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 cometʹs 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 ≈ 5 μm. 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 ≈ 30 μm and ≈ 3 mm, produced at a rate of M ≈ 22 kg s−1, and ejected at outflow velocities of ≈ 0.5 m s−1. Only upper limits on the production rates of smaller grains are obtained. The nucleus of fragment K was not observed directly but its size is restricted to lie within a rather narrow interval 0.4 ≲ Rf ≲ 1.2 km.