When more than twenty fragments from Comet Shoemaker-Levy 9 collided with the upper atmosphere of Jupiter, the event offered scientists the opportunity to witness the consequences of the collision of extraterrestrial objects and provided insights into the likely effects of asteroid or comet impacts on Earth.
From July 16 to July 22, 1994, more than twenty fragments of Comet Shoemaker-Levy 9, which had been ripped apart by Jupiter’s gravity during an earlier encounter with the planet, collided with the atmosphere of Jupiter. These collisions deposited more energy into the atmosphere of Jupiter than would be produced by all of the nuclear weapons in the military arsenals around the world. These impacts, which were observed from Earth and from spacecraft, provided astronomers with their first opportunity to witness a cosmic collision of a size capable of causing global consequences. The impacts produced bright fireballs in Jupiter’s atmosphere and new cloud features visible from Earth.
Comet Shoemaker-Levy 9 was discovered by Eugene Merle Shoemaker, Carolyn Shoemaker, and David H. Levy in photographs taken on March 24, 1993. These photographs were taken using a 0.46-meter diameter Schmidt camera, a low-power telescope with a high light-gathering capability, at the Mount Palomar Observatory in Southern California. A Schmidt camera is designed so that it can see dim objects, and it has a wide field of view, making it an ideal instrument to search a large area of the sky for faint objects such as comets and asteroids. Shoemaker-Levy 9 was the ninth comet discovered by this group of researchers.
The comet was very dim—having a brightness of 13.8, much fainter than the dimmest object that can be seen with the human eye or with binoculars—when it was discovered. Still, it appeared unusual in the photographs, being slightly elongated. Once the position of Shoemaker-Levy 9 was determined, other observers looked at it with telescopes having higher magnification. Photographs taken by John Scotti,
Within days of its discovery, the comet had been observed by astronomers at the University of Hawaii and the McDonald Observatory in Texas. By April, 1993, these and other observations allowed Brian G. Marsden to determine that Shoemaker-Levy 9, instead of orbiting the Sun as is typical for comets, was actually in orbit around Jupiter.
Other researchers were able to trace the history of the comet’s orbit. They determined that the comet passed only 15,500 miles above the clouds of Jupiter, within 1.4 Jupiter radii of the planet’s center, on July 7, 1992. They suggested that during this close approach, the difference between the gravitational force Jupiter exerted on the near and far sides of the comet had ripped the weak comet into many pieces.
Shoemaker-Levy 9 had been in a rapidly changing orbit around Jupiter for several decades. The comet did not fragment during earlier encounters with Jupiter because it had approached no closer than about five million miles in its previous orbits. Analysis of high-resolution photographs taken by the National Aeronautics and Space Administration (NASA) Hubble Space Telescope
An ultraviolet image of Jupiter taken by the Hubble Space Telescope on July 21, 1994, shows a number of fragments from Comet Shoemaker-Levy 9 struck the planet’s southern hemisphere. These fragments were embedded in clouds of debris, which appear very dark in the ultraviolet photo, as dust absorbs sunlight.
By late May, 1993, it appeared that Shoemaker-Levy 9 was likely to hit Jupiter in 1994, and the fragments would be moving at a speed of about 130,000 miles per hour relative to the planet. At that point, the comet became the subject of intense study by astronomers around the world, since a cosmic collision of that magnitude had never been observed before. Based on the size and speed of each of the fragments, the impacts were expected to produce an explosive effect equivalent to between 6 million and 250 million megatons of TNT.
In the two-year period between the breakup of the comet and the collision with Jupiter, the fragments had spread out along the comet’s orbit. The impacts, which took place over a one-week period from July 16 through July 22, 1994, caused enormous fireballs in the atmosphere of Jupiter and produced large, dark storms. Fragments G and H, each about 1.5 miles in diameter, caused the most destruction.
The first two impacts occurred on a part of Jupiter that was facing away from the Earth, so the impacts were not directly observable from Earth. However, astronomers were able to see the bright clouds of debris as they rose over the edge of the planet. An hour later, as Jupiter rotated, the impact sites became visible, and the extent of the damage was clear. The impacts had left dark scars in the atmosphere of Jupiter.
The observation of Shoemaker-Levy 9’s impact with Jupiter was a once-in-a lifetime event for astronomers. The disruption of a comet into many fragments is an unusual event. Capture of a comet into an orbit around Jupiter is even more unusual, and the collision of a large comet with a planet is extremely rare, estimated to occur only once in a thousand years.
Sixty-five million years ago, Earth was struck by a large asteroid, an event which may have brought about the extinction of the dinosaurs. However, exactly what took place as the object passed through Earth’s atmosphere has only been modeled, never verified, by experiment. The enormous release of energy into the atmosphere cannot be produced by humans, even with the use of nuclear bombs. Thus the Shoemaker-Levy 9 impacts into the atmosphere of Jupiter provided the first opportunity to observe this type of event and to validate the models, allowing scientists to determine with better accuracy the likely effects of impacts of asteroids or comets on Earth.
Shoemaker-Levy 9’s impact on Jupiter provided graphic visual evidence of the destructive power of comet and asteroid impacts on a planet. Governments around the world began to recognize the consequences such an event would have if it occurred on Earth. The event resulted in a more ambitious effort to discover and track asteroids and comets that approach Earth.
The impacts also allowed scientists to study Jupiter. The dark spots in the atmosphere were quickly distorted in shape, serving as a tracer to map the winds on Jupiter. Ultraviolet observations from the Hubble Space Telescope showed the debris sinking into Jupiter’s atmosphere, providing the third dimension in the motion of Jupiter’s winds. Observations also led to the suggestion that linear chains of craters, previously observed on two of Jupiter’s moons, Ganymede and Callisto, might have formed by the impact of bodies disrupted in the same way as Shoemaker-Levy 9.
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Levy, David H. Impact Jupiter: The Crash of Shoemaker-Levy 9. New York: Basic Books, 2003. Well-illustrated, nontechnical account of the impact of Comet Shoemaker-Levy 9 into Jupiter. Discusses how the event changed the understanding of comets and cosmic cataclysms. -
_______. Shoemaker by Levy: The Man Who Made an Impact. Princeton, N.J.: Princeton University Press, 2000. Account of the life of Eugene Shoemaker, including the events that led to the discovery of Shoemaker-Levy 9, and Shoemaker’s work on the effects of comet and asteroid collisions with Earth and other heavenly bodies. -
Noll, Keith, Harold A. Weaver, and Paul D. Feldman, eds. The Collision of Comet Shoemaker-Levy 9 and Jupiter. New York: Cambridge University Press, 2006. A 388-page collection of scientific reports presenting the major scientific results from observation of the impacts. -
Shoemaker, Gene, Carolyn Shoemaker, John R. Spencer, and Jacqueline Mitton. The Great Comet Crash: The Collision of Comet Shoemaker-Levy 9 and Jupiter. New York: Cambridge University Press, 1995. Collection of images from telescopes around the world showing Comet Shoemaker-Levy 9 and the effects of its collision with Jupiter.
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