Asteroid Yields Planetary Clues

Scientists say a loosely packed asteroid with the consistency of a melting snowman may help explain how planets formed in the early solar system: by gobbling up other objects.

The asteroid, a charcoal-black, irregularly shaped sphere 32 miles across that is known as Mathilde, orbits the sun between Mars and Jupiter and is made of rock and dust so porous that it seems to swallow up meteors like a pillow absorbing a punch.

Because asteroids like Mathilde are believed to be primordial leftovers from the early solar system, the research could help astronomers understand how planets coalesced from swarms of smaller bodies.

Primitive asteroids like Mathilde might have scooped up material "like a celestial Pac-Man," Erik Asphaug of the University of California at Santa Cruz wrote in a commentary accompanying the study.

The research also suggests it may be more difficult than Hollywood might imagine to blast an asteroid like Mathilde off a collision course with Earth.

The study was published in the latest issue of the journal Nature. It was conducted by Boeing Co. scientists Kevin R. Housen and researchers from the University of Washington.

The researchers tried to experimentally recreate Mathilde's exotic surface to find out why its meteor craters were dent-like and lacked the debris rings that encircle meteor craters seen on other planets and moons.

In their experiments, Housen and colleagues filled a rapidly spinning centrifuge with 120 pounds of sand and pearlite. As the centrifuge pushed the porous mixture onto its walls, they fired projectiles into it and were able to create the same type of compacted craters that mark Mathilde.

Asphaug also said the research suggests that knocking an Earth-bound asteroid off course with an explosion might falter because asteroids like Mathilde which dominate the outer asteroid belt between Mars and Jupiter seem to easily absorb a blow.

Clark R. Chapman, an asteroid expert with the Southwest Research Institute in Boulder, Colo., said the findings could mean landing a robot spacecraft on a similar, porous asteroid would be tricky.

"We may need snowshoes to land on such a surface," he said.