Comet Built Like an Asteroid, Scientists Find

Anne Minard
for National Geographic News
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.

The University of Washington's Don Brownlee, principal Stardust investigator, said Wild 2 is decidedly a comet.

The whole point of the Stardust mission was to sample a comet, he said. There is no doubt about Wild 2's identity.

"It's coming apart like crazy at its present distance," he said, explaining that Wild 2 hails from the outer solar system—despite its composition.

If it had always been in the inner solar system, it would have lost so much dust and ice by now that it probably wouldn't have survived to the present day, scientists said.

Blurring the Boundary

The new findings—which appear in the journal Science—highlight a growing body of gray area between comets and asteroids.

The conventional theory held that asteroids formed from rock in the warmer, inner areas of the solar system, and comets came from rock and ice in the colder, more distant regions.

Several theories are emerging to explain increasing recent findings of comet-like asteroids and asteroid-like comets.

Newer theories propose that some material was formed in the hot inner solar system and was then transported outward, either by fierce solar winds, physical phenomena such as eddies, or gravitational instabilities.

That would allow for some Kuiper Belt objects to contain inner solar system materials and others to contain pristine samples that predate the solar system.

Go Fish

The surprise at finding asteroid-like materials in Wild 2 was underscored by a bit of disappointment, even in the published paper.

"81P/Wild 2 has been widely anticipated to be a reservoir of presolar material,
including stardust, cryogenically preserved since the accretion of the planets," the authors wrote, using the comet's full name.

Adds the project's Brownlee: "That's one of the reasons this [craft] was called Stardust."

The researchers thought they would detect ancient stardust that collapsed to form our own sun and planets.

Astronomers know pristine comets are out there, Ishii says. Indeed, airplanes in our own stratosphere have collected their dust.

"Wild 2 is more on the asteroid end of the spectrum—more asteroid-like than cosmically primitive comets," she said.

"We know there have to be comets that are on the more pristine end of it," she said. "We'd sure like to go out to one of those."

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