Shoemaker-Levy 9

Comet Shoemaker-Levy 9 was discovered already orbiting Jupiter — a highly unusual circumstance. Jupiter’s tidal forces had fragmented the nucleus into 21 pieces during a close pass in July 1992. Over six days in July 1994 (16-22 July), all 21 fragments impacted Jupiter in succession, each releasing energies of millions of megatons of TNT and leaving dark impact scars in Jupiter’s atmosphere larger than Earth — the first directly-observed collision of two Solar System bodies, and a demonstration of the kind of event that can resurface planets.