Surprise! That Comet Is an Asteroid, Sort Of

Two years ago, NASA's Stardust mission returned to Earth with a payload of comet dust, harvested from the small Wild 2 Comet. Researchers are still studying what the probe brought back, and now say the little comet barely looks like a comet at all.

By early last year, they'd realized that a good portion of the dust returned by Stardust seemed to have formed close to our own young Sun, rather than originating with other stars or elsewhere in the universe. That was a first surprise, since comets are generally believed to be a kind of time capsule, holding stardust and other ancient material predating the creation of our own solar system.

Now researchers say that the Wild 2 sample seems to be strangely devoid of any of this pre-solar material that they expected. As such, it looks much more like a piece of rock coming from the Asteroid Belt, rather than a conventional comet.

"The material is a lot less primitive and more altered than materials we have gathered through high altitude capture in our own stratosphere from a variety of comets," said (Lawrence Livermore National Laboratory's) Hope Ishii. "As a whole, the samples look more asteroidal than cometary."

So was there a mistake? Is Wild 2 just an asteroid that strayed into the cometary Kuiper Belt sometime early in the solar system's history, and has fooled scientists since. Turns out the answer isn't so simple.

Wild 2 gets to keep its designation as a comet, because it has a tail of vaporizing ices. But these findings show that researchers can't draw a black-and-white line between comets and asteroids, Ishii says. In fact, there may be a spectrum, or continuum of objects that share traits once thought to define one or the other.

A paper on the research is being published in the Jan. 25 edition of Science.