COLLISION OF COMET SHOEMAKER-LEVY 9 WITH JUPITER - IMPACT STUDY OF 2 FRAGMENTS FROM THE TIMING OF PRECURSOR EVENTS

The impacts of fragments K and R of comet Shoemaker-Levy 9 are examine d with the aims to interpret the timing of the observed precursors to the main thermal emission event and to correlate the results of ground -based infrared observations with a variety of observations made onboa rd the Galilee spacecraft. Analysis of the phenomena associated with t he impact and explosion of fragment K shows that there is no discrepan cy in the timing of the Earth- and Galileo-based observations and that the time of 53 +/- 3 seconds between the emission peak of Precursor I and the onset of Precursor 2, as recorded by terrestrial observers, c an be interpreted as the interval between the impactor's disappearance behind the Jovian limb and the first appearance of the ejecta's plume over the limb following the explosion of the fragment's residual mass . It is concluded that the impactor exploded at an altitude of 45 to 5 0 km above the pressure level of 1 bar and that the residual mass invo lved in the explosion, approximately 6 to 7 million tons and about 400 meters across, represented only a fraction of 1 percent of the fragme nt's preatmospheric mass. The explosion is calculated to have taken pl ace under a dynamic pressure of several hundred bars and the explosion energy is found to have been on the order of 10(26) erg. The results for fragment R show it to be smaller and less massive than fragment K, exploding slightly higher in the Jovian stratosphere, 50-60 km above 1 bar. Most of the entry kinetic energy of either impactor was rapidly dissipated during atmospheric flight by prolific mass ablation - domi nated by the object's fragmentation - before the point of explosion wa s reached. The rate of ablation of these massive fragments was compara ble with that observed in the Earth's atmosphere for category IIIb fir eballs, which belong to the population of ''soft'' cometary material a nd whose bulk density was determined to be typically 0.2 g/cm(3). Vigo rous fragmentation - both continuous and occurring in discrete events of extremely short duration is known for such impactors to be independ ent of their mass and to surpass vastly (especially for massive object s) ablation effects due to evaporation and spraying. Evidence from sev eral other observations of comet Shoemaker-Levy 9 appears to corrobora te the present conclusions on the bulk properties and the penetration depths of the major fragments of comet Shoemaker-Levy 9.