Comet Shoemaker-Levy 9: an active comet

The important elements of the debate over the activity versus dormancy of comet Shoemaker-Levy 9 (S-L 9) are reviewed. It is argued that the circularity of the isophotes in the inner comae of S-L 9 as well as the spatial dependencies of the comae brightness profiles are indicators of sustained dust production by S-L 9. It is also shown that the westward tail orientations, which were formerly interpreted as a sign of the comet's dormancy, are not a good indicator of either activity or dormancy. Rather. the tail orientations simply place constraints on the dust production rate for grains smaller than approximate to 5 mu m. All the available evidence points to S-L 9 as having been an active, dust-producing comet. Synthetic images of an active comet are fitted to Hubble Space Telescope images of the S-L 9 fragment K, and its grain size and outflow velocity distributions are extracted. These findings show that the appearance of the dust coma was dominated by large grains having radii between approximate to 30 mu m and approximate to 3 mm, produced at a rate of M over dot approximate to 22 kg s(-1), and ejected at outflow velocities of approximate to 0.5m s(-1). Only upper limits on the production rates of smaller grains are obtained. The nucleus of fragment Ii was not observed directly but its size is restricted to lie within a rather narrow interval 0.4 less than or similar to R(f) less than or similar to 1.2 km. (C) 1997 Elsevier Science Ltd. All rights reserved.