ALTHOUGH comets have been studied throughout most of recorded history, a detailed understanding of their internal properties is still lacking. Recent observations1 of the split comet Shoemaker–Levy 9—actually a spectacular string of cometary fragments that resulted from the tidal disruption of a single parent body as it passed close to Jupiter2–5—have therefore stimulated much interest, as they provide an unprecedented opportunity to investigate the physical properties of comets more generally6–8. I report here simulations of the tidal breakup of the parent comet, which I assume to have been an assemblage of a large number of spherical components bound together only by gravity. Following the initial tidal disruption of the assemblage, the particles coalesce rapidly by mutual gravitation into a chain of larger fragments, the morphology of which depends critically on the density of the components. By comparing the size, number and distribution of the stimulated fragments with observations of Shoemaker–Levy 9, I determine an average comet density of about 0.5gcm-3 and a parent comet diameter of about 1.8 km.