In 1994, the comet Shoemaker-Levy 9 collided with Jupiter, creating the largest collision ever observed between two Solar System objects. The impact raised awareness of the threat of asteroid impacts on Earth. The comet was torn apart in 1992 and on a collision course with Jupiter. The fragments collided with Jupiter over five days, producing large fireballs and dark spots visible for months. The impact has made it easier for people to take the threat of asteroid impacts seriously.
Shoemaker-Levy 9 (SL9) was a comet that collided with Jupiter in 1994. The impact was one of the most-watched astronomical events of the century and was the largest collision ever observed between two Solar System objects. The high public visibility of the collision helped spread the idea that Earth was vulnerable to impacts from space.
Shoemaker-Levy was originally a short-period comet, with a nucleus about three miles (five kilometers) in diameter. Over the course of the 20th century, it was captured by Jupiter’s powerful gravitational field, entering a long free orbit around Jupiter. Eight months before Shoemaker-Levy’s discovery, in July 1992, the comet passed so close to Jupiter that it was torn apart in a train of fragments. When the comet was discovered in March 1993 by the Shoemakers and David Levy, its strange appearance marked it as unusual, and astronomers quickly discovered it was on a collision course with Jupiter.
The Shoemaker-Levy fragments ranged in size, from a few hundred feet (one hundred meters) to a mile (two kilometers) in diameter. They were expected to collide with Jupiter over a period of five days, and although the impacts would have been on the side of Jupiter facing away from Earth, many astronomers still watched the giant planet in hopes of seeing something. The Hubble Space Telescope, the Galileo spacecraft and the ROSAT X-ray observatory have all turned to Jupiter to see the aftermath of the collisions.
When the comet fragments began colliding with Jupiter on July 16, 1994, the size and speed of the impacts produced large, incandescent fireballs, which rose above Jupiter’s limb and were seen by ground-based telescopes. The fireballs were so hot that they glowed for over thirty seconds after the impacts, and the explosions produced huge dark spots in Jupiter’s atmosphere, which were visible for months afterward. The astronomers subsequently analyzed the chemical composition of the spots, hoping to glean information about the structure of Jupiter’s atmosphere.
The excitement surrounding the impact has made it easier for people to take the threat of an asteroid impact seriously. Collisions like this are not at all unusual, taking place on Jupiter roughly once every thousand years. Voyager 2 even found long chains of craters on Jupiter’s moons Callisto and Ganymede, presumably caused by other destroyed comets. Although Earth impacts are rarer due to Earth’s smaller size and mass, catastrophic collisions have occurred in the past, most notably the Chicxulub impactor which is thought to have caused the extinction of the dinosaurs.
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