FOR a week beginning 16 July 1994, fragments of comet Shoemaker-Levy 9 win collide with Jupiter each day. Although the fragments are probably smaller than originally estimated(1), the impacts may nevertheless have observable consequences that will provide valuable insight into the properties of comets and the dynamics of planetary atmospheres. Interpretation of these observations will depend sensitively on parameters such as the mass, density and overall structure of the fragments. To deduce some of these parameters, we have simulated the event that created the fragments-the passage of the parent comet through the tidal field of Jupiter in 1992. Modelling the comet as a strengthless aggregate consisting of a large number of grains, we find that the tidally disrupted body condenses rapidly into clumps, driven by their self-gravity. Formation of a fragment chain resembling Shoemaker-Levy 9 occurs for a narrow range of the simulated comet’s bulk density, 0.3-0.7 g cm(-3). A chain of similar to 20 similar-sized fragments matching observations is obtained for a non-rotating parent comet of 1.5 km diameter and bulk density 0.5 g cm(-3), suggesting that the clusters will each liberate similar to 10(27) erg on impact. A slightly larger initial density leads to significant mass variation among the clusters and the possibility of a few similar to 10(28)-erg events.