On 27 March 1993, shortly after its discovery, Brian Marsden calculated that the broken nucleus of comet Shoemaker-Levy 9 was on a collision course with Jupiter. Tidal forces from a close Jupiter flyby in July 1992 had shattered the comet into at least 21 fragments strung along a chain like pearls. Between 16 and 22 July 1994, the 19 major fragments — labelled A through W — slammed into Jupiter's southern hemisphere at 60 km/s, leaving dark impact scars visible for months. It was the first direct observation of a extraterrestrial collision in the Solar System and galvanised planetary defence efforts worldwide.