When a Comet Hits a Planet: Big Fireworks

UNLIKE the dinosaurs, who never saw what hit them, astronomers are watching today's planet-bashing comet like hawks as it closes in on Jupiter. Moreover, they are preparing to use observatories around the world and out in space for intensive scrutiny of what will happen when Comet Shoemaker-Levy 9 - a strung-out group of about 20 comet fragments - reaches the giant planet.

The comet pummeling is expected to be one of the greatest astronomical events of the century (although not visible to the naked eye from Earth) - or, perhaps, one of the biggest fizzles.

The leading fragment is due to hit Jupiter July 16. The others will impact over the following six days.

Yet that's the only forecast concerning this event about which astronomers feel certain right now. If those comet fragments contain solid nuclei, mainly of water ice a kilometer to several kilometers in diameter, as some astronomers think, each of their impacts could have an explosive energy on the order of a million times the energy of a typical nuclear warhead, according to University of Chicago astrophysicist Mordecai-Mark Mac Low. That kind of impact may have done in the dinosaurs. It should set off massive disruptions in Jupiter's atmosphere. But if the fragments are just crumbly puff balls of dust and gases, Jupiter may swallow them without even a burp.

Scientists' expectations are fuzzy because the comet fragments are fuzzy. Not even the sharp-eyed instruments of the repaired Hubble Space Telescope have seen through the dust that veils them. Their size, mass, density, and structure are uncertain. So, even though the fragments will slam into Jupiter at 60 kilometers a second (130,000 miles an hour), scientists cannot estimate how much energy that implies.

Nevertheless, this is a once-in-a-lifetime opportunity for planetary scientists. They have seen the results, after the fact, of asteroid and comet impacts on planets and moons all over the solar system. Now ``we have, for the first time, been able to predict a major impact in advance and prepare scientifically to observe its effects,'' comet co-discoverer Eugene Shoemaker of the United States Geological Survey at Flagstaff, Ariz., explained during a press briefing in Washington in May.

Dr. Shoemaker - together with his wife, Carolyn, and Tucson amateur astronomer David Levy - using an 18-inch telescope on Mt. Palomar in California, discovered the fragmented comet on March 24 last year. It had come within 21,000 kilometers (13,000 miles) of the planet's cloud tops July 8, 1992, when tidal forces tore the comet apart.

Shoemaker-Levy 9 (SL9) probably started out as an average comet, sweeping in from the outer reaches of the solar system. According to current theory, a vast reservoir containing billions of comets extends beyond the planets. The inner part of that reservoir - the Kuiper belt - is shown around the edge of the accompanying diagram. Gravitational perturbations, perhaps due to passing stars, occasionally throw comets out of the reservoir into the inner solar system. Some just pass through and on into outer space. Others like Haley's Comet are captured into solar orbits that bring them back close to the sun periodically. Jupiter apparently captures a few.

At Jupiter, the comet fragments will plunge into a massive atmosphere made up of about 89 percent molecular hydrogen and 11 percent helium, with small amounts of ammonia, methane, water, ethane, acetylene, carbon monoxide, hydrogen cyanide, and some other compounds. Clouds of ammonia crystals float over the atmosphere's top. Below them, there may be lower clouds of such exotic compounds as ammonium hydro-sulfide, as well as frozen and liquid water.

Comet SL9 will come in over Jupiter's south pole and impact at about 45 degrees south latitude. From Earth, impacts will be just out of sight.

Jupiter's fast rotation, however, should bring each impact site into view within 20 minutes. Earth-based instruments then can look for residual effects. They may also see reflections of impact flashes on one of Jupiter's moons or its encircling ring. As comet fragments plunge into Jupiter's chemical stew, astronomers will be looking for light flashes from explosive energy release, ripples spreading out from the impact sites, and any chemical or circulation changes in the atmosphere. They will also monitor the fragments' last minutes of flight to see if their dust is stripped away to reveal any nuclei. Some of that dust may form a new ring around the planet.

The Galileo spacecraft, however, now heading for Jupiter, will have a clear view.

It should be able to record images of the impacts for later playback to Earth. Several other spacecraft will monitor the impacts. The European Ulysses solar probe will listen for changes in Jupiter's radio emissions. Voyager 2 - which once surveyed Jupiter at close range - will also monitor the radio noises even though it will be about 4 billion miles away. And back at Earth, several astronomical satellites, including the Hubble telescope, as well as ground- based observatories, will maintain close watch on the comet show.