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

THE spectacular 'string-of-pearls' appearance of periodic comet Shoema ker-Levy 9 (1993e) and its impending encounter with Jupiter's atmosphe re have caused a flurry of speculation on the likely effects of the im pact1. The magnitudes of the predicted effects depend critically on th e masses of the fragments composing the chain, although these are curr ently poorly constrained. However, some limits on the comet's total si ze, and hence mass, can be obtained by considering its dynamical histo ry. On its previous swing past Jupiter, the comet apparently passed wi thin the Roche limit of the planet2,3, and it therefore seems likely t hat 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 estim ate to have been only about 2 km in diameter.