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 . Click on the
“Crushing blow (11 November 1999)”.]