The Hubble space telescope has revealed a bizarre asteroid, U.S. astronomers say -- one trailing a tail of dust more than 600,000 miles long.

The object found soaring through the asteroid belt was first thought to be a comet because of its long, well-formed tail, they said, but has been confirmed as a rare phenomenon dubbed an "active" asteroid.

Its odd, X-shaped trailing debris field could be evidence the object, known as Asteroid P/2010 A2, collided head-on with another asteroid at some point in its past, while another theory holds it's breaking itself apart due to an unsustainable spin.

"It's hard to pin it down," astronomer Jayadev Rajagopal, with the National Optical Astronomy Observatory, told Discovery News.

"This one certainly looks like it's a collision," but there are a number of mechanisms that could explain its odd configuration, he said at the American Astronomical Society conference in Indianapolis this week

Only about 12 "active" asteroids have been observed to date, but A2 is in a class by itself, Rajagopal said.

"We are watching the death of an asteroid," he said. "This is the only one which is showing the event as it is happening."

How long A2 lasts -- it was discovered in January 2010 -- will depend on the size of its particles and how fast they are moving, he said.

"I expect it to hang around for quite a while."