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Recent N--body simulations of a tidally disrupted comet indicate that gravitational instabilities may be responsible for the appearance of comet Shoemaker--Levy 9 (S--L 9). These models are physically distinct from other models which assume tides broke the comet into the observed twenty or so separate fragments. The N body simulations show that if a comet catastrophically fragments into N>>20 parts, the particles can gravitationally recondense into n fragments a few hours after periapse. Since n depends sensitively on the parent comet's density, catastrophic disruption of a comet provides a sensitive measure of its density. We show, via a tidal impulse model, that if a tide disrupts a comet into a cloud of many particles, the resulting cloud is Jeans unstable and condenses to ~20 fragments about 10 hours after periapse for a comet density ~0.6 gm/cm. Whether these instabilities condense to compact objects or remain as extended swarms depends upon the rate at which collisions might damp their random motions. We also show that a varicose instability in an incompressible medium cannot account for S-L 9's twenty fragments, for this instability quickly breaks-up a tidally elongated cylinder into only n~1 or 2 fragments. These results indicate that a particle gas equation of state can be representative of catastrophically disrupted comets.
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