Number 150 (Story #1), November 5, 1993 by Phillip F. Schewe and Ben Stein CHAINS OF CRATERS ON CALLISTO AND GANYMEDE , two of Jupiter's moons, are now explained as being mostly due to split comets like Comet Shoemaker-Levy, which has broken into a chain of 22 fragments and is headed for a smashup with Jupiter in July 1994. Jay Melosh of the University of Arizona and Paul Schenk of the Lunar and Planetary Institute cite as evidence the fact that nearly all of Callisto's crater chains are on the Jupiter-facing hemisphere. In a separate paper, Melosh and James Scotti use a tidal-breakup model to calculate the size of the Shoemaker-Levy parent comet as being about 2 km. (Nature, 21 Oct. 1993.) Hubble Space Telescope pictures of the fragments suggest a size more like 3-4 km. The uncertainty in the size estimates translates into an uncertainty factor of 1000 for the energy of the collision. In any case, the ensuing explosions will cause Jupiter to ring, which might provide some rare seismological information about the planet's interior. (Nature, 28 Oct. 1993.) Study of reverberating waves in Jupiter's atmosphere should allow scientists to sharpen their views on the nature of the Great Red Spot. (Science, 22 Oct.)