Big Splat on a Small Planet

BOSTON--Astronomers are used to seeing plenty of planetary mayhem, but the discovery of a 450-kilometer impact crater on the asteroid Vesta, which is only 525 kilometers in diameter, has them wowed. The huge impact could have flung pieces of Vesta on the first step of a journey toward Earth, explaining the origin of one common type of meteorite.

Peter Thomas of Cornell University and his colleagues reported at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society here this week that repeated imaging of Vesta using the Hubble Space Telescope had revealed a jagged bite out of the southern hemisphere of the asteroid. By recording the shape of Vesta in silhouette as it rotated, Thomas and his colleagues mapped a classic impact crater 450 kilometers across and 8 kilometers deep with a 13-kilometer-tall peak in the center.

Such a crater could help explain how bits of Vesta have ended up on Earth. So-called basaltic achondrite meteorites look very much like small, 5-kilometer asteroids near the zone in the asteroid belt where Jupiter can gravitationally fling rocks toward Earth. The asteroids, in turn, resemble Vesta itself. But astronomers had not been able to come up with a way to get the 5-kilometer chunks off Vesta. An impact producing a 450-kilometer crater on a body with a gravitational grasp as weak as Vesta's should hurl chunks of the asteroid several kilometers in size into space, according to the group.

Meteoriticist Michael Gaffey of Rensselaer Polytechnic Institute in Troy, New York, says the big impact completes a plausible connection between Vesta and its presumed meteorites. Proving the connection, however, may require sending a probe to Vesta, he adds. That's a project Gaffey and others will be proposing as part of NASA's smaller, faster, cheaper approach to solar system exploration.

Posted in Space