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Elongated Asteroids Believed To Be Weaker

Public Affairs Office
Lawrence Livermore National Laboratory

Contact: Stephen Wampler
Phone: (925) 423-3107
E-mail: wampler1@llnl.gov

FOR IMMEDIATE RELEASE: June 1, 1999

NR-99-05-03

ELONGATED ASTEROIDS BELIEVED TO BE WEAKER

LIVERMORE, Calif. -- Elongated or stretched asteroids in space are apparently
weaker than spherical asteroids, a scientist at the Department of Energy's
Lawrence Livermore National Laboratory will report today. Mark Hammergren,
a planetary scientist at Livermore's Institute of Geophysics and Planetary
Physics, came to this conclusion after studying nearly 850 asteroid
observations by dozens of astronomers.

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.

Lawrence Livermore National Laboratory is managed by the University of
California for the U.S. Department of Energy.


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