Comet Shoemaker Levy

Shoemaker Levy was a Comet discovered on 1993 March 25 by Eugene and Carolyn Shoemaker and David Levy on a photographic patrol plate exposed at Mount Palomar, San Diego, USA. The discovery image appeared unusually elongated, and further investigation revealed that the comet had, in fact, been broken into a number of fragments. Analysis of the comet’s motion showed it to be in a two-year orbit around Jupiter, into which it had probably been captured as long ago as 1929. At a close (21,000 km/13,000 mi) perijove in 1992 July, tidal stress had disrupted the comet’s fragile nucleus into at least 21 sub-kilometer-sized fragments.

Calculations showed that these fragments would impact on to Jupiter over the course of the week of 1994 July 16–22. The fragments were named alphabetically in order of anticipated impact, and as they continued along their terminal orbit they became spread out into a ‘string of pearls’ imaged from ground-based observatories and the Hubble Space Telescope. Ahead of 1994 July, there was much speculation as to what effect, if any, the impacts would have on the giant planet.

Intensive observing programs were established worldwide. The impacts themselves occurred just beyond the Jovian limb, out of sight from Earth (but visible from the Galileo spacecraft at that time en route to Jupiter). The planet’s rapid rotation would carry the impact sites into view from Earth within about an hour of each event. Observers were surprised by the scale and violence of the impacts. Cameras on Galileo revealed the entry fireballs as bright flashes, and infrared observations from the terrestrial viewpoint showed huge plumes of material thrown high above the Jovian cloud-decks following the impacts. The energies involved were most graphically shown by the easily visible Earth-sized dark spots that marked each impact site on Jupiter. Over the course of the week of the comet’s demise a series of dark impact scars peppered Jupiter’s clouds at 44ºS latitude, with impacts coming, on average, about seven hours apart. The impact scars remained visible for some weeks, eventually merging into a new dark belt on Jupiter, persisting for the next 18 months.

Searches through historical observations of Jupiter have produced only flimsy candidates for previous similar impacts during the telescopic era since the early 17th century. It has been suggested that such events may occur once in a thousand years. The Shoemaker–Levy 9 impacts again emphasized the fragility of comet nuclei and the catastrophic energy scales involved in cosmic collisions. Many of these fragments appear to have been captured as distant, small satellites by both Jupiter and Saturn.

Questions to Ponder

• What caused Shoemaker-Levy 9 to split up?
• Why was the Shoemaker-Levy 9 impact so important to astronomers?