The impact of comet Shoemaker-Levy 9 onto Jupiter represents the first time in human history that people have discovered a body in the sky and been able to predict its impact on a planet more than seconds in advance. The impact will deliver more energy to Jupiter than the largest nuclear warheads ever built, and up to a significant fraction of the energy delivered by the impact which is generally thought to have caused the extinction of the dinosaurs on Earth, roughly 65 million years ago. Earth-bound observers are taking this opportunity to observe and study the comet's collision with a planet to gain more understanding of one of the fundamental physical processes within the solar system, impacts. The discovery has spawned scientific thinking about the frequency with which comets fragment and implications related to the inventory of small bodies in the Solar System and how they modify the surface and atmospheres of the planets.
The fast approaching collision of segmented Periodic Comet Shoemaker-Levy 9 with the planet Jupiter has peaked the interest of professional and amateur astronomers worldwide. Scientists expect a spectacular 51/2-day event from July 16-22 and anticipate some observations. For the first time in history, scientists have advance notice of such a collision and the technological capabilities to observe it.
Astronomers predict the comet's 20+ segments will hit Jupiter's dark night side, where they will be hidden from telescopes on Earth. Some observers may be able to view the phenomenon indirectly in light reflected from Jupiter's inner moon or off ring particles. Other observers anticipate viewing the impacts and expected explosions through observations from NASA's Galileo and other spacecraft or by studying the aftereffects on Jupiter's atmosphere.
Thousands of planet-watchers are readying observatories on the ground and in space for what they hope will be a remarkable encounter. For comet experts and planetary specialists around the world, this may be the most important event of their careers because of the discoveries they may make about the nature of comets and the makeup of Jupiter's atmosphere and magnetosphere. This knowledge may help them explain similar high-energy events on Earth.
The fragmented comet was discovered by Eugene and Carolyn Shoemaker and David Levy on March 24, 1993. It was the ninth periodic comet found by this team of professional and amateur astronomers. They identified the comet through a photograph taken with the 18-inch Schmidt telescope at Mt. Palomar Observatory near Los Angeles, California. Subsequent imaging conducted by James Scotti at the Spacewatch Telescope on Kitt Peak in Arizona, and by Jane Luu and David Jewitt at the Mauna Kea Observatory in Hawaii revealed the comet's peculiar form: it is actually a string of numerous fragments of comet, "a string of pearls."
P/Shoemaker-Levy 9 probably split apart during July 1992, when scientists think it traveled within 113,000 kilometers of Jupiter's center. During this pass in its orbit around Jupiter, the planet's tidal forces tore it apart. The comet is designated, "P," for "periodic," because even before its capture in a death grip by Jupiter, its original orbit around the Sun was closed and contained within our solar system.
Its fragments vary in size, with about six relatively large pieces, a dozen medium-sized ones, and assorted smaller debris. The average chunk is estimated to be two kilometers in diameter, although no one knows for certain. The size or mass of the fragments however, will determine the nature of their impact on Jupiter's atmosphere.
As the comet string nears Jupiter, its associated dust coma will be bombarded by charged particles trapped in Jupiter's magnetosphere. Gas and dust ejected from the comet may be swept up by Jupiter's magnetic field, possibly causing large changes in the density and composition of Jupiter's aurora. The comet fragments will plummet into the planet one by one, like a freight train falling off a bridge. The explosions within Jupiter's atmosphere may inject atmospheric ingredients into the magnetosphere, altering Jupiter's radio emissions. As the comet fragments enter Jupiter's stratosphere, they will be heated and lose some mass and energy by aerodynamic forces. Because they are fragile, they may break up after penetrating about 300-400 kilometers into the atmosphere. The largest and strongest fragments will descend another 50-200 kilometers. At this point, the fragments will release the majority of their kinetic energy in a spectacular explosion in a little over a second. This explosion may create a fireball, like a nuclear burst, that could rise above Jupiter's cloud tops in a matter of minutes.
Ordinarily, events occurring 150-200 kilometers below visible cloud tops would be invisible beyond the planet. In this case, however, the shock wave from the airburst may blow through the planet's atmosphere carrying the gases far above Jupiter's clouds. This deeper gas contains volatile materials that ultimately will condense high in the atmosphere. The gas may form unusual clouds, which could last a long time if the comet's ice particles are small. The impacts could create a thermal anomaly or a tremendous storm, similar to Jupiter's Red Spot but not as big. This new turbulence or spot might be visible through the most powerful Earth-based telescopes.
When these comet fragments dissipate in the atmosphere, they could be very bright - possibly as bright or brighter than Jupiter itself. The light from the impact of the largest fragments could brighten a well-placed inner moon of Jupiter enough to be detected by powerful Earth telescopes. The Hubble Space Telescope is scheduled to make more pictures of Jupiter during the days of impact. Thousands of Earth-bound astronomers throughout the world will point their telescopes towards Jupiter to look for some evidence of the crash.
In the United States, NASA and The National Science Foundation have jointly funded a coordinated program to support research efforts for this event, using many ground-based observatories and several spacecraft: Galileo, Hubble Space Telescope, the International Ultraviolet Explorer, the Extreme Ultraviolet Explorer, Ulysses, and Voyager 2. The program includes listening for radio signals, visible and thermal imaging, modeling, theory, and data analysis. Since 1993, an electronic bulletin board on the Internet, organized by astronomer Michael A' Hearn at the University of Maryland, has kept the world's planetary-scientific research community advised of the latest information on the pending collision. In January 1994, 175 astronomers from the United States and Europe met to identify the specific physical phenomena they would try to observe to gain the most knowledge from the event. They established a continuous worldwide series of Jupiter observations during the collision's six-day time frame. They also agreed to disseminate their scientific information within three to six months after obtaining it. Throughout the planning process, scientists in varied U.S. and international organizations have displayed a spirit of cooperation in their quest for discoveries. Astronomers hope to gain more knowledge about the composition of comets and the makeup of Jupiter's atmosphere. Analysis of the new data may teach us more about the role of comets, meteors, and other space objects in the disappearance of the dinosaurs more than 65 million years ago. Additional measurements and observations may test theories of other mass extinctions on Earth, the behavior of high-energy shock waves and cloud formation in planetary atmospheres, the makeup of comets, and even the origin of planets.
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