Not as solid as she looks. Gravity data suggest Mathilde's interior is disrupted.

Slimming Mathilde

LAUREL, MARYLANDAdorned with incredibly deep, shadowed craters, Mathilde may look like your average asteroid. But to the surprise of researchers, she's a lightweight: The 52-kilometer asteroid has only a third of its expected mass. The discovery, reported at a press conference here today, lends support to the claim that most asteroids are heaps of rocks loosely held together only by their own gravity. Eons of banging about in the asteroid belt, it seems, have reduced Mathilde and perhaps most asteroids to piles of flying rubble.

Mathilde, the largest asteroid to date to be viewed up close, was imaged by the Near Earth Asteroid Rendezvous (NEAR) spacecraft, which is on its way to an encounter with the asteroid Eros in 1999. The spacecraft carries no instrument that could directly probe an asteroid's interior, but NEAR's radio managed to do so indirectly. As the spacecraft passed within 1200 kilometers of Mathilde, the asteroid's feeble gravity slightly deflected NEAR's path. By monitoring the Doppler frequency shift in the spacecraft's radio signal during the flyby, NEAR researchers could infer that Mathilde slowed down the spacecraft by a third of a millimeter per second—about the speed of a sluggish ant, said NEAR team member Donald Yeomans of the Jet Propulsion Laboratory (JPL) in Pasadena, California.

From that minuscule slowing, Yeomans calculated a mass for Mathilde of 1017 kilograms, or a millionth the mass of Earth's moon. Assuming an average diameter of 52 kilometers (a preliminary value determined from NEAR images), that yields a density of just 1.3 grams per cubic centimeter—not much more than water. But meteorites thought to have been chipped off this type of common asteroid are typically twice as dense, 2.6 grams per cubic centimeter. "We've got an object considerably lighter than we thought," says celestial mechanicist Yeomans. "If it were any lighter, it could float."

Although Mathilde's apparent low density may rise somewhat as researchers take better account of its irregular shape, it's unlikely that the final estimate will approach the density of solid rock, notes asteroid specialist Alan Harris of JPL. The most likely explanation, he notes, is that Mathilde is a conglomeration of blocks, boulders, and loosely compacted debris. There have been earlier signs that asteroids are rubble piles, but this is the most direct evidence yet, says Harris. And that bears on problems as diverse as the collision history of asteroids and strategies for protecting Earth from an asteroid threatening to strike.