Hammergren will present his results during the opening day of the Centennial meeting of the American Astronomical Society in Chicago, Ill. During his study of asteroid observations, Hammergren found that very elongated asteroids are never seen to be rotating faster than once every four hours. In contrast, more spherical asteroids can rotate as fast as once every 2.3 hours.

Such evidence, Hammergren said, provides strong support for the theory that most asteroids are not tightly-bonded solid chunks of rock, but rather are loose aggregates of material, sometimes called "rubble piles."

"Solid asteroids can rotate as fast as they want and still hold together, but if rubble piles rotate too fast, they'll fly to pieces," Hammergren said. "It makes sense that more elongated asteroids, which have weaker gravity on their ends, would be even more prone to break up."

Based on his research, Hammergren theorizes that "rubble pile" asteroids in space are governed by the same processes that lend stability to piles of sand on Earth.

"In effect, loose and weak materials in rubble piles have the ability to support large surface features on asteroids, just as loose sand and weak dirt can support huge mountains on Earth," he said.

Hammergren also offered two other findings based on his theory of rubble pile stability.

One is that changes in a rubble-pile asteroid's shape would probably not occur gradually, but would instead happen cataclysmically, as a series of massive landslides.

The other is that if such landslides occur on the surfaces of rapidly- rotating asteroids, parts of the asteroids' surfaces may be thrown off into space.

Such break-ups provide a possible explanation for the formation of asteroid moons, which orbit the original asteroid similar to the way the moon orbits the Earth.