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Lessons from Jupiter's black eye

I. de Pater (Berkeley) / H. Hammel (SSI) / T. Rector (U. of AK-Anchorage) / Gemini Obs.
This mid-infrared image of the impact site on Jupiter was captured by the Gemini

North telescope in Hawaii. The yellow arrow points to the color-coded "bruise."

Astronomers are continuing to watch the Great Black Spot on Jupiter to figure out how it was made and what the aftermath tells us about Jupiter's makeup. Over the next month, you can expect big telescopes to gather data on the chemical composition of the spot. Those observations may tell scientists whether the "black eye" was caused by a comet like Shoemaker-Levy 9, which bruised Jupiter 15 years ago, or an asteroid, or perhaps some weird internal process.

In the meantime, experts are working out the implications of Jupiter's smackdown for our own planet. Former Apollo astronaut Rusty Schweickart says last month's event could help re-energize his international campaign to keep a closer watch on potential cosmic threats.

Scientists say there's little chance that a comet will zoom in from the solar system's far reaches and smack Earth, as was the case for Shoemaker-Levy 9's encounter with Jupiter. However, the chances of a catastrophic asteroid impact are significantly greater: That kind of thing likely happens roughly every 1 million years, astronomers say.

An asteroid strike on the scale of the one that killed off the dinosaurs 65 million years ago - say, bigger than a kilometer (half a mile) wide - would set off a deep wave of extinction. So far, astronomers have cataloged about 800 of such near-Earth objects, none of which appear to pose a serious threat. Schweickart points out that even a smaller blast could stir up a "cosmic Katrina" of hurricane proportions, however. We're just starting to tally the somewhat larger number of somewhat smaller cosmic killers. 

For years, Schweickart and his colleagues at the B612 Foundation and the Association of Space Explorers have been urging the United Nations to take a more formal approach to assessing cosmic threats. Because the effects of an impact could be global, the deliberations about what to do in case an impact should have an international scope as well.

The first step is to identify potential threats: Just in the past week, Schweickart and others sent a letter urging the Australian government to restart funding for a near-Earth object search in the Southern Hemisphere. The government cut support to Spaceguard Australia in 1996, and since then asteroid-watchers have worried about the huge "blind spot" in their coverage area. (Fortunately, Australian amateur astronomers such as Anthony Wesley, who first spotted the Jupiter impact, have helped fill the gap.)

"Australia is arguably the most advanced country in the hemisphere," Philip Chapman, NASA's first Australian-born astronaut, was quoted as saying in The Australian. "Failure to contribute to the international effort is grotesquely irresponsible."

Schweickart told me in a follow-up e-mail that the jury is still out on the precise cause of the latest Jupiter impact:

"I think that the comet claim is simply a default position and perhaps a carryover from the Shoemaker-Levy 9 comet impact.  Since no one saw it prior to impact, who knows whether it was a comet or asteroid.  In my view, it's very much more likely to be an asteroid impact, simply due to the much higher population.

"As to why no one saw it prior to impact or knew it was going to impact… that’s pretty easy.  Any asteroids that don’t come near Earth are difficult to see.  The likelihood of spotting a 1-kilometer (this was probably not larger than that… most likely smaller) asteroid which circulates from the main belt out to halfway between Jupiter and Saturn is vanishingly small. The only way we’d have to know about this ahead is if it happened to be a near-Earth asteroid with an aphelion [maximum distance from the sun] greater than Jupiter's distance. That's a very small percentage of the near-Earth asteroid population.

"Still… any evidence of current impacts on any other body help to emphasize that it's only a matter of time till it's our turn. So the more the merrier!"

To keep up with the status of the Jupiter impact investigation, check in with this Web page at the University of Central Florida. And to weigh in on the wider question of Earth's vulnerability to cosmic impacts, just leave a comment below.

Update for 5:38 p.m. ET Aug. 4: passes along a fresh picture of the Great Black Spot from Australian discoverer Anthony Wesley, indicating that Jupiter's wind shear has already stretched the spot into a not-quite-as-great black line.

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