Comet Built Like an Asteroid, Scientists Find

January 24, 2008

Astronomers were thrilled to finally get their hands on a real comet when NASA's Stardust mission returned to Earth in 2006.

Now studies of samples the craft brought back from the comet Wild 2 are causing some of those same astronomers to reconsider what comets actually are. (Read about initial findings from the mission.)

For instance, although Wild 2 orbits like a comet, it's built like an asteroid, scientists said Thursday.

A chemical analysis of the Stardust samples resembled objects from the inner solar system's asteroid belt instead of the pristine and ancient materials expected to be deep-frozen in the much more distant Kuiper Belt, beyond Neptune, said Hope Ishii, the California-based Lawrence Livermore National Laboratory physicist who led the research.

"The first surprise was that we found inner solar system materials, and the second surprise was that we didn't find outer solar system materials," Ishii said.

(Related story: "Hot" Rocks Found in Icy Comet [March 14, 2006])

For some astronomers, the results come as a relief that a handful of prevailing computer models aren't seriously flawed. The models had been indicating a major shake-up in the formation of the solar system that would have scattered materials far and wide—causing, for instance, inner solar system materials to reach the outer solar system.

"It's clear that material must have moved around the [developing] planetary nebula quite a bit," said Will Grundy, an astronomer at Lowell Observatory in Flagstaff, Arizona.

Grundy was not affiliated with the research.

Clearly a Comet

Wild 2 is named after Swiss astronomer Paul Wild (pronounced "Vilt"), who discovered the comet in January 1978.

Ever since Jupiter's gravity slung the comet toward the sun three decades ago, Wild 2's three-mile-wide (five-kilometer-wide) nucleus has orbited the sun every 6.39 years on an elliptical path. The new orbit brings it closer to the sun than Mars on the close end, and beyond Jupiter's orbit on the far end.

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