Rendezvous gets more personal with Eros

Last week, as it ventured closer to a space rock than any satellite has before, the Near Earth Asteroid Rendezvous (NEAR)–Shoemaker mission took the sharpest images ever of an asteroid.

A 350-m-wide patch of Eros viewed Oct. 26, when NEAR was 7 km from the surface; smallest rocks are about 1.4 m across. JHU/APL

Nearly 9 months after entering orbit around the near-Earth asteroid 433 Eros, NEAR swooped within 5.3 kilometers of the potato-shape rock—as close as a commuter plane flies over Earth. From that altitude, the craft discerned details smaller than a yardstick across.

Researchers intended the 30-minute close encounter on Oct. 26 as a practice run for early next year, when the craft will orbit even closer to Eros and attempt to land on the asteroid.

Last week’s maneuver also provided a glimpse of small-scale processes on Eros, says Joseph Veverka of Cornell University. Dropping down from a 50-km orbit, NEAR revealed tiny rocks as well as small scars from impacts that may have occurred just tens of millions of years ago. Larger impacts are rarer and are likely to be much older, Veverka notes.

The extreme close-ups show a surface littered with rocks as well as finer debris. Some of the debris has settled at the bottoms of craters. Veverka speculates that seismic shaking from more recent impacts spread the debris.

Angular shards of rock—material presumably excavated when space debris pummeled the asteroid—add to the evidence that Eros is a solid chunk rather than a pile of rubble. Gravity maps constructed from changes in the speed of the craft as it flew over the asteroid reveal that Eros has a remarkably uniform density. That’s another indication that the asteroid isn’t an amalgam of loosely bound fragments, researchers reported last week in Pasadena, Calif. at a meeting of the American Astronomical Society.

In addition, a 15-km-long ridge, which wraps around half the asteroid, also suggests the asteroid is solid. To create such a ridge, the shock waves from impacts must have propagated through a single slab of rock, says Louise Prockter of Johns Hopkins Applied Physics Laboratory in Laurel, Md.

The close-ups also show that material presumably excavated by impacts have the same color as original surface material. “The rocks from below look just like the stuff on top,” Veverka notes. This suggests that Eros has the same composition throughout and hasn’t undergone any significant internal transformation since it formed during the birth of the solar system. Other asteroids, which were subject to intense heating early on, now have varied compositions.

Astronomers have variously compared the asteroid’s bland color to butterscotch and baby vomit. Spectra show little variation in color over Eros’ surface, notes Lucy-Ann A. McFadden of the University of Maryland in College Park. Eros belongs to a subclass of so-called S asteroids, the most common type of asteroid. Its spectra match those of a group of iron-poor ordinary chondrites, the most common type of meteorite that falls to Earth. The match clinches the case that this subclass of asteroids is the parent of these ordinary chondrites.

NEAR has returned to an orbit 200 km from Eros. In December, it will drop down to 35 km, close enough for the craft’s gamma-ray spectrometer to analyze the composition of material several centimeters beneath the surface. On Feb. 12, if enough fuel remains, scientists will direct NEAR to land. The landing could provide data on the strength of the rock, says missionproject scientist Andrew Cheng of Johns Hopkins.

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