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PRESENTATION PHYSICS (GRAVITATIONAL  SATELLITE)

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From Cassini's launch through the mission's Grand Finale, "gravity assists" were essential to making the spacecraft go where the scientists wanted it to go.

This propellant-saving, mission-enabling technique has been used in solar system exploration since the early 1970s. The key to the gravity assist technique is that it involves three bodies - the spacecraft, the solar system body that is providing the "assist", e.g., Venus, Earth, Titan, and the central body about which the spacecraft's path is being controlled. So early in Cassini's flight, the three bodies were Cassini, Venus, and the sun. More recently, the three bodies were Cassini, Titan, and Saturn. It is frequently asked how it can be that the gravity assist technique doesn't violate the law of conservation of energy, since it might appear that energy is being gained for nothing. This fundamental law of physics is of course fully in effect during all such maneuvers. In the two-body problem, e.g., Cassini and Titan, there was an exchange between potential and kinetic energy as Cassini sped up relative to Titan on approach, and then slowed again to the same speed as on approach during departure. But the total energy was constant throughout this period, and Cassini left Titan with exactly the same energy relative to Titan as it had before the encounter. In the three-body problem, however, with Saturn at the center, the interaction between Titan and the spacecraft caused an exchange of energy between the two bodies, so while the two bodies still have the same energy relative to each other, their individual energies relative to the central body have changed. Relative to Saturn, Cassini gained energy and Titan lost it, or vice versa, depending on how the flyby trajectory was designed, but the total system energy (Cassini, Titan, and Saturn) is unchanged. Since the energy of a body is determined by its mass and speed, and the masses aren't changing in the gravity assist scenario, only the speed changes need be considered here. For a fixed energy change for Titan, only an infinitesimal speed change was needed since Titan is so much more massive than Cassini was. Likewise, Cassini needed a rather large speed change to get a matching but opposite energy change since its mass was bordering on insignificant relative to that of Titan. A typical close Titan flyby changed Cassini's speed by around 800 m/sec relative to Saturn (but zero relative to Titan), and such a speed change translated into substantial orbital parameter changes in Cassini's orbit about Saturn. Titan, on the other hand, experienced a speed change of around 7 cm per million years, so no one need worry about any changes to the orbital parameters of Titan!

The rocket that launched Cassini in 1997 was the most powerful available to NASA, but it still wasn't powerful enough to send the nearly 6,000-kilogram (13,200-pound) spacecraft on a direct course to Saturn. Instead, mission designers planned multiple flybys of Venus, Earth and Jupiter, using each planet's gravity to boost Cassini's sun-relative speed and send the spacecraft out to Saturn.

Cassini's main onboard rocket engine was needed to brake the spacecraft and allow it to be captured into an orbit about Saturn on arrival in 2004. But since then, most of the "steering" of the spacecraft was done through carefully designed flybys of Saturn's large moon Titan, which has more gravitational influence to offer than any of the other moons. Knowledge of Titan's mass and orbit allowed mission planners to choose flyby conditions that resulted in the desired amount of change in Cassini's direction and speed. Each Titan flyby was targeted to return Cassini to the next Titan flyby. Close flybys of other interesting moons were made along the way as the spacecraft continued to loop around Saturn.

Titan gravity assists were used to achieve significant changes in the inclination of Cassini's orbit as well so that instead of staying nearly in the equatorial plane, the spacecraft's flight path was inclined well out of the plane of the rings. This change in the viewing geometry brought many new findings of previously unseen ring dynamics and atmospheric phenomena at Saturn's poles.

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Cassini Legacy: 1997-2017 : Gravity Assists

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From Cassini's launch through the mission's Grand Finale, "gravity assists" were essential to making the spacecraft go where the scientists wanted it to go. This propellant-saving, mission-enabling technique has been used in solar system exploration since the early 1970s.

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HubbleSite - The Telescope - Hubble Essentials

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The Hubble Space Telescope's launch in 1990 sped humanity to one of its greatest advances in that journey. Hubble is a telescope that orbits Earth. Its position above the atmosphere, which distorts and blocks the light that reaches our planet, gives it a view of the universe that typically far surpasses that of ground-based telescopes.

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Scientists are eagerly poring over new observations of Saturn's moon Enceladus ?beamed to Earth from NASA's Cassini spacecraft from a flyby last week that measured the moon's gravitational tug in an experiment to determine its interior structure and composition.

The flyby, which occurred April 27 at 8:10:17 p.m. EDT (0010:17 GMT on April 28), took Cassini through the water-rich plume that flares out from Enceladus' south polar region. At closest approach, Cassini was about 100 kilometers (60 miles) above the surface of Enceladus, at a speed of 15,000 mph relative to the moon.

In a new Enceladus photo released by NASA, the moon looks like a crisp orb in space with one end ? the south pole known to be home to icy geysers ? appearing hazy through the plumes there.

Cassini's low flyby would have been ideal for snapping new photos of Enceladus, but the primary objective of the mission was to collect information for the gravity experiment. The new photo, for example, is a raw, unprocessed image.

"Radio science was prime through this flyby, meaning that it got to control spacecraft pointing, which precluded pointing the optical instruments at Enceladus," said Robert Mitchell, Cassini program manager. "So, unfortunately, the imaging camera did not take up-close pictures."

Instead, the imaging camera was only able to obtain more distant pictures.

As part of Cassini's 26-hour gravity observation, a steady radio link to NASA's Deep Space Network on Earth enabled Cassini scientists to use the radio instrument to measure the variations in the gravitational pull of Enceladus.

Detecting any changes will help scientists understand what lies beneath Enceladus' famous "tiger stripe" fractures, which spew water vapor and organic particles from the moon's south polar region.

The experiment was also expected to help scientists learn if the south polar region's sub-surface resembles a lava lamp. Scientists have hypothesized that a bubble of warmer ice periodically travels up to the crust and repaves it, explaining the quirky heat behavior and intriguing surface features of this region. The Cassini probe launched in 1997 and arrived at Saturn in 2004, where it dropped the European Huygens probe on the cloudy surface of the planet's largest moon Titan. Cassini was slated to be decommissioned in September of this year, but has received an extended mission that now runs through 2017.

The Cassini-Huygens mission is a joint project of NASA, the European Space Agency and the Italian Space Agency. It is managed by NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Images - The Rings and Moons of Saturn

Cassini's Latest Discoveries

Special Report: Cassini's Mission to Saturn

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Secrets of Saturn Moon's Interior Probed by Cassini Spacecraft

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Scientists are eagerly poring over new observations of Saturn's moon Enceladus ?beamed to Earth from NASA's Cassini spacecraft from a flyby last week that measured the moon's gravitational tug in an experiment to determine its interior structure and composition. The flyby, which occurred April 27 at 8:10:17 p.m.

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Just after a violent flare on its parent star bathed it in intense X-ray radiation, the planet's atmosphere gave off a powerful burst of evaporation.

The observations give a tantalising glimpse of the changing climates and weather on planets outside our Solar System.

Astronomer Alain Lecavelier des Etangs and his team used Hubble to observe the atmosphere of exoplanet HD 189733b during two periods in early 2010 and late 2011, as it was silhouetted against its parent star.

While backlit in this way, the planet's atmosphere imprints its chemical signature on the starlight, allowing astronomers to decode what is happening on scales that are too tiny to image directly.

Scroll down for video:

The observations were carried out in order to confirm what the team had previously seen once before in a different planetary system: the evaporation of an exoplanet's atmosphere.

HD 189733b has a blue sky, but that's where the similarities with Earth stop.

The planet is a huge gas giant similar to Jupiter, but it lies extremely close to its star, just one thirtieth the distance Earth is from the Sun.

Even though its star is slightly smaller and cooler than the Sun, this makes the planet's climate exceptionally hot, at above 1000 degrees Celsius, and the upper atmosphere is battered by energetic extreme-ultraviolet and X-ray radiation.

As such, it is an excellent candidate to study the effects of a star on a planetary atmosphere.

'The first set of observations were actually disappointing,' Lecavelier said, 'since they showed no trace of the planet's atmosphere at all. We only realised we had chanced upon something more interesting when the second set of observations came in.'

The team's follow-up observations, made in 2011, showed a dramatic change, with clear signs of a plume of gas being blown from the planet at a rate of at least 1000 tonnes per second.

Lecavelier added: 'We hadn't just confirmed that some planets' atmospheres evaporate - we had watched the physical conditions in the evaporating atmosphere vary over time. Nobody had done that before.'

Despite the extreme temperature of the planet, the atmosphere is not hot enough to evaporate at the rate seen in 2011.

Instead the evaporation is thought to be driven by the intense X-ray and extreme-ultraviolet radiation from the parent star, HD 189733A, which is about 20 times more powerful than that of our own Sun.

Taking into account also that HD 189733b is a giant planet very close to its star, then it must suffer an X-ray dose three million times higher than the Earth.

Evidence to support X-ray driven evaporation comes from simultaneous observations of HD 189733A with the Swift satellite, which, unlike Hubble, can observe the star's atmosphere-frying X-rays.

A few hours before Hubble observed the planet for the second time, Swift recorded a powerful flash of radiation coming from the surface of the star, in which the star briefly became 4 times brighter in X-rays.

See video here:

Co-author Peter Wheatley, from the University of Warwick, said: 'X-ray emissions are a small part of the star's total output, but it is the part that it is energetic enough to drive the evaporation of the atmosphere.

'This was the brightest X-ray flare from HD 189733A of several observed to date, and it seems very likely that the impact of this flare on the planet drove the evaporation seen a few hours later with Hubble.'

X-rays are energetic enough to heat the gas in the upper atmosphere to tens of thousands of degrees, hot enough to escape the gravitational pull of the giant planet.

A similar process occurs, albeit less dramatically, when a space weather event such as a solar flare hits the Earth's ionosphere, disrupting communications.

While the team believes that the flash of X-rays is the most likely cause of the atmospheric changes they saw on HD 189733b, there are other possible explanations.

For example, it may be that the baseline level of X-ray emission from the star increased between 2010 and 2011, in a seasonal process similar to the Sun's 11-year sunspot cycle.

Regardless of the details of exactly what happened to HD 189733b's atmosphere, which the team hope to clarify using future observations with Hubble and ESA's XMM-Newton X-ray space telescope, there is no question that the planet was hit by a stellar flare, and no question that the rate of evaporation of the planet's atmosphere shot up.

This research has relevance not only for the study of Jupiter-like planets. Several recent discoveries of rocky 'super Earths' near their parent stars are thought to be the remnants of planets like HD 189733b, after the complete evaporation of their atmospheres.

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Thank God this wasn't our sun! Hubble spots planet shedding atmosphere following solar flare outburst

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Just after a violent flare on its parent star bathed it in intense X-ray radiation, the planet's atmosphere gave off a powerful burst of evaporation. The observations give a tantalising glimpse of the changing climates and weather on planets outside our Solar System.

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Seasons on other planets

Martian poles and cloud cover

The seasons are caused by the regular, repeating way a planet changes its orientation with respect to the Sun, as it follows its orbital path. Both the amount of solar radiation and its point of entry into the planet's atmosphere change at any given spot.

By having an inclined rotation axis or following an elliptical orbit, or both, a planet itself can cause such changes. On Earth we experience the four, familiar seasons of spring, summer, autumn and winter. These seasons can be broadly applied to the other planets as well.

On Earth, the seasons are largely driven by the rotation axis of the planet being inclined at about 23 degrees to the upright. This means that different hemispheres of Earth are presented to the Sun at different times of the year.

Seasons on Mars are influenced by the tilt of its axis and by its varying distance from the Sun. Earth is always about the same distance from the Sun, but the orbit of Mars is more elliptical so, at certain times of its year, it receives more energy than at others, because it is closer to the Sun.

Mars is closest to the Sun (and moving fastest in its orbit) during southern summer. This makes summer in the south of Mars shorter and hotter than summer in the north. Mars has 687 days in a year compared to our 365 days, so martian seasons are longer.

An intriguing discovery from the long-term study of Saturn with the NASA/ESA Hubble Space Telescope (HST) was that, by 2002, the wind speed had dropped from 1700 to 1000 kilometres per second since the Voyager 2 fly-by measured it during the early 1980s. This change could be indicative of the onset of winter on Saturn, which began in March 2002. It was still summer for the planet when Voyager 2 cruised past.

There is also some speculation among scientists that shadowing from the ring system of the planet may superimpose a more complex seasonal pattern on the longer, more-familiar spring-summer-autumn-winter cycle.

Neptune too has been shown to exhibit seasonal changes. Like Saturn, it is sliding into a winter season, from which Neptune will not emerge for another twenty Earth years. Again, Hubble data have shown that the planet is steadily reflecting more sunlight back into space due to the build up of clouds in its atmosphere.

Beyond the straightforward seasons, long-term orbital changes also affect the amount of sunlight received and its entry-point into a planet's atmosphere. These effects are called 'Milankovitch cycles' on the Earth and take place over tens or hundreds of thousands of years. They have been shown to be a driving factor in Earth's ice ages. Martian 'Milankovitch cycles' are thought to be more extreme than Earth's.

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The seasons are caused by the regular, repeating way a planet changes its orientation with respect to the Sun, as it follows its orbital path. Both the amount of solar radiation and its point of entry into the planet's atmosphere change at any given spot.

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Every planet in the solar system has seasons. Most have four like the Earth -- called Winter, Spring, Summer and Fall -- but that's where the similarities end. Extraterrestrial seasons are hardly noticeable on some planets (Venus), mindbogglingly extreme on others (Uranus) and in some cases simply impossible to define (Mercury).

The table below gives the dates of the seasons for 8 of the 9 planets in the solar system. Only Pluto is missing. It's so far away that we don't know much about seasons on that distant world.

In the table the equinoxes and solstices are named after the corresponding season in the northern hemisphere. This is the convention that astronomers often use to discuss planetary seasons. When the north pole of a planet is tilted toward the sun, astronomers call it the Summer Solstice; when the south pole is tilted toward the sun it is called the Winter Solstice. Nevertheless, the seasons are always opposite in the two hemispheres. On Earth, for example, when it is summer in New York, it is winter in Sydney. On a spring day in Paris, autumn leaves are falling in Argentina, and so on...

When the Vernal Equinox takes place on March 20, Earth will join Venus and Jupiter as the only planets in the solar system where it is now northern Spring.

Planetary seasons are caused by two factors: axial tilt and variable distance from the sun (orbital eccentricity). Earth's orbit is nearly circular and so has little effect on climate. It's our planet's axial tilt that causes almost all seasonal changes. When the north pole is tilted toward the Sun, it's northern summer. Six months later the north pole tilts away from the Sun and we experience northern winter.

The other two planets where it is northern spring, Jupiter and Venus, have very small axial tilts -- just 3 degrees compared to Earth's 23.5 degree tilt. Seasonal changes on those planets are correspondingly small. Spring on Venus isn't much different from autumn. The planet's dense, acidic atmosphere produces a runaway greenhouse effect that keeps the surface at 750 K year-round -- that's hot enough to melt lead. Spring fever on Venus is really hot!

Our second-nearest planetary neighbor Mars has the highest orbital eccentricity of any world except Pluto. Its distance from the Sun varies between 1.64 and 1.36 AU over the Martian year. This large variation, combined with an axial tilt greater than Earth's gives rise to seasonal changes far greater than we experience even in Antarctica.

Right: Over the past six months, the southern hemisphere of Mars has passed through spring and into summer. Spring started in early August 1999 and summer arrived toward the end of December 1999. Mars Global Surveyor is in a polar orbit, thus the spacecraft's camera has had an excellent view of seasonal changes. Shown here are three views of the same portion the layered terrain near the Martian south pole. They show how the landscape thaws and defrosts as summer approaches. [more information]

From the point of view of an Earth-dweller, one of the strangest effects of seasons on Mars is the change in atmospheric pressure. During winter the global atmospheric pressure on Mars is 25% lower than during summer. This happens because of the eccentricity of Mars's orbit and a complex exchange of carbon dioxide between Mars's dry-ice polar caps and its CO2 atmosphere. Around the summer solstice when the Martian north pole is tilted away from the sun, the northern polar cap expands as carbon dioxide in the polar atmosphere freezes. At the other end of the planet the southern polar cap melts, giving CO ₂ back to the atmosphere. This process reverses half a year later at the winter solstice.

At first it might seem that these events occurring at opposite ends of Mars would simply balance out over the course of the Martian year, having no net effect on climate. But they don't. That's because Mars is 10% closer to the Sun in winter than it is in summer. At the time of the winter solstice the northern polar cap absorbs more CO ₂ than the southern polar cap absorbs half a year later. The difference is so great that Mars's atmosphere is noticeably thinner during winter.

Seasons on Mars vs. Seasons on Earth

Above: The orbit of Mars is very eccentric, unlike Earth's which is more nearly circular. Its orbital motion is slowest when it is at aphelion (the farthest point from the Sun) and fastest at perihelion (the closest point to the Sun). This effect, combined with the planet's axial tilt, makes Martian seasons vary in duration more than those on Earth. The length of the seasons in this table are give in Earth days and Martian days. The two are almost exactly the same duration. An Earth day is 24 hours long, a Martian day is 24.6 hours long. [more information]

Martian seasons are peculiar by Earth standards, but they probably pale in comparison to seasons on Uranus. Like Earth, the orbit of Uranus is nearly circular so it keeps the same distance from the Sun throughout its long year. But, Uranus's spin axis is tilted by a whopping 82 degrees! This gives rise to extreme 20-year-long seasons and unusual weather. For nearly a quarter of the Uranian year (equal to 84 Earth years), the sun shines directly over each pole, leaving the other half of the planet plunged into a long, dark, frigid winter.

Left: A dramatic time-lapse movie by NASA's Hubble Space Telescope shows seasonal changes on Uranus. Once considered one of the blander-looking planets, Uranus is now revealed as a dynamic world with the brightest clouds in the outer Solar System. more info.

The Northern Hemisphere of Uranus is just now coming out of the grip of its decades-long winter. As the sunlight reaches some latitudes for the first time in years, it warms the atmosphere and triggers gigantic springtime storms comparable in size to North America with temperatures of 300 degrees below zero. In the animation pictured left the bright clouds are probably made of crystals of methane, which condense as warm bubbles of gas well up from deep in the atmosphere of Uranus.

Uranus does not have a solid surface, but is instead a ball of mostly hydrogen and helium. Absorption of red light by methane in the atmosphere gives the planet its cyan color. Uranus was discovered March 13, 1781, by William Herschel. Early visual observers reported Jupiter-like cloud belts on the planet, but when NASA's Voyager 2 flew by in 1986, Uranus appeared as featureless as a cue ball. In the past 13 years, the planet has moved far enough along its orbit for the sun to shine at mid-latitudes in the Northern Hemisphere. By the year 2007, the sun will be shining directly over Uranus' equator.

Parents and Educators: Please visit Thursday's Classroom for lesson plans and activities related to this story.

Mercury's seasons -- if they can be called that -- are also remarkable. Until the 1960's it was thought that Mercury's "day" was the same length as its "year" keeping the same face to the Sun much as the Moon does to the Earth. This was shown to be incorrect by Doppler radar observations. We now known that Mercury rotates three times during two of its years. Mercury is the only body in the solar system tidally locked into an orbital-to-rotational resonance with a ratio other than 1:1.

This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an observer on Mercury's surface. [ref] At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving three times faster across the sky. Observers at other points on Mercury's surface would see different but equally bizarre motions.

Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K at night to 700 K during the day.

Web Links
Earth's Seasons -- A table of solstices and equinoxes from the US Naval Observatory

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Every planet in the solar system has seasons. Most have four like the Earth -- called Winter, Spring, Summer and Fall -- but that's where the similarities end. Extraterrestrial seasons are hardly noticeable on some planets (Venus), mindbogglingly extreme on others (Uranus) and in some cases simply impossible to define (Mercury).

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The Huygens lander made it all the way down to the surface of Titan on Jan. 14, 2005, which was exciting for scientists in a few ways. First, researchers knew almost nothing about Titan's surface at the time; pictures of Titan from both Voyager spacecraft showed a blurry orange haze shrouding whatever was below. Huygens got beneath that haze and found a moon with a solid surface dotted with lakes, oceans and rivers made of hydrocarbons (mostly methane and ethane). Huygens also gave scientists a lot of information about Titan's environment, from the winds in the upper atmosphere to the dunes on the surface that form from a strange material.

Huygens died within hours of landing (it was only designed to last a short while), but Cassini's observations of Titan continued with many close flybys. Perhaps its greatest discovery was finding evidence of prebiotic chemistry on Titan - meaning chemistry that could, in theory, give rise to life. Among many other discoveries, Cassini tracked the seasonal changes in Titan's hydrocarbon lakes and seas, and also watched the appearance and disappearance of mysterious "islands" in one of those lakes, which scientists now think could be streams of bubbles.

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Cassini's 13 Greatest Discoveries During Its 13 Years at Saturn

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The Huygens lander made it all the way down to the surface of Titan on Jan. 14, 2005, which was exciting for scientists in a few ways. First, researchers knew almost nothing about Titan's surface at the time; pictures of Titan from both Voyager spacecraft showed a blurry orange haze shrouding whatever was below.

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Source: http://en.wikipedia.org/wiki/Impact_event

A bullet shot out of a high-powered rifle can travel at speeds up to 2500 miles per hour. In one second it would travel one kilometer in distance. Whatever it strikes would be pulverized on impact.

Now imagine a boulder-sized asteroid arriving from outer space, and hitting the earth at twenty times this speed (roughly 20 kilometers per second). The impact energy released would be comparable to an atomic bomb exploding.

Now imagine an asteroid that is one kilometer in diameter, weighing roughly 1.4 billion tonnes, hitting the earth at this speed. The force of impact would be so great that it would mostly vaporize the asteroid. And the impact energy released would be so tremendously powerful that it would be much greater than all the energy released if all the atomic bombs in the world went off at once. It would truly be like Armageddon.

You can easily estimate the impact energy of an asteroid. You do this by calculating the kinetic energy of the asteroid just before it strikes the earth. This is equal to the impact energy.

Where:

M is the mass of the asteroid just before it strikes the earth

V is the velocity of the asteroid just before it strikes the earth

For example, consider an asteroid that is one kilometer in diameter and weighs 1.4 billion tonnes ( M = 1.4×10 ¹² kilograms), and is traveling at 20 kilometers per second ( V = 20,000 m/s). The kinetic energy would be equal to (1/2)×1.4×10 ¹²×(20,000) ² = 2.8×10 ²⁰ Joules.

It is hard to imagine such immense energy. But over the course of geologic time, the planet earth has been struck many times by numerous asteroids, some very large. In fact, it is believed that the extinction of the dinosaurs was likely caused by a massive asteroid (or comet) impact and its catastrophic effects on the global environment.

A popular tourist attraction is the Barringer Crater in the Arizona desert. It is about 1200 meters in diameter and 170 meters deep. The crater is shown below.

Source: http://en.wikipedia.org/wiki/Meteor_Crater

This crater was created by an asteroid impact about 50,000 years ago. It is estimated that the asteroid was about 50 meters across.

Many asteroids explode in the air before they ever reach the ground. As they travel through the air at a velocity of 15-20 kilometers per second, they experience huge pressure forces due to the enormous air resistance felt at this high speed. These pressure forces cause the asteroid to explode violently. When asteroids explode in the air in this manner, they are called airbursts. The explosion energy of an airburst is comparable to the energy released by a ground strike.

If a large asteroid, one kilometer in diameter, impacts a large body of water, such as an ocean, the consequences can be just as disastrous as a ground strike. The impact would create waves hundreds of feet high which can flood and devastate coastal communities all around the world.

There are scientists currently monitoring outer space for asteroids that threaten to impact the earth. Fortunately, the vast majority do not. But occasionally, some do. On February 15, 2013, an asteroid 20 meters in size entered the earth's atmosphere over Russia, at a speed of about 20 kilometers per second. It exploded about 30 kilometers above the earth in a massive airburst explosion estimated to be about 20-30 times more powerful than that of the atomic bomb dropped on Hiroshima. It created a very bright flash, brighter than the sun, and generated a powerful shock wave that blew out many windows. 1500 people were injured. The asteroid was estimated to weigh over 10,000 tonnes.

Here is a video of the asteroid as it explodes.

This is the largest known natural object to have entered earth's atmosphere since the 1908 Tunguska event which flattened 2000 square kilometers of remote forest in Siberia. Its blast energy was about 1000 times greater than that of the atomic bomb dropped on Hiroshima.

Early warning of an asteroid impact is crucial to saving as many lives as possible. It is currently far too easy for a planet-killing asteroid to be undetected until it's too late. Attempts are currently being made to ramp up the monitoring efforts for improved early warning, since this will save as many lives as possible.

In 2008, a successful early detection was made. The asteroid, called 2008 TC3, was 4.1 meters in diameter and weighed 80 tonnes. It entered earth's atmosphere on October 7, 2008 and exploded as an airburst at an altitude of about 37 kilometers above the Nubian Desert in Sudan.

More ambitious plans involve preventing deadly asteroids, which are on a collision course with earth, from hitting the earth. This would involve deflecting the path of the asteroid somehow, perhaps by sending out a rocket to collide with it, and knock it off course. Other more elaborate strategies are possible, such as using focused solar energy to vaporize material from the asteroid surface, creating a small thrust, which, over the course of months and years, can deflect the asteroid trajectory sufficiently to prevent it from colliding with earth. This however, requires that knowledge of its impending collision be known well ahead of time.

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Source: http://en.wikipedia.org/wiki/Impact_event A bullet shot out of a high-powered rifle can travel at speeds up to 2500 miles per hour. In one second it would travel one kilometer in distance. Whatever it strikes would be pulverized on impact. Now imagine a boulder-sized asteroid arriving from outer space, and hitting the earth at twenty times this speed (roughly 20 kilometers per second).

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NASA maintains a constant monitoring programme on asteroids, gigantic space rocks that can come close to Earth and even strike the planet.

So should we be worried about them and the approach of the latest three-mile wide asteroid, Florence - and what would happen if one struck our planet?

What is an asteroid?

An asteroid is a small, airless rocky mass which orbits the sun, but is too small to be classed as a planet.

Most of them are found in the Asteroid Belt - a ring which lies between the orbits of Mars and Jupiter.

It's thought that asteroids are debris left over at the formation of the universe about 4.6 billion years ago.

Some can be very large, but others can be tiny - even as small as a grain of sand - because of their smaller size, asteroids lack the gravitational pull to shape themselves into a sphere - meaning they usually have a rocky, irregular appearance.

What would happen if one struck the Earth?

When an asteroid crashes into the Earth, it's known as a meteorite - and it can cause potentially huge damage, depending on the size.

For example, it was an asteroid that wiped out the dinosaurs, as the impact threw up a dust cloud blocking out sunlight and destroying life.

Meanwhile, in 2013, a meteorite exploded over Russia - with more than 1,000 people injured by the explosion and following shockwave.

At the time, experts said it released the same amount of energy as a small atomic weapon.

So, an asteroid's effects on Earth completely depend on the individual asteroid - but it is clear that if a large one did strike the Earth, the consequences could be catastrophic.

Which is why the space agency has unveiled its historic plan to redirect any space rocks heading for our planet.

What are experts doing to prevent asteroids from causing destruction?

Nasa have said they are planning to smash a spaceship into an asteroid as part of an emergency rehearsal mission to save mankind.

The organisation has also warned it's "not a matter of if - but when" we will be forced to deal with an asteroid slamming into the Earth's surface.

As many worry what the world may look like if a celestial body hit our planet, astrophysicist Lord Martin Rees has warned that the government should spend millions on asteroid defence.

What other asteroids have been spotted recently?

When will the next asteroid hit?

An asteroid called Florence is scheduled to fly by Earth on September 1, 2017, but at a safe distance quite far away.

It will bring a "close encounter" as it zooms by about 4.4 million miles from Earth.

According to Nasa it is the closes an asteroid of this size has come near Earth since it first began tracking near-Earth objects.

A closer asteroid which is expected to come much closer to the planet is Apophis.

It is predicted to speed past Earth on April 13, 2029, at just 18,600 miles away - a hair's width in astronomical terms.

The gap is just one tenth of the distance between us and the moon, but scientists have ruled out the possibility of impact.

However the rock, which is 370-metres in diameter, will fly past again on April 13 2036.

If it hit, it would create an explosion 65,000 times as powerful as the nuclear bomb dropped on Hiroshima.

So, should we be worried about asteroids?

Nasa reassures us that the most dangerous asteroids are very rare. According to the organisation, about once a year an asteroid the size of a car hits the atmosphere, burning up before it reaches the surface.

However, asteroids large enough to wipe out life - which would need to be over a quarter of a mile wide, according to Space.com - only strike once every 1,000 centuries.

The one to watch out for is called Gliese 710. When it arrives at Earth (in 1.35million years), it is expected to pull asteroids from the outer reaches of the solar system towards Earth, having a potentially catastrophic impact.

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The lowdown on asteroids - including the house-sized rock that will whizz close to Planet Earth in October

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NASA maintains a constant monitoring programme on asteroids, gigantic space rocks that can come close to Earth and even strike the planet. So should we be worried about them and the approach of the latest three-mile wide asteroid, Florence - and what would happen if one struck our planet?

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A black hole is a massive object whose gravitational field is so intense that no light (electromagnetic radiation) can escape it. When two orbiting black holes merge, a massive amount of energy is released in the form of jets. Meanwhile, the movement of these massive bodies disturbs the fabric of space-time around them, sending ripples of gravitational waves radiating outward.

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With Cassini nearing its demise, it will be some time before we can again so closely observe Saturn's largest moon, Titan. Cassini was built in large part to answer questions about this enigmatic moon, and today Titan will seal its fate.

The probe's last Titan flyby on 21 April bent Cassini's orbit to enable it to fly between Saturn and the planet's rings. Today's final slingshot past that moon will speed up the spacecraft and set it on an irreversible collision course with Saturn in five days' time.

"With that final kiss goodbye from Titan, we are going so deep into Saturn's atmosphere that the spacecraft is not going to have any chance to come back out," says Earl Maize, Cassini project manager at NASA.

Cassini is taking a last look at the moon and its weather before speeding towards Saturn. Through Cassini's eyes, scientists have learned that Mercury-sized Titan is a world bathed in liquids, with a hydrological cycle very similar to that of Earth. Hydrocarbons rain from the sky, etching Titan's surface with rivers, lakes and seas. Methane waves lap on its shores.

"Right now, somewhere on Titan, it is raining. That's the only place in the solar system where I can say that," says Alex Hayes at Cornell University in Ithaca, New York.

Titanic landscape

While Cassini carries instruments designed to study the entire Saturn system, it brought along an entire lander just for examining Titan.

The Huygens probe, which landed on Titan in 2005, is the furthest object ever delivered to the surface of another world. It sent back images of a ruddy, mountainous surface, and dark pebbles in what appeared to be a dry lakebed. The probe provided the first direct evidence that Titan experiences rainfall.

Over the years, Cassini's radar instruments allowed scientists to peer beneath Titan's hazy atmosphere and study its surface and lakes, which they found are made of liquid methane and ethane. Methane is common on Earth but breaks down quickly, so its abundance on Titan was surprising and remains hard for scientists to explain. It is often a byproduct of life, but could also provide energy to sustain strange forms of life unimagined on Earth.

NASA/JPL-Caltech/Space Science Institute

In addition, Cassini unveiled mountains and dunes that are much like sand dunes on Earth, but that scientists think are made of hydrocarbon-coated ice pebbles. Evidence even emerged that Titan may harbour an internal ocean of liquid water and ammonia.

"We now know that there is another world in the solar system that has abundant liquids and organics, and therefore harbours the potential for life," said Sarah Hörst at Johns Hopkins University in Baltimore, Maryland, in a recent editorial published in Nature Astronomy.

Placing bets

Hayes says he often tells a story of a friendly betting pool that scientists shared before Huygens landed, when researchers scrawled their choices of what the probe would encounter on a bulletin board at Caltech in Pasadena, California.

"People signed up for different categories that included things like ice, tar, liquid, or dead on arrival; one scientist made his own category and called it 'eaten by a sea monster'," he says.

"'What is Huygens going to land on?' is the story of Titan. Every time we learn something new, another series of just-as-interesting questions crops up as a result of that new knowledge. That's been the paradigm of Titan exploration for the past 13 years with Cassini."

The next decade is much less certain. Although a couple of Titan missions are in the early stages of planning, none are imminent. So once Cassini sails to its certain doom, we may only have its data to pore over for some time.

Read more: Cassini: Fiery grand finale of the mission to Saturn; After Cassini - Where next in the search for alien life

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Slingshot around Titan is the beginning of the end for Cassini

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With Cassini nearing its demise, it will be some time before we can again so closely observe Saturn's largest moon, Titan. Cassini was built in large part to answer questions about this enigmatic moon, and today Titan will seal its fate.

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LIGO detects gravitational waves for third time

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The Laser Interferometer Gravitational-wave Observatory (LIGO) has made a third detection of gravitational waves, ripples in space and time, demonstrating that a new window in astronomy has been firmly opened. As was the ...

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Scientists say they have proven the existence of gravitational waves - the ripples in space-time that stem from objects moving throughout the Universe. If true, it's the first time these waves have been measured directly and marks one of the biggest scientific discoveries in decades. But there have been a few false alarms before.
If true, it marks one of the biggest scientific discoveries in decades
The discovery was made by the large group of researchers who make up the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration. The LIGO observatories picked up the wave signals on September 14th, 2015, coming from two distant black holes merging 1.3 billion years ago. The black holes spun around each other several times per second before merging in a massive explosion - a process that generates huge gravitational waves. These waves peter out on the way to Earth, requiring delicate instrumentation that must adjust for any interference on our planet. Though LIGO's observations are ground-breaking, other scientists will need to pore over the data to make sure it's correct. The study was published today in Physical Review Letters. There will also be more studies in the coming weeks and months.

Gravitational waves were predicted by Albert Einstein in 1916, as part of his theory of general relativity. Finding them would confirm a big part of that theory - and would also be the first step toward a new way of observing the cosmos. Black holes and neutron stars are too dim to be seen by modern telescopes, but researchers might be able to measure the gravitational waves they produce. That would tell scientists more information about how these objects manipulate gravity, and may even lead to the discovery of new objects and concepts we've never even thought about. "It's not just that we're checking on a theory," says Ira Thorpe, an astrophysicist at NASA, who was not involved with LIGO. "The real important part of this is it gives us a completely new tool for understanding our Universe."

That is, assuming today's finding is right. Scientists have been wrong about gravitational waves a few times before. In the 1960s, American physicist Joseph Weber claimed he had detected gravitational waves, but his findings were discredited when no one could reproduce them. And in 2014, a team using the BICEP2 telescope near the South Pole said they had found evidence of the waves by studying the early Universe; their results, however, turned out to be from cosmic dust. The tools that LIGO uses are very susceptible to outside interference, which means that today's results could be influenced by interference generated here on Earth.
Scientists have been wrong about gravitational waves before
But for now, today is a day of celebration for scientists. "Ladies and gentlemen, we have detected gravitational waves. We did it," said David Reitze, the LIGO executive director, at the press conference announcing the discovery. His declaration was met by cheers and applause by the scientists and press in attendance. "The things we've surmised and speculated about will become the subjects of detailed study," Marc Kamionkowski, a theoretical physicist at Johns Hopkins University, told The Verge before the announcement. "It's just amazing that they can detect this."

What are gravitational waves?

An artist rendering of gravitational waves from two black holes. (NASA)
Einstein's theory of general relativity completely altered how we perceive the Universe. Originally, scientists thought that space and time were two fixed and independent concepts. But Einstein's theory combined space and time together into one four-dimensional model called space-time. And space-time isn't fixed at all.
Einstein's theory completely altered how we perceive the Universe
The theory also altered our understanding of gravity. According to Newtonian physics, all objects in the Universe are innately attracted to one another. Einstein instead proposed that objects actually warp the space-time around them, creating gravitational "pull." Imagine a bowling ball on top of a stretched blanket. The ball bends the blanket, creating an impression in the fabric. Planets and stars are the bowling ball; space-time is the blanket. Now, imagine rolling a marble across that blanket with the bowling ball on it. The marble is going to follow the curve of the blanket down toward the ball. Smaller objects like asteroids act like the marble when near a bigger mass: they following the curvature of space-time the larger object creates.

When objects move through the Universe, they also warp space-time as they go. This causes ripples: gravitational waves. The movement of every object - from a person to a supermassive black hole - produces gravitational waves.
Gravitational waves are usually incredibly weak
Most everyone in the scientific community believe gravitational waves exist, but no one has ever proved it. That's because the signals from gravitational waves are usually incredibly weak. "When we move, the gravitational wave is so weak it is effectively zero," says Kamionkowski. "The Earth going around the Sun produces a gravitational wave signal, but it is still very, very weak."

But the more mass an object has, the bigger the wave it produces. Super-dense objects like black holes or neutron stars moving at rapid speeds can produce big enough gravitational waves that can be measured here on Earth. So that's what LIGO's scientists targeted.

LIGO

The LIGO collaboration has two observatories in Louisiana and Washington State, both funded by the National Science Foundation. Each facility is shaped like a giant "L;" the "arms" of the L are two vacuum-sealed tubes stretching 2.5 miles long, with mirrors at each end. The mirrors are used to measure how gravitational waves warp space-time. When a gravitational wave passes, one mirror gets closer while the other retreats; scientists measure this phenomenon by bouncing lasers off the mirrors. Changes in the amount of time it takes a laser to bounce off a mirror indicate a gravitational wave.

The gravitational wave measurements from the black holes were also converted into audible form, what LIGO calls a "chirp." Right as the black holes merge, the frequency of the resulting gravitational waves increases up until the moment of collision. As a sound, that movement becomes a high-pitched note that raises octaves really quickly. The chirp is perceptible in the recording below. Scientists shifted its frequency so it's easier to hear.

The gravitational waves only change LIGO's instruments by about one ten-thousandth the size of a proton. This means Earth isn't the ideal place to look for waves, since movements from people or traffic can potentially cause interference. For instance, LIGO kept getting "readings" that were actually the result of cars rolling over a nearby bump in the road.

One of LIGO's test mass mirrors. (LIGO)
But LIGO has tried to adjust for possible interferences. The collaboration even has three members dedicated to messing with the data. These scientists purposefully create fake signals to make sure the collaboration can discern what real measurements are supposed to look like. At one point, LIGO was ready to announce they had detected gravitational waves based on one of these fake readings. The team had to start all over again when they found out the data had been manipulated.

Moving forward

An artist rendering of what the LISA gravitational wave detector in space could look like. (NASA)
If LIGO's measurements hold up, the collaboration could start its own ripple effect - one within the scientific community. Gravitational waves offer ways to study objects that don't give off light - which could mean more research institutions might invest in ways to detect them. "When radio telescopes were first discovered, we found all these powerful radio sources we'd never known before," says Kamionkowski. "We're getting new eyes to look at the sky."
Gravitational wave detection could move beyond Earth into space
Likely, gravitational wave detection will move beyond Earth into space. Thorpe is currently working on the LISA Pathfinder mission, a partnership between NASA and the European Space Agency. The project launched a spacecraft in December to test the technologies needed for future space-based gravitational wave detectors. Thorpe says that by working in space, the disturbances associated with Earth aren't a problem. Also, there's a lot more room to work with: a space detector could use laser beams millions of miles long to pick up gravitational wave movements. "By going to space, we can really take advantage of the fact that we can make our detector bigger," he says.

Realistically, though, such technologies are decades away. But Thorpe says today's discovery will help spur efforts like the LISA Pathfinder mission. "The success of a project like LIGO will inspire us to keep going and trying."

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Scientists have finally proven Einstein's century-old theory about gravitational waves

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Scientists say they have proven the existence of gravitational waves - the ripples in space-time that stem from objects moving throughout the Universe. If true, it's the first time these waves have been measured directly and marks one of the biggest scientific discoveries in decades. But there have been a few false alarms before.

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Physicists have announced the discovery of gravitational waves, ripples in the fabric of spacetime that were first anticipated by Albert Einstein a century ago.

"We have detected gravitational waves. We did it," said David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory (Ligo), at a press conference in Washington.

The announcement is the climax of a century of speculation, 50 years of trial and error, and 25 years perfecting a set of instruments so sensitive they could identify a distortion in spacetime a thousandth the diameter of one atomic nucleus across a 4km strip of laserbeam and mirror.

The phenomenon detected was the collision of two black holes. Using the world's most sophisticated detector, the scientists listened for 20 thousandths of a second as the two giant black holes, one 35 times the mass of the sun, the other slightly smaller, circled around each other.

At the beginning of the signal, their calculations told them how stars perish: the two objects had begun by circling each other 30 times a second. By the end of the 20 millisecond snatch of data, the two had accelerated to 250 times a second before the final collision and a dark, violent merger.

The observation signals the opening of a new window on to the universe.

"This is transformational," said Prof Alberto Vecchio, of the University of Birmingham, and one of the researchers at Ligo. "We have observed the universe through light so far. But we can only see part of what happens in the universe. Gravitational waves carry completely different information about phenomena in the universe. So we have opened a new way of listening to a broadcasting channel which will allow us to discover phenomena we have never seen before," he said.

"This observation is truly incredible science and marks three milestones for physics: the direct detection of gravitational waves, the first detection of a binary black hole, and the most convincing evidence to date that nature's black holes are the objects predicted by Einstein's theory."

The scientists detected their cataclysmic event using an instrument so sensitive it could detect a change in the distance between the solar system and the nearest star four light years away to the thickness of a human hair.

And they did so within weeks of turning on their new, upgraded instrument: it took just 20 milliseconds to catch the merger of two black holes, at a distance of 1.3 billion light years, somewhere beyond the Large Magellanic Cloud in the southern hemisphere sky, but it then took months of meticulous checking of the signal against all the complex computer simulations of black hole collision to make sure the evidence matched the theoretical template.

The detector was switched off in January for a further upgrade: astronomers still have to decipher months of material collected in the interval. But - given half a century of frustration in the search for gravitational waves - what they found exceeded expectation: suddenly, in the mutual collapse of two black holes, they could eavesdrop on the violence of the universe.

Prof B S Sathyaprakash, from Cardiff University's school of physics and astronomy, said: "The shock would have released more energy than the light from all the stars in the universe for that brief instant. The fusion of two black holes which created this event had been predicted but never observed."

Gravitational waves

The finding completed the scientific arc of prediction, discovery and confirmation: first they calculated what they should be able to detect, then decided what the evidence should look like, and then devised the experiment that clinched the matter. Which is why on Thursday scientists around the world were able to hail the announcement as yet another confirmation of their "standard model" of the cosmos, and the beginning of a new era of discovery.

Astronomers have already exploited visible light, the infrared and ultraviolet, radio waves, x-rays and even gamma-rays in their attempt to understand the mechanics of stars, the evolution of the galaxies and the expansion of the universe from an initial big bang 13.8bn years ago.

Unequivocal

Thursday's announcement was the unequivocal first detection ever of gravity waves. The hope is that gravity wave astronomy could start to answer questions not just about the life of stars but their deaths as well: death by collision, death in a black hole, death in some rare stellar catastrophe so fierce that, for a few thousandths of a second, the blast is the brightest thing in the universe.

Even before the Ligo detectors in two US states reopened for business late last year, researchers were confident that a detection would follow swiftly. The announcement came after months of speculation, and decades of theoretical and practical work by an international network of more than a thousand scientists and engineers in Britain, Europe, the US and around the world.

Professor Kip Thorne, of the California Institute of Technology, and one of the founding fathers of Ligo, said that until now, astronomers had looked at the universe as if on a calm sea. All of that had changed.

"The colliding black holes that produced these gravitational waves created a violent storm in the fabric of space and time, a storm in which time speeded up and slowed down, and speeded up again, a storm in which the shape of space was bent in this way and that way," he said.

Prof Neil Turok, director the Perimeter Institute for Theoretical Physics at Waterloo in Canada, and a former research colleague of Prof Stephen Hawking, called the discovery "the real deal, one of those breakthrough moments in science".

Not only had the detector picked up the collision of two enormous black holes across a distance of almost a billion light years of space, it recorded the distinctive "chirp" as the two spiralled towards each other.

The discovery, he said, completes a scientific arc of wonder that began 200 years ago, when the great British scientist Michael Faraday began to puzzle about how action was transmitted across the distance of space; how the sun pulled the Earth around. If the sun moved 10 yards, very suddenly, would the Earth feel the difference?

He reasoned that something must cross space to transmit the force of gravity. Faraday's reasoning inspired the great British mathematician James Clerk Maxwell to think about how an electric force travelled, and arrive at an understanding of light and a prediction of radio waves.

"Einstein, when he came to write down his theory of gravity, his two heroes were Faraday and Maxwell," said Turok. "He tried to write down laws of the gravitational field and he wasn't in the least surprised to discover that his predictions had waves, gravitational waves."

The Ligo discovery signals a new era in astronomy, he said.

"Just think of radio waves, when radio waves were discovered we learned to communicate with them. Mobile communication is entirely reliant on radio waves. For astronomy, radio observations have probably told us more than anything else about the structure of the universe. Now we have gravitational waves we are going to have a whole new picture of the universe, of the stuff that doesn't emit light - dark matter, black holes," he said.

"For me the most exciting thing is we will literally be able to see the big bang. Using electromagnetic waves we cannot see further back than 400,000 years after the big bang. The early universe was opaque to light. It is not opaque to gravitational waves. It is completely transparent.

"So literally, by gathering gravitational waves we will be able to see exactly what happened at the initial singularity. The most weird and wonderful prediction of Einstein's theory was that everything came out of a single event: the big bang singularity. And we will be able to see what happened."

* The headline to this article was amended on 12 February 2016. An earlier version said the discovery was a breakthrough after two centuries of expectation. This has been corrected.

Topics

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Gravitational waves: breakthrough discovery after a century of expectation

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Physicists have announced the discovery of gravitational waves, ripples in the fabric of spacetime that were first anticipated by Albert Einstein a century ago. "We have detected gravitational waves. We did it," said David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory (Ligo), at a press conference in Washington.

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3.2 - Newton's Law of Universal Gravitation

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Check out http://www.mrayton.com/access.html for hundreds of AP Chemistry multiple choice and free response practice questions! Check out Learnerator for thousands of AP Chemistry and AP Physics practice questions! Enter the code AYTON10 for 10% off your order! Just click http://learnerator.com?utm_source=youtube&utm_medium=link&utm_campaign=bobayton

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This video shows how polar orbiting satellites build up a picture of the earth from consecutive passes, known as 'swaths', as the Earth rotates. EUMETSAT's Metop satellites fly in polar orbit around the Earth, with an inclination of 98.7° to the Equator.

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Kepler's Law and Newton's Law of Gravitation

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Kepler's Law and Newton's Law of Gravitation

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Kepler's Laws and How Newton Got Cool | Doc Physics

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Newton derived the proportionality constant from Kepler's Third Law using his newly formed Law of Universal Gravitation. That's how I know you're just as attracted to me as I am to you...which is not very much.

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Kepler's Third Law of Motion - Law of Periods  (Astronomy)

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Understanding Kepler's 3 Laws and Orbits

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In this video you will be introduced to Kepler's 3 laws and see how they are relevant to orbiting objects. Visit https://sites.google.com/site/dcaulfssciencelessons/ for more! The video also makes use of computer simulations created by PhET Interactive Simulations University of Colorado http://phet.colorado.edu.