The impact of Periodic Comet Shoemaker-Levy 9 on Jupiter

We have performed computational shock-physics simulations of the hypervelocity (60 km/s) impact of 1-3 km, water-ice spheres entering a hydrogen-helium Jovian atmosphere, conditions that simulate current estimates for the collision of fragments of Periodic Comet Shoemaker-Levy 9 with Jupiter. Observatories in space and around the world observed the events as they occurred in July, 1994. The Hubble Space Telescope obtained high resolution images of the impact-generated fireballs that appeared above the limb of Jupiter and of the visibly dark ejecta patterns distributed over broad regions of the Jovian stratosphere. Time-resolved radiometric measurements from spacecraft and Earth-based observatories detected multiple arrivals for each impact: the entry flash as the incoming fragment entered the atmosphere, arrival of the debris front (fireball) as it emerged above the Jovian limb (which varies as a function of wavelength), the arrival of the debris front into sunlight and finally, the emergence of the impact site as it rotated into view. We used the Eulerian shock-physics code CTH, and its parallel version PCTH to perform 2-D analyses of penetration and breakup, and 3-D analyses of the growth of the resulting fireball during the first 120 seconds after fragment entry. For sufficiently large fragments, impact-generated fireballs rise into line-of-sight over the Jovian limb. For 1- and 3-km fragments impacting 6 degrees beyond the limb, this occurs approximately 75 and 50 seconds after impact, respectively. Measurement of the time interval between fragment entry and limb arrival provides information that places strong restrictions on equivalent explosive yield (from which fragment mass can be estimated). Measurements of more arrivals help constrain the effective penetration depth, thereby lending insight into the material and mechanical properties of the cometary fragments. We believe that matching high resolution imagery and time-resolved photometry with numerical simulations will provide some of the best means by which Shoemaker-Levy 9 fragment masses and material properties will be determined.