Comet Shoemaker-Levy 9 was discovered by Eugene and Carolyn Shoemaker and David Levy in 1993. Shortly after its discovery it was determined to be in a highly elliptical path near Jupiter and on a collision course. It was difficult to calculate its orbit prior to its 1992 pass near the giant planet.
In 1992, SL 9 passed by Jupiter within the Roche limit. It was broken into at least 21 separate fragments which were dispersed several million kilometers along its orbit.
The size and mass of the original body and the individual fragments is as of this writing still highly uncertain. The estimates from 2 to 10 km in diameter for the original body and 1 to 3 km for the largest fragments.
Between 16 July 1994 and 22 July 1994 the fragments impacted the upper atmosphere of Jupiter. This was the first time that scientists had an opportunity to witness the collision of two extraterrestrial bodies.
The impacts were observed by virtually every large ground based telescope, thousands of small and amateur telescopes, and several spacecraft including HST and Galileo.
The pictures were posted to the Net within hours of the impacts and caused severe overloading on some ftp and WWW sites.
The after-effects of the impacts were visible on Jupiter for nearly a year after the event.
There are linear chains of craters on Ganymede and Callisto that are believed to have been formed by the impacts of bodies similar to SL 9.
SL 9 is no more, but its scientific legacy will be studied for years.