Plans change for NEAR visit to an asteroid

Science News, Jan 2, 1999 by R. Cowen

Fragments of asteroids have bombarded Earth ever since the birth of the solar system. Now, an emissary from Earth is about to embark on the first extended visit to an asteroid. Loss of radio contact during a crucial maneuver will delay the mission for at least a month, however.

According to the original plan, on Jan. 10 a spacecraft would have met its intended mate, a near-Earth asteroid--the second largest--known as 433 Eros. Entering orbit about 1,000 kilometers from the rock and eventually coming within a few kilometers of its surface, the Near Earth Asteroid Rendezvous (NEAR) craft would determine the composition, shape, and density of Eros to an accuracy unprecedented for any asteroid. A suite of six instruments is poised to take millions of images and measurements over the asteroid's entire surface.

Late on Dec. 20, during a time when NEAR was to fire its engine and head for Eros, the craft lost contact with Earth. Contact was reestablished a day later, but the earliest that NEAR could now enter orbit around Eros is Feb. 6--if it has enough fuel left to complete the firing maneuver. Alternatively, the craft could swing by Earth for a gravitational kick and attempt a meeting with Eros in 2002.

Planetary scientists expect that NEAR, during its year-long mission, will reveal the answer to several riddles. Recent observations have suggested that many asteroids--including 253 Mathilde, briefly visited by NEAR last June (SN: 7/12/97, p. 29)--are not solid objects but porous amalgams of rocky debris, loosely bound by gravity. This year's observations of Eros should determine whether or not this potato-shaped body is a solid chunk of rock.

In an article recently posted on the Internet, William Bottke of Cornell University and his colleagues argue that Eros is probably a pile of rubble. The researchers note that the asteroid is highly elongated. The team's computer simulations suggest that Eros' orbit once took it close enough to Venus or Earth to experience a gravity effect known as a tidal force, the same force that raises tides on Earth's oceans. The asteroid's rapid spin also suggests that the body once ventured close to Earth or Venus.

A tidal force occurs when a massive body exerts significantly different forces on the near and far sides of an extended object. A solid object can usually withstand tidal forces, but a liquid or a rubble pile would deform under the stress. Bottke and his colleagues suggest that this is how Eros got its elongated shape.

Other researchers are more skeptical. NEAR scientist Joseph Veverka of Cornell suggests past collisions could account for Eros' elongated appearance. Moreover, he suspects Eros is a fragment blasted off a much larger body. A mere rubble pile could not have survived such an impact.

NEAR scientists say they should have a rough estimate of the asteroid's density shortly after the craft begins orbiting Eros. Moreover, they expect to identify surface minerals by analyzing reflected light. If the overall density is considerably less than that of the surface materials, it's likely that Eros has numerous interior voids between chunks of debris gently stuck together by gravity.

A second puzzle concerns the descendants of Eros and other members of the so-called S class, the most common asteroid type. Researchers have long argued that ordinary chondrites, primitive chunks of solar system material that are the most common meteorites to fall to Earth, are chips off the S-asteroid block.

Although the color and mineral composition of S asteroids are similar to those of ordinary chondrites, the match is not exact. Scientists have proposed that bombardment by space debris has weathered S asteroids, altering their original surface composition, which persists in meteorites. High-resolution spectra of Eros may determine if S asteroids really are primitive and the parents of ordinary chondrites.