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A comet is shown.

The comet-like body P/2010 A2 (LINEAR), as seen by the Spacewatch Telescope in Arizona.

Photograph courtesy Robert McMillan and Jim Scotti, University of Arizona

Brian Handwerk

for National Geographic News

Published January 21, 2010

A curious, comet-like object recently found in pictures from a ground-based telescope might actually be fallout from a high-speed asteroid collision, planetary scientists report.

If these suspicions are confirmed, the object would represent the first time astronomers have witnessed the immediate aftermath of such a cosmic smashup.

Dubbed P/2010 A2 (LINEAR), the fuzzy, tailed mystery object is about 130,000 miles (210,000 kilometers) to 190,000 miles (305,000 kilometers) long, stretching across part of our solar system's main asteroid belt.

The belt contains thousands of asteroids that orbit between Mars and Jupiter, some 250 million miles (402 million kilometers) from Earth.

It's believed most comets come from the cold, distant reaches of the solar system and travel on long, elliptical orbits, which keep the icy bodies far from the sun most of the time.

As a comet nears the sun, heat turns the comet's volatile ices into gases, and solar radiation pushes on those gases to create a tail. (Related: "New Comet Found, Vaporized.")

But the newfound object suddenly appeared within the warmer asteroid belt and may even have originated there, puzzling astronomers.

"We're still trying to really figure out what it is," said University of Arizona planetary scientist Jim Scotti, who is part of one of the teams observing the object from the Kitt Peak National Observatory outside Tucson.

Explosive Asteroid Impact?

The object's oddities have some astronomers, including Scotti, thinking that the bright "tail" is actually a debris field created just after a small asteroid had smashed into a larger one. (See pictures of comets and asteroids.)

A 650-foot-wide (200-meter-wide) space rock apparently still sitting near the object's head could be one of the collision victims.

Odds are that the smaller impactor would have been only a few meters across, since asteroids of this size are far more common in the main belt.

If a collision occured, it's most likely that the space rocks didn't meet head on, Scotti said. Still, the impact speed could have ranged from 0.6 to 6 miles (1 to 10 kilometers) a second—fast enough to create a debris field visible from Earth.

Asteroid Scars

Astronomers have yet to witness an actual asteroid collision. But there's plenty of evidence to suggest that the smashups happen all the time.

For instance, all known asteroids display telltale scars in the forms of impact craters. And some groups of asteroids are thought to have been born from collisions that fragmented their original "parents."

(Related: "Comet Swarm Delivered Earth's Oceans?")

"With such evidence of collisions in the asteroid belt, it's not surprising that eventually we will see one," Scotti said.

The trick is, "I'm not sure we know what an asteroid collision really would look like in detail," he admitted.

"We have some ideas, but I'm not sure anyone has really sat down and modeled the size and velocity of the debris, or where all that debris goes and how long it would remain potentially observable."

For now, scientists can only wait and watch to see if P/2010 A2 (LINEAR) slowly dissipates, like debris from an explosion, or continues to act like a comet—which would pose a new round of puzzling questions.

A rare handful of comet-like bodies are known to orbit in the main asteroid belt. But if P/2010 A2 (LINEAR) is actually a comet, how did it conserve its water ice so close to the sun for some 4.5 billion years—roughly the age of the solar system—only to begin releasing gases now due to some unseen event?

"That's a long time to bake an object," Scotti said.

"It's hard to imagine how an object would maintain a reservoir of volatiles that it could use to suddenly start producing a tail. But you know, stranger things have happened."

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