So Much Rock, So Little Gravity

Now that scientists have gotten a good look at the asteroid 433 Eros (right), they're more confused than ever: gravity on this tiny, 21-mile-long body should be so low that a ball thrown from the surface would escape into space. But pictures sent back by NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft show that Eros--which has probably been banging around our solar system since its beginning--is home to lots of little craters and big piles of rubble. "Intuition and calculation tell you that most of the debris produced in a collision would have escaped," says Joseph Veverka of Cornell University," but the surface is full of it."

Veverka, principal investigator on NEAR's multi-spectral imager and infrared spectrometer, describes the Eros mystery in today's issue of Science, which includes three additional reports from other NEAR teams studying the asteroid. "We simply don't understand this," Veverka says, offering what he thinks are two "equally bizarre" possibilities. "One is that ... somehow the debris gets thrown out at very low speeds" and so cannot escape even Eros's gravitational pull. "Or the ejected material ends up in the same orbit as Eros, and over time the asteroid runs back into its own debris and gathers it up."