Crash course on a comet bound for Jupiter.

Lined up like pearls on a string, some 20 comet-like fragments will slam one by one into Jupiter next July. The impacts will allow at least two spacecraft -- Galileo and Voyager 2 - to observe directly the most powerful series of collisions ever predicted for the solar system.

On that much, astronomers agree. But the amount of energy unleashed by the fragments, known collectively as Comet Shoemaker-Levy 9, remains a matter of intense debate. That's because researchers don't know the size, and hence the kinetic energy, of any of the pieces, which were discovered last March and are thought to originate from a parent body ripped apart by Jupiter's gravity in July 1992 (SN: 6/26/93, p.410).

Several studies reported this week may help astronomers estimate the size of the largest fragments. Researchers described their findings during a crash course on Comet Shoemaker-Levy 9 - a marathon four-hour session at the annual meeting of the American Astronomical Society's Division for Planetary Sciences in Boulder, Colo.

At the meeting, Harold A. Weaver of the Space Telescope Science Institute in Baltimore presented several snapshots of Comet Shoemaker-Levy 9 taken on July 1 with the Hubble Space Telescope. Although the images represent the highest resolution of the fragments to date, astronomers still can'[ clearly distinguish the hard core of each body from its comet-like shroud of dust and gas. Weaver estimates that the highly reflective shrouds, known as comas, account for some 70 percent of the luminosity of the fragments in the Hubble pictures. By subtracting this estimated contribution, Weaver and his colleagues calculate that the largest pieces have a core no greater than about 5 kilometers in diameter - about half the size of early estimates.

The kenetic energy of each fragment is proportional to its mass, which in turn is proportional to the cube of the fragment's diameter. Thus, the smaller size indicated by the Hubble images suggests that the fragments might dump into the Jovian atmosphere only about one-eight the energy originally calculated. Nonetheless, Weaver notes, the total energy unleashed would still equal about 100 mega-tons of TNT.

If the Hubble study provides a maximum size for the largest fragments of Shoemaker-Levy 9, another study, reported in the Oct. 21 NATURE by James V. Scotti and H. Jay Melosh of the University of Arizona in Tucson, may provide a minimum diameter. By tracing the orbits of the 20-odd fragments back in time, the astronomers infer that the present body had a diameter of 2 kilometers. Thus the largest fragments might measure just 1 kilometer across and impart only one-thousandth the energy proposed in earlier studies.

At the meeting, Paul Chodas, Zdenek Sekanina, and Donald K. Yeomans of NASA's Jet Propulsion Laboratory in Pasadena, Calif., reported that their orbital calculations - based on a larger set of data - indicate that the parent body might have a diameter of 9 kilometers, consistent with the Hubble study. They predict that the Jovian collisions will take place over about five days, centered on July 21, 1994.

How often does a comet break into a string of pieces near Jupiter? According to Melosh and Paul Schenk of the Lunar and Planetary Institute in Houston, the answer may lie in Voyager 1 images of the large Jovian moon Callisto, which show 13 straight-line chains of creaters. They say a string of cometary fragments sequentially striking the moon best explain these crater chains, as well as three others identified on the Jovian moon Ganymede. Schenk and Melosh estimate that comets with a diameter of a few kilometers break up near Jupiter once every 80 years.