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Crushing Blow

Nature News Service

Press Release: November 11, 1999

Crushing Blow 

In June 1997, the Near Earth Asteroid Rendezvous spacecraft flew past
the asteroid 253 Mathilde, sending back images of a crater-battered world
about 52 kilometres in diameter, with five giant craters each over 20 km
in diameter. Craters of such a size are generally surrounded by blankets of
ejected material several kilometres deep, but on Mathilde there are no
signs of such material.

The asteroid's unusually low density is thought to be part of the
explanation for this lack of ejected material, and this is now confirmed
by hypervelocity impact experiments carried out by Kevin R. Housen
of The Boeing Company, Seattle, Washington, and colleagues. These
experiments -- which involve firing projectiles at very high velocities
at samples of a porous material -- suggest that the craters are produced
by compaction, rather than excavation. Such a compaction process would
result in relatively little ejected matter being lost into space, explaining
why material from highly porous asteroids is a rarity in meteorites
reaching Earth.

Erik Asphaug of the University of California at Santa Cruz, California
discusses these findings in an accompanying News and Views article.

[NOTE: The News and Views article and full text of the paper are available
at http://www.nature.com/cgi-bin/wbsp-home.cgi .  Click on the item
"Crushing blow (11 November 1999)".]

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