When scientists from NASA's Near Earth Asteroid Rendezvous mission took their first close-up look at 433 Eros on Feb. 14, they had more questions than facts on their target asteroid. Now, after eight months of examining Eros with the NEAR Shoemaker spacecraft's array of instruments and sensors, NEAR team members say they have solid data on the history, makeup and topography of the complex, oddly shaped space rock.
"This has been a very successful mission so far," says Andrew Cheng, NEAR project scientist from the Applied Physics Laboratory, which manages the NEAR mission for NASA. "We have seen all of Eros from a variety of orbit distances, taken millions of measurements, and we've answered the big-gest questions from when the mission began."
Cheng and NEAR scientists from several institutions discussed their findings with the media Oct. 24 at the American Astronomical Society's Division for Planetary Sciences meeting in Pasadena, Calif. Their data continues to confirm that Eros is a fractured chip off a larger body, made of some of the most primitive materials in the solar system. Its uniform density (about the same as Earth's crust) and global fabric of grooves and ridges imply Eros is a cracked but solid rock, not a gravity-bound collection of rubble. Its cratered surface has steep cliffs and is covered by a deep layer of moving regolith, the loose rocks and dust left over from collisions with other objects. Eros also shows no variation in colors or spectral properties, except where downslope motion has exposed bright, underlying material on the steep walls of larger craters.
"We're learning that surface processes on small bodies like Eros can be very complicated," says Joseph Veverka, NEAR imaging team leader from Cornell University.
Combining digital images, Doppler tracking and data from NEAR Shoemaker's laser rangefinder--which measures Eros' surface height with laser pulses--scientists have built the first detailed maps and three-dimensional model of an asteroid. They also have used the laser instrument to determine that regolith on Eros is nearly 300 feet deep in some places and that its uneven distribution affects the asteroid's gravity. Further data indicate the regolith has moved on the bumpy surface, smoothing over rougher areas and spilling into craters.
The craters themselves have raised questions, especially the square craters likely shaped by Eros' underground fabric of cracks and grooves. NEAR scientists have counted more than 100,000 craters wider than 50 feet and about 1 million house-sized or bigger boulders--testament to the battering Eros received while traveling in the main asteroid belt and its current orbit. To keep better track of the asteroid's main features, team members have proposed nearly 40 names (based on romantic figures in history and literature) to the International Astronomical Union, which approves names on planetary bodies.
"Eros has a very interesting surface, unlike anything we've ever seen on the moon or on similar types of asteroids observed from spacecraft," says Clark Chapman, a NEAR imaging team member from the Southwest Research Institute. "Even the rocks on Eros are larger than those on the moon."
The elements in that surface have yielded further clues. Using NEAR Shoemaker's X-ray/gamma-ray spectrometer, or XGRS--which reads the unique fluorescent "colors" that elements emit when hit with the sun's X-rays--scientists have fortified their data on Eros' elemental composition, which links it to some of the most primitive meteorites in the solar system, the ordinary chondrites. The chondrites are a homogeneous mixture of heavy and light materials, a sign that the asteroid from which they originated was never subjected to intense melting.
"We have seen no variation in Eros' composition from place to place," says Steven Squyres, an XGRS team member from Cornell. "We have also found that we get the same readings whether the X-ray emissions come during a solar flare or a 'quiet' sun, which gives us considerable confidence in our analysis."
Yet even with this definitive data, some of NEAR's most exciting work lies ahead. NEAR Shoemaker made a low pass over Eros on Oct. 26, flying about three miles above the surface and snapping images five times sharper than mission scientists have ever seen of the asteroid. The car-sized spacecraft then will travel at altitudes from 125 miles to 22 miles and lower until the yearlong orbit mission ends in February 2001.
Launched on Feb. 17, 1996, NEAR was the first in NASA's Discovery Program of low-cost planetary missions. APL designed and built the NEAR Shoemaker spacecraft. To see the latest images of Eros, visit the NEAR Web site at http://www.jhuapl.edu.