Tryst in space: Craft, asteroid rendezvous

August 16, 2002 at 2:19 pm

Even NASA sometimes gets a second chance at love. On Feb. 14, a spacecraft 256 million kilometers from Earth finally cozied up to its intended, a peanut-shaped lump of rock called 433 Eros. After 4 years of relentless pursuit through the solar system, including a flubbed embrace a year ago (SN: 1/2/99, p. 7), NASA’s NEAR spacecraft slipped into orbit about the asteroid. This marks the first time that a satellite has rendezvoused with a tiny body. Eros has about twice the area of Manhattan.

Composite image of the asteroid 433 Eros taken by NEAR shortly after the craft entered orbit shows crater (center). NEAR Science Team/APL, NASA

“Today is Valentine’s Day to most people, but to me, it’s Christmas Eve,” NEAR project scientist Andrew F. Cheng told reporters at a briefing held Monday at the Johns Hopkins Applied Physics Laboratory in Laurel, Md. “I’m watching all the presents pile under the tree.” If all goes according to plan during its yearlong visit, NEAR will map the gravitational field of Eros, determine if the body has a magnetic field, and infer whether the object is solid or a pile of sand. By spiraling in close enough to take the asteroid’s fingerprints—detailed infrared, X-ray, and gamma-ray spectra—the craft may settle an enduring mystery. Astronomers have conflicting evidence on whether asteroids such as Eros, classified as S types, are the parent bodies of ordinary chondrites, the most common meteorites that fall to Earth.

NEAR may not shed much light on such questions until later in the mission, when it zooms within a few kilometers of the asteroid’s surface. Ultimately, it may come within 0.5 km of Eros and, for a grand finale, touch down on the rock.

Even at its current distance, about 300 km from Eros, the craft is revealing a complex, heavily cratered surface, notes NEAR scientist Joseph Veverka of Cornell University. The pockmarked facade, as well as the lack of sharp edges, suggests that the surface of Eros is billions of years old, he notes.

The satellite recorded layers of material along crater walls. The layering could indicate that Eros doesn’t have a uniform composition, Veverka says. The asteroid might have undergone melting and cooling that separated heavier minerals from lighter ones, perhaps when it was part of a larger chunk of rock. The layers, however, may simply be a sprinkling of fine particles that settled back onto the surface after being splashed up by projectiles slamming into Eros.

Peering inside one crater, about 5 km wide and 800 meters deep, NEAR found a 50-meter-wide boulder that appears to have rolled downhill. Since Eros’ gravity is only one-thousandth as strong as Earth’s, the boulder may have traveled for days before reaching the crater’s bottom.

Infrared spectra of Eros’ northern face, taken Feb. 13 just before NEAR began orbiting, reveal two absorption bands showing different ratios of the minerals olivine and pyroxene, Veverka told Science News. This is the first time that scientists have had such information about an asteroid, he says. By correlating these bands with craters and other geological features, researchers hope to determine if Eros truly varies in composition.

Ridges and linear fractures suggest that Eros is mostly solid rock, Veverka says. Michael J. Gaffey of Rensselaer Polytechnic Institute in Troy, N.Y., argues that it could be an amalgam of rocky fragments and fluffier material.