ESTIMATE OF THE SIZE OF COMET SHOEMAKER-LEVY-9 FROM A TIDAL BREAKUP MODEL

THE spectacular 'string-of-pearls' appearance of periodic comet Shoemaker-Levy 9 (1993e) and its impending encounter with Jupiter's atmosphere have caused a flurry of speculation on the likely effects of the impact1. The magnitudes of the predicted effects depend critically on the masses of the fragments composing the chain, although these are currently poorly constrained. However, some limits on the comet's total size, and hence mass, can be obtained by considering its dynamical history. On its previous swing past Jupiter, the comet apparently passed within the Roche limit of the planet2,3, and it therefore seems likely that the present chain of about twenty nuclei was created when a single progenitor was split by tidal forces during closest approach. Here we describe a simple model of tidal splitting which is able to reproduce closely both the length of the observed chain and its position angle on the sky as a function of time. The length of the fragment chain is directly proportional to the size of the parent object, which we estimate to have been only about 2 km in diameter.