ASTEROID MATHILDE , viewed from a distance of only 1200 km by the Near-Earth Asteroid Rendezvous (NEAR) space probe, is heavily cratered from millions of years of rough travel through the solar system. By sensing the Doppler effect on radio waves returning to Earth from NEAR owing to the (very slight) gravitational tug between asteroid and spacecraft, Mathilde's mass could be estimated. Surprisingly, its density turns out to be not much greater than that of water, suggesting that it is not a solid object but rather a compacted pile of debris. NEAR's next assignment is to meet and orbit the asteroid Eros in 1999 (Science, 4 July 1997.)