Pieces of the comet Shoemaker-Levy 9 will begin impacting the atmosphere of Jupiter at around 19:56UTC (12:56pm, PDT) on Saturday July 16. This is the first time in human history that a major astronomical impact has been predicted far enough in advance to be observed (if only indirectly) and modeled by astronomers and astrophysicists. There are several on-line sources of info about this event, including one at Arizona and one at JPL.
It is important to know how deep the comet will penetrate the Jovian atmosphere before being disrupted, and the rate at which the comet's kinetic energy will be converted into heat and deposited in the atmosphere. These results have a significant bearing on the size and duration of the "fireball" that will rise from the impact point. The simulation presented is of the approximately 3 second period during which time the fragment deposits its kinetic energy into Jupiter's atmosphere.
We have simulated the impact of a spherical bolide (comet) 1km in diameter with the Jovian atmosphere. An mpeg animation (length, approx 1.5Mb) is available. We are using a fully three-dimensional code and "smooth particle hydrodynamics" (SPH) to model the dynamics of this system. SPH uses discrete particles to represent the continuous system, and transforms the equations that govern fluid flow into a set of equations that represent the motion of the fluid and the rate of change of the fluid's internal energy. Since it uses particles, it is particularly well suited to problems with very large density contrasts and shocks, such as are present in the Shoemaker-Levy impact. However, cometary impacts are a relatively new application area for SPH codes. In particular, the large mass differences between the bolide particles and the atmosphere particles can be problematical. The animation shown is our best simulation of the impact to-date, but should not be taken as our final scientific statement on the subject.
The code we are using has been developed at Caltech with support the NSF/CRPC and and at LANL with NASA/HPCC funding. It was designed from scratch as a parallel code for "fast" gravitational N-body summations. Subsequent evolution, and abstraction of the underlying algorithms and approaches to parallelism have allowed us to use the same code to address problems in vortex dynamics, smooth-particle hydrodynamics and boundary-element methods on both parallel and serial supercomputers. Ths simulation shown was run on the Intel Delta and Intel Paragon systems at Caltech on up to 128 processors.
In order to keep the simulation's size tractable, we supply new particles at the bottom, and remove them from the top of the computational volume. Although the image appears to be oriented vertically, in fact, the comet is descending through the atmosphere at an angle of 45 degrees to the local vertical. The atmosphere is modeled with high resolution (approximately 0.07km interparticle separation) in a 2km diameter cylinder along the comet's trajectory, and lower resolution (approximately 0.14 km interparticle separation) out to diameter of 4.4km. There are typically about 66000 particles in the computational volume at any one time. The comet is initially a spherical gas ball with temperature 3.5K, and density 0.5g/cm^3, modeled by approximately 8000 particles with 0.03km interparticle separation. In the images, color represents density, which extends over approximately four orders of magnitude, from 5e-4 g/cm^3 to about 5g/cm^3.