COMET SHOEMAKER-LEVY-9 - FRAGMENT AND PROGENITOR IMPACT ENERGY

Initial observational data from the impact of fragments of Comet Shoem aker-Levy 9 (SL9) are compared with smoothed particle hydrodynamic (SP H) calculations to determine their pre-impact diameters and the equiva lent diameter of the SL9 progenitor. Diameters (solid ice) of 2.0+/-0. 1, 2.0+/-0.05, 2.1+/-0.04 and 1.9+/-0.05 km for fragments A, E, G1, an d W are obtained from impact-induced plume heights from the Hubble Spa ce Telescope (HST) data. Applying these values to scale apparent diame ters for the balance of 18 SL fragments in Weaver et al.'s [1995] cata log of 22 objects yields a SL9 progenitor diameter of 5.0+/-1.8 km. Th is corresponds to total impact energy of 1.2 (+1.8 - 0.8) x 10(30) erg . Such an energetic event occurs on Jupiter and Earth at least every 4 ,900 +4,700, -2,700, and similar to 0.5 x 10(8) years, respectively.