Comet Shoemaker-Levy 9 dust size and velocity distributions

Pre-impact observations of Comet Shoemaker-Levy 9 (S-L9) obtained with the Hubble Space Telescope are examined, and a model of an active, dust-produci ng comet is fitted to images of fragments G, H, K, and L. The model assumes steady isotropic dust emission from each fragment's sunlit hemisphere. Bes t-fit results indicate that the dominant light-scatterers in these fragment s' comae were relatively large dust grains of radii 10 mu m less than or si milar to R less than or similar to 3 mm. The fragments' dust size distribut ions were rather flat in comparison to other comets, dN(R) proportional to R-2.3+/-0.1, and the dust ejection speeds were similar to 0.5-1.5 m/s. The S-L9 fragments themselves were not detected directly, and upper limits an t heir radii are 1.0-1.5 km assuming an albedo a = 0.04. However, these fragm ents' vigorous production of dust, which ranges from 6 to 22 kg/s, places a lower limit of similar to 100 m on their radii at the moment of tidal brea kup. Any fragments smaller than this limit, yet experiencing similar mass l oss rates, would have dissipated prior to impact. Such bodies would fail to leave an impact scar at Jupiter's atmosphere, as was realized by fragments F, J, P-1, P-2, T, and U. (C) 2000 Academic Press.