1994

Comet SL-9 Slams Into Jupiter

Eugene Shoemaker (1928–1997), Carolyn Shoemaker (b. 1929), David Levy (b. 1948)

Impacts have been a fundamental force in shaping the surfaces and atmospheres of the planets, and have even influenced the history of climate and life in our own world. But impacts are rare and unpredictable events in the solar system and thus have been impossible to study directly—or at least they were until a group of astronomers discovered a year in advance exactly when a comet would smash into the planet Jupiter.

In the summer of 1993, the American astronomers Eugene Shoemaker and Carolyn Shoemaker and the Canadian astronomer David Levy reported finding a strange “string of pearls” comet that their observations showed had recently passed so close to Jupiter that it had been broken apart into dozens of fragments. Incredibly, their data showed that the fragments, now collectively known as comet Shoemaker-Levy 9 (SL-9), were on return trajectories to Jupiter, and that they would crash into the planet in July 1994.

This was the first time that an impact event was known in advance, and astronomers worldwide mobilized telescopes and space observatories to monitor the events. As predicted, 21 cometary fragments 0.6–1.2 miles (1–2 kilometers) across slammed into Jupiter at speeds of 134,000 miles per hour (60 kilometers per second) between July 16 and July 22. The results were spectacular and surprising—that such small, icy objects could produce huge fireballs, plumes, and Earth-size blemishes lasting for months was not widely predicted. Indeed, some astronomers had predicted that Jupiter would simply swallow up the tiny fragments without any noticeable effect at all. How wrong they were!

Fireball from the impact of SL-9 fragment G on July 18, 1994, as seen from Mount Stromlo Observatory, Australia.

The SL-9 impacts into Jupiter were a humbling reminder of the destructive power of small bodies traveling at very high speeds. They were also a public and media sensation, shared worldwide in near real time via a relatively new communication medium known as the Internet.

Earth-size blotches—scars from the impacts of fragments of comet SL-9—dot the midlatitudes of Jupiter south of the Great Red Spot in this July 1994 Hubble Space Telescope photo.