Comet Shoemaker-Levy 9
Comet Shoemaker-Levy 9, discovered in March 1993, by amateur astronomers, first impacted Jupiter July 16, 1994. It was orbiting the giant planet and had been torn apart probably in 1992, when Jupiter's gravity tore it apart.
Twenty-one fragments struck the planet over the next 6 days. The largest came on July 18.