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.
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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