In a few weeks, Earthbound astronomers will have a chance to witness an unprecedented celestial display when the remains of an icy comet, born in the outer reaches of the solar system, crashes into the planet Jupiter and explodes with the force of 250,000 H-bombs -- or about 500 billion tons of TNT. Though the fireworks won't be directly visible from Earth -- the impacts will occur on the side facing away from us -- scientists hope to gather important information about the physics of such cosmic collisions.
The comet, named Shoemaker-Levy 9 after its co-discoverers, was snatched from its periodic path around the sun by Jupiter's strong gravitational field sometime in the 1970s. Since then it has been spiraling ever closer to the giant planet in an orbit that brings it nearly to Jupiter's turbulent cloud tops approximately every two years. During the last such close encounter, in July 1992, the interloper was literally torn to pieces by powerful tidal forces generated by Jupiter's gravity; when the dozen or so fragments that remain again approach on July 16, they will crash into the planet one by one over the span of about a week.
Astronomers have never had an opportunity to observe a predicted collision in our solar system. To make the most of this opportunity, they are turning their most sophisticated instruments skyward. Giant infrared telescopes owned by NASA and the United Kingdom atop Mauna Kea in Hawaii will devote major blocks of observing time to the event. The Kuiper Airborne Observatory, a converted 707 jet airliner that carries a 36-inch telescope for observations above most of the Earth's atmosphere, also will join the hunt. Even the big guns, like the 200-inch reflecting telescope at Mt. Palomar Observatory and the telescopes of Kitt Peak National Observatory, are being readied for impact week.
Alongside these mighty windows on the heavens will be several space-based eyes. In addition to the orbiting Hubble Space Telescope, the event will be monitored by the Galileo probe, which currently is some 150 million miles from Jupiter, and by the venerable Voyager 2, now more than 4 billion miles on the opposite side of the planet from Earth. Despite their great distance, both these probes have the tremendous advantage of being in position to observe the impact directly.
No one knows exactly what will happen when the comet hits its mark -- it could be the splash of the century or a dud of cosmic proportions. Either way, scientists are confident they have much to learn from an impending catastrophe that is also a once-in-a-lifetime scientific opportunity.