Comet Shoemaker-Levy 9 (SL9) and Jupiter, diagram. Due to a previous close approach to Jupiter, SL9 had broken up into a mass of cometary fragments travelling in the same orbit. Twenty-three such fragments are here labelled a to w, heading towards Jupiter (upper left). The impacts took place in July 1994, over a period of 6 days. The impacts were the first such observed by humans, and were studied for evidence as to the composition of Jupiter's atmosphere.