Comet Shoemaker-Levy 9 was discovered on 24th March 1993, and was soon found to be in orbit of Jupiter; it was most likely captured by Jupiter 20-30 years earlier. Tracing back the orbit revealed that it had passed very close to Jupiter on 7th July 1992, and it is thought that the comet fragmented during this event. As predicted from the orbit, the fragments of Shoemaker-Levy 9 collided with Jupiter over the course of a week in July 1994.
By simulating the breakup of the original comet nucleus we can constrain the density of the comet. These simulations use hexagonal closest packed 'rubble piles' with a timestep of ~6 minutes, and are run for 500 steps.