Most astronomers--but surprisingly few of us regular folks--realize that our solar system, with its average-sized star and nine planets, is still in a state of flux. Far from being stable, the solar system is still sorting itself out, sweeping up the chaotic debris of comets and asteroids left over from its earliest beginnings. Recently, two astronomers--Brian Marsden, of the Harvard-Smithsonian Center for Astrophysics, and Paul Chodas, of California's Jet Propulsion Laboratory--announced the first predicted collision of such cosmic leftovers with a planet. The recently discovered Comet Shoemaker-Levy is on a beeline course for Jupiter and is expected to plow into the planet next July. At the most recent meeting of the American Astronomical Society, in Berkeley, California, NASA astronomers Chris Chyba and Kevin Zahnle described the predicted event in some detail: the resultant explosion will be equivalent to an explosion of 200 million megatons of TNT--more powerful than the simultaneous detonation of all the world's nuclear weapons.
The catastrophe that Chyba and Zahnle described may rival the collision between the earth and a comet 65 million years ago, an event which many believe killed off much of the life on our planet, including the dinosaurs. Zahnle explains that because of the comet's unusual nature, the event on Jupiter could cause the giant gaseous planet to flare up repeatedly to twenty-five times its usual brightness over the course of a few hours. Comet Shoemaker-Levy has been captured by Jupiter's tremendous gravitational field and is now revolving in an orbit around the planet. On July 8, 1992, Shoemaker-Levy passed 30,000 miles above Jupiter's cloud tops, within what astronomers call the Roche Limit: the critical distance from the center of a planet at which tidal forces are sufficient to disrupt and tear apart a satellite (or, in this case, a comet). As a result of its close approach, Shoemaker-Levy was ripped up into a train of smaller pieces-like "pearls on a string" as one astronomer described it. The best orbital computations available at press time suggest that a part or all of the cometary debris will collide with Jupiter at speeds of 40 miles per second sometime between July 23 and July 27, 1994. Donald Yeomans, an orbit specialist at the Jet Propulsion Laboratory, has calculated a probability of 64 percent that the center of the comet train will collide with Jupiter. In the case of a glancing strike, the pieces on the trailing end of the comet train might survive to escape Jupiter's gravity and go into a more typical orbit around the sun.
Comet Shoemaker-Levy, once a twelve-mile-wide chunk of ice and rock, was discovered last March 24 by the comet-hunting team of Carolyn and Eugene Shoemaker, David Levy, and Philippe Bendjoya with the 18-inch telescope on Palomar Mountain. When James Scotti saw the comet with the University of Arizona's Spacewatch Telescope on March 26, he said it looked more like a "comet stream." When Jane Luu and David Jewitt imaged it with a camera mounted on the 86-inch telescope on Mauna Kea in Hawaii, they were amazed to discover seventeen separate cometary nuclei. That figure is now at twenty-one, and the largest of these--at this writing--imaged on July 1, may be six miles across.
To the immense disappointment of earthly sky watchers, this entire catastrophic event will occur on the night side of Jupiter, which faces away from us. But because Jupiter rotates on its axis once every ten hours, we should be able to see the effects of the devastation a few hours after it occurs. And the reflected light of the explosion on Jupiter's nearest satellites may be visible with small telescopes--perhaps even with binoculars.
As each piece of cometary material plunges into Jupiter's thick atmosphere, it should break into rubble and then, 100 to 200 miles down, be destroyed by intense atmospheric resistance. This destruction will cause an enormous release of energy in the upper layers of the planet. …