Comet Aims for the Planet Jupiter In Our Cosmic `Shooting Gallery'

OUR solar system is a cosmic ``shooting gallery'' with comets sometimes whizzing uncomfortably close to planets. Now one of them is heading straight for Jupiter.

Comet Swift-Tuttle caused a stir when it looked as though it might slam into Earth in the year 2126. But by the end of last year, refined orbital calculations predicted that this so-called ``doomsday'' comet would continue to miss our planet. There's no such reprieve for Jupiter as Comet P/Shoemaker-Levy 9 approaches.

Comet-tracker Harold Weaver at the Space Telescope Science Institute in Baltimore says astronomers know when and where that comet will hit. They expect it to slam into Jupiter's southern hemisphere next July. Moving at 60 kilometers a second (130,000 miles an hour), it carries a total impact energy equivalent to the explosive energy of 100 million megatons of TNT. That's many thousands of times the explosive energy of the world's total inventory of nuclear weapons. But it's comparable to the impact energy of the object or objects that many geophysicists think hit Earth and did in the dinosaurs.

Jupiter, unlike Earth, is a great gas bag. It shouldn't suffer much damage. But Dr. Weaver says this will be ``a pretty major event'' for solar-system scientists to study. ``Here is our first opportunity to observe the impact of a comet on a planetary atmosphere,'' he says. He adds: ``It's useful to study something like this. It helps us to understand what our [own] potential risks are.''

What makes this impact especially intriguing to scientists is that the comet has already broken into many pieces. This confirms that a comet can deliver a ``multiple-warhead'' strike. Some geophysicists think that may have happened to Earth 65 million years ago.

A Hubble Space Telescope image released last week, shown here, reveals at least 20 fragments. There may be more. Weaver says Hubble currently can't detect fragments smaller than 2 kilometers in diameter so ``the size distribution of the planetesimals is still indeterminate.''

Astronomers have been tracking this comet since its discovery last March by Carolyn Shoemaker and Eugene Shoemaker - with Northern Arizona University and the United States Geological Survey in Flagstaff, Ariz., respectively - and amateur comet-observer David Levy of Tucson. Ground-based telescopes showed the fragmentation - hence the comet's nickname ``string-of-pearls.'' But they haven't been able to resolve the detail revealed by the Hubble image.

THE Shoemakers, Mr. Levy, and Weaver are part of a 19-member Hubble telescope team that is following the comet. They estimate that each fragment could produce a 200-kilometer diameter blue-white fireball.

Each of these fireballs would, in turn, blow a hole in the Jovian atmosphere as big as Texas, according to the Space Telescope Science Institute.

But to forecast exactly what is likely to happen, scientists need to know more about the comet.

This is one of the big unsolved mysteries of comet science - what are comets made of?

The standard model of a comet is the ``dirty snowball'' - a mass of ices laced with dust. But the few encounters spacecraft have had with comets have thrown doubt on that model. Some comets, at least, may be more like ``frozen mud balls'' with as much as 75 percent of their mass made up of nonvolatile materials. That, apparently, is what the European Space Agency (ESA) Giotto probe found when it came within 200 km of the nucleus of Comet Grigg-Skjellerup in July 1992. Commenting on the report of an ESA team published in Nature last April, comet scientist Mark V. Sykes of the University of Arizona said the results ``continue to undermine the ice-dominated `dirty-snowball' model.''

Knowledge of the nature of the ``string-of-pearls'' comet is crucial to interpreting the Hubble image. Weaver explains that the estimate of the fragments' sizes depends on their albedo - that is, on how much light they reflect. His group has assumed a 4-percent albedo like that of the very dark Comet Halley nucleus. But if the fragments are bright, the size estimate would be too large. For example, an albedo of 100 percent (perfect reflectivity) would reduce the estimated size of the ``string-of-pearls'' fragments to under a kilometer.

The Hubble telescope team is looking forward to December's shuttle mission, which is scheduled to repair the orbiting telescope's faulty optics. The comet's image shown here is somewhat obscured by fine dust. When the telescope has a sharper view, Weaver says the comet fragments ``should really pop out.'' Then scientists may be able to pin down their size and nature.