Asteroids and meteorites: a new link.

Astronomers agree that the vast majority of meteorites that fall to Earth represent fragments of asteroids, rocky bodies that reside in a belt between Mars and Jupiter. But it isn't always easy to determine which asteroid group gave rise to a easy to determine which asteroid group gave rise to a particular set of meteorites or how these fragments could have traveled to Earth.

Consider the meteorite class called basaltic achondrites, which make up about 6 percent of all meteorites recovered on Earth and which formed from once-molten material that originated on or under the surface of a small body.

Only one known asteroid, 4 Vesta, has a surface composition similar to that of basaltic achondrites, researchers showed in the 1970s. Thus, 4 Vesta seems likely to be the parent of this group of meteorites. But that poses a problem, because 4 Vesta resides in a region of the asteroid belt from which material can't easily reach the inner solar system and Earth.

Now, Richard P. Binzel of the Massachusetts Institute of Technology and graduate student Shui Xu say they have solved the puzzle. In recording the spectra of eight small asteroids near Vesta, they found that each has a composition similar to Vesta's. This suggests the asteroids, each measuring about 7 kilometers across, formed when another asteroid collided with Vesta, gouging out fragments that escaped Vesta's gravitational field with a speed as great as 500 meters per second.

Binzel and Xu calculate that if the eight asteroids near Vesta had substantial velocities as they exited Vesta, then smaller fragments, measuring only about a kilometer in width, would have left Vesta at speeds exceeding 1,000 meters per second. Such speeds, forbidden by previous models, are great enough for the fragments to reach locations in the asteroid belt from which material could travel to Earth. Binzel and Xu conclude that 4 Vesta is indeed the parent body of most basaltic achondrites. They reported their work last week in Munich, Germany, at a meeting of the American Astronomical Society.