Step outdoors at twilight this month and you can see the planet Jupiter, a serene white pearl glowing in the night sky. It's hard to believe anything could disturb such heavenly tranquillity. But soon something will, and with a bang. Beginning July 16, nearly two dozen remnants of a fragmented comet, some perhaps as big as mountains, will hurtle at 134,000 miles per hour into the solar system's largest planet.
Researchers are looking forward to the celestial cataclysm as to few other events in modern space science. Jupiter won't sustain permanent damage; no matter how big, the comet pieces are tiny next to their monster target. But some of the collisions are likely to churn up enormous plumes of comet debris and Jovian gas and send shock waves rippling through the orb's interior. The temporary effects of these planetary insults could give scientists a unique window into the mysteries of Jupiter's interior and atmosphere. Astronomers will also get a rare close- up view of a comet -- a "dirty snowball" that wanders the heavens and occasionally ends its days in a spectacular collision. Indeed, a cosmic encounter between a comet and Earth nearly a century ago may have been the cause of a huge explosion that flattened trees for miles in Siberia (box, Page 50). Scientists are hoping to learn from the crash on Jupiter whether even small comet bits can inflict heavy damage on a planet such as our own.
Unfortunately, astronomers will not be able to watch this Jovian joust directly, since the comet will hit Jupiter on the side facing away from Earth. But astronomers will have a grandstand seat anyway, thanks to the spacecraft Galileo, which will be perched a relatively intimate 143 million miles from the giant planet at impact time. On its way to a rendezvous with Jupiter in 1995, when it will drop a probe into the planet's atmosphere, Galileo will aim its battery of imaging sensors toward the impact site, giving scientists their best view of the collision. And since Jupiter spins quickly, the site of each collision should come into view from Earth some 10 to 20 minutes after the actual strike. Any lingering effects of the collisions, such as giant plumes, may still be visible to earthbound observers. Almost every major telescope will be trained on the crash, including the orbiting Hubble Space Telescope.
Strangers in the night. The crash could hardly involve two more interesting -- and mysterious -- players. Jupiter is the king of the planetary neighborhood, bigger by far than any other body in the solar system except the sun. Strange, colored bands of swirling clouds girdle the planet. An immense cyclone called the Great Red Spot whirls endlessly in Jupiter's southern hemisphere, its stability a puzzle to generations of scientists.
The shattered comet, found in 1993 and christened Shoemaker- Levy 9 after its discoverers, is thought to have been captured by Jupiter's gravity several decades ago. Scientists estimate that the comet remained in orbit around Jupiter until 1992, when it ventured too close to the giant. The planet's enormous gravitational pull dashed the comet to pieces and put the remaining debris on its current path to oblivion.
The stream of scientific information resulting from the collision will keep astronomers busy for years. By far the most dreamed-of prize is a peek at Jupiter's interior. Beneath its 60- mile-thick layer of upper clouds, the planet is made of mostly hydrogen and helium. Scientists believe that the tremendous pressures in Jupiter's interior create a sea of liquid hydrogen whose swirling currents endow Jupiter with its powerful magnetic field.
To plumb the depths beneath Jupiter's surface, astronomers will watch for specific changes as shock waves generated by the crash ripple through the planet's interior. Scientists hope these seismic waves, similar to earthquake tremors, will travel deep into the planet's center and then bounce back up to the surface. The energy of the waves should theoretically create tiny changes in temperature in Jupiter's atmosphere, which may give clues to the location of the planet's liquid hydrogen layer.
Scientists also hope to discover how much of a more earthly substance, water, is in the Jovian atmosphere -- a finding that could give new insights into the origins of the solar system. The shards of comet will be going so fast that, as they plunge downward, they will heat the surrounding atmosphere to as high as 70,000 degrees Fahrenheit. Some of the water in the atmosphere may vaporize, rise to the planet's surface and form clouds that are visible from Earth. Water might also be detected indirectly, because the sheer energy of the inrushing comet will cause ripples of gas to spread through Jupiter's atmosphere. The faster the waves move, the more water there is.
Solar shake-up? The scientists' findings could call into question the leading hypothesis about the solar system's birth. Astronomers currently think that the sun and planets all formed from one giant cloud of dust and gas. Since Jupiter is a gaseous planet -- closer in composition to the sun than it is to arocky planet like the Earth -- scientists are able to estimate how much water should exist there; so far, however, they have detected only a fraction of thepredicted amount. If more water isn't found, says California Institute of Technology planetary scientist Andrew Ingersoll, "We have to go back to the drawing board."
The perpetrator of this planetary hullabaloo is coming in for scrutiny as well. Astronomers would especially like a list of Shoemaker-Levy's ingredients, which they plan to look for by analyzing the composition of the impact's plume. The brightness and timing of the plume may hold clues to the pieces' sizes and densities, too. Scientists will also watch for the flashes made by the comet pieces as they strike the top of Jupiter's atmosphere. A series of equally bright flashes would lend support to the theory that comets are an agglomeration of many smaller, uniformly sized chunks rather than one solid mass.
But what happens if nothing happens? In the 15 months since the comet was first sighted, some of its pieces have broken into even smaller chunks; others seem to have disappeared altogether. Some scientists worry that the remaining pieces may be too small to create a bang of any scientific interest. Still, even a heavenly fizzle would suggest that Jupiter's atmosphere is far more stable than scientists have believed. And it would affirm backyard observers' perception that the pearly planet really is as tranquil as it appears in the evening sky.
The path of destruction Beginning this week, pieces of a shattered comet will smash into the planet Jupiter with potentially cataclysmic results. Scientists hope that this natural experiment will yield new insights about Jupiter's atmosphere and interior and shed light on the mysterious ways of comets. Such opportunities are rare, since crashes of this kind come only once in a thousand years or so.
March 24, 1993 Comet discovered
A doomed wanderer Discovered in 1993, comet Shoemaker-Levy 9 probably spent its early days wandering the solar system. It was eventually captured by Jupiter's gravitational pull, and scientists calculate that in 1992 it swung too close to the giant planet and was torn apart by gravitational forces.
Mysterious remains Observers originally saw 21 comet pieces. Since then, some pieces appear to have vanished and others have fractured further, so the true number is uncertain. The estimated size of the pieces ranges from minute to mountainous.
July 16, 1993 Comet's farthest point from Jupiter: 31 million miles
All eyes on Jupiter Four different instruments will watch the collisions from Galileo's scan platform. One will take pictures and others will detect heat and other invisible waves of energy, providing a glimpse of how the giant planet's atmopshere reacts to the crash.
Close-up, sort of On its way to a rendezvous with the giant planet in 1995, the spacecraft Galileo has a grandstand seat to the collisions. Even tho
