Anthony Wesley

Small hit, big planet. Dark debris marks the recent impact of an asteroid or comet on Jupiter.

Jupiter's Been Hit!

Dick writes about Earth and planetary science for Science magazine.

A large object has slammed into Jupiter, leaving behind a giant black smudge that was first reported yesterday by an amateur astronomer. The find is only the second time in recorded history that scientists have glimpsed an impact scar in the atmosphere of a giant planet. "I never expected I'd get to see something like this," says astronomer Leigh Fletcher, a postdoc at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

Amateur astronomer Anthony Wesley had been taking routine images of Jupiter through his 37-centimeter telescope (pros would be on a 1000-centimeter instrument) in Murrumbateman, Australia, at about 11:30 p.m. local time, when he noticed something unusual: a dark spot several thousands of kilometers across rotating into view high in Jupiter's south polar region (see picture). Wesley had been about to end his observing run, and he initially considered passing the spot off as a typical dark polar storm. But he decided to keep at it, and in 15 minutes more he believed he was seeing something else entirely. (Read Wesley's observation report here.)

Wesley suspected an impact and soon contacted Fletcher and JPL astronomer Glenn Orton. As luck would have it, the duo had previously scheduled time on the NASA Infrared Telescope on Hawaii (remotely operated from JPL), so they took a closer look. They found the same distinctive infrared signature as Orton and others saw 15 years ago this week when the 21-plus fragments of disrupted comet Shoemaker-Levy 9 slammed into Jupiter one after the other (Science, 29 July 1994, p. 601). "We've been incredibly fortunate to have a talented amateur report this within hours," says Fletcher. Such amateurs "are doing some of the fundamental work of observing what's happening on Jupiter," adds Orton.

The impact "was a bit of a surprise," says astronomer Heidi Hammel of the Space Science Institute in Boulder, Colorado, who observed the 1994 impacts with the Hubble Space Telescope. "We all thought these were a little more rare." This one--a solitary event so far--looks like one of Shoemaker-Levy 9's medium-size impacts, says Hammel. How large the rocky asteroid or icy comet was is hard to estimate, says astronomer Harold Weaver of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. There was never any consensus on the size of the Shoemaker-Levy 9 objects, but this one might have been several hundred meters across--a kilometer at most--and traveling at tens of thousands of kilometers per hour.

If scientists are to retrieve any new information about jovian impacts, they'll have to be quick about it. Winds are tearing the black splotch apart even as astronomers race to submit their emergency proposals for telescope time--including time on the recently renovated Hubble Space Telescope.

Posted in Space