people have launched into space to do different jobs.

The first man-made satellite was called Sputnik 1 and it was sent into space by the Soviet Union in 1957. The first US satellite was Explorer 1 and it was launched in 1958.

Some of the satellites we rely on today include:

Weather satellites to help watch the climate and predict the weather all over the world, including your hometown

Spy satellites that help the government watch what enemies are doing

Communications satellites that transmit some radio and TV signals

Global Positioning Satellites (GPS) that tell you where you are on Earth and help you get where you want to go

What Does a Satellite Carry?

Man-made satellites all usually have an antennae and source of power because they have to make their own power in order to stay in their orbit. Since the Sun puts out so much energy, satellites use that solar energy to make the power they need.

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satellite lesson for kids: definition & facts | study.com

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Satellite Lesson for Kids: Definition & Facts | Study.com

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Imagine laying out in the grass on a summer night and looking up into the big, dark sky. You see some twinkling stars, but what really catches your eye is the bright, full moon. It's so bright that it seems to light up your back yard like a night light.

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Space travel has given us a wealth of knowledge which has in turn helped us create inventions and technologies that have made human life easier and helped us learn more and explore further into the universe.

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20 Inventions We Wouldn't Have Without Space Travel

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Space travel has given us a wealth of knowledge which has in turn helped us create inventions and technologies that have made human life easier and helped us learn more and explore further into the universe.

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Inventions we use every day that were actually created for space exploration

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Unlike inventions we no longer use, these inventions are employed daily to save lives, improve environmental sustainability, and keep humans healthy.

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Jon Nguyen: Tour the solar system from home

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Want to navigate the solar system without having to buy a spacecraft? Jon Nguyen demos NASA JPL's "Eyes on the Solar System" -- free-to-use software for exploring the planets, moons, asteroids, and spacecraft that rotate around our sun in real-time.

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Fabio Pacucci: Could the Earth be swallowed by a black hole?

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From asteroids capable of destroying entire species to supernovae that could exterminate life on Earth, outer space has no shortage of forces that could wreak havoc on our planet. But there's something in space that is even more terrifying than any of these -- something that wipes out everything it comes near.

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Douglas Thomas: How a typeface helped launch Apollo

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When humanity first landed on the moon in 1969, the typeface Futura was right there with them. In this fascinating history of typography, designer Douglas Thomas shares Futura's role in launching the Apollo 11 spacecraft -- and how it became one of the most used fonts in the world.

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Phil Plait: The secret to scientific discoveries? Making mistakes

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Phil Plait was on a Hubble Space Telescope team of astronomers who thought they may have captured the first direct photo of an exoplanet ever taken. But did the evidence actually support that? Follow along as Plait shows how science progresses -- through a robust amount of making and correcting errors.

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youtube solar system magic school bus - Bing video

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The Magic School Bus Lost in the Solar System The Entire Magic School Bus Episodes YouTube Magic School Bus Lost in Space Free Magic School Bus Solar System Magic School Bus Episodes YouTube Magic School Bus Planets Magic School Bus Uranus Magic School Bus Lost in the Snow Winter Magic School Bus Weather The Magic School Bus Cold Feet

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YouTube Magic School Bus Lost in Space - Bing video

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Magic School Bus Videos Online Free Magic School Bus Outer Space Magic School Bus Space Episodes Ms. Frizzle Magic School Bus Episodes Full The Magic School Bus Lost in the Solar System Joanna Cole Magic School Bus Gets Eaten YouTube Magic School Bus Episodes List Magic School Bus Episodes YouTube Magic School Bus Goes to Space