A surprising encounter of the NEAR kind.

An analysis of pictures and data obtained during last summer's flyby of the asteroid 253 Mathilde reveals that this carbon-rich, heavily-cratered body is only about half as dense as rocky asteroids. This finding--the first accurate measurement of the density of a carbonaceous asteroid--could prompt a reassessment of the geophysical history of these wandering planetesimals.

The Near Earth Asteroid Rendezvous (NEAR) spacecraft skimmed within 1,212 kilometers of Mathilde on June 27, 1997. An initial analysis of the data hinted that the dark body, which reflects light only about half as well as a charcoal briquette, has a low density (SN: 7/12/97, p. 29).

Now, two reports in the Dec. 19, 1997 SCIENCE indicate that the asteroid is highly porous, suggesting that it either was formed from loosely packed fragments or has been pulverized into a "rubble pile" by repeated impacts with other celestial bodies.

One study, led by Donald K. Yeomans of NASA's Jet Propulsion Laboratory in Pasadena, Calif., analyzed the gravitational pull of the asteroid on the NEAR spacecraft as it sped along at about 10 km per second. By measuring the slight acceleration of the craft as it approached Mathilde and the deceleration after it passed, the scientists determined the mass of the asteroid to be about 110 trillion tons.

NEAR's optical measurements, part of a study led by Joseph F. Veverka of Cornell University, reveal Mathilde to be a potato-shaped body measuring some 66 km by 48 km by 46 kin. Together, the two teams' measurements indicate that the asteroid's density is only about 1.3 times that of water.

A large amount of water ice preserved within Mathilde would help explain its low density, but Earth-based spectroscopic measurements of the asteroid show that there is no water locked within it.

During its flyby, NEAR observed about half of the asteroid's surface and spotted five craters with diameters of 19 km to 33 km. The largest and best-imaged of the craters is wider than, the asteroid's mean radius, says Veverka. That crater, whose bottom remained in shadow throughout the 25-minute flyby, may be 5 or 6 km deep, he adds.

Although Mathilde could have been reduced to a pile of rubble by a long history of collisions with other asteroids, it's also possible that its low density is primordial, Veverka says. Mathilde's small size and low gravity create a pressure of less than 2 Earth atmospheres at its center, which is not enough to compress loose materials into a solid.

NEAR's close encounter with Mathilde took place halfway through a 3-year journey to the asteroid 433 Eros, which the spacecraft will orbit for 13 months beginning in early 1999. On Jan. 23, NEAR will swing by Earth and get a gravitationally assisted course correction, says Robert W. Farquhar, mission director at Johns Hopkins University's Applied Physics Laboratory in Laurel, Md. During the maneuver, NEAR is scheduled to pass about 330 miles above the border between Iraq and Kuwait at 10:23 a.m. local time.