NEAR mission gets close to asteroid

This October, NASA's Near Earth Asteroid Rendezvous (NEAR) mission made the closest flyby ever to the surface of an asteroid. The target of the mission is 433 Eros, the first-discovered near-Earth asteroid (NEA)—defined by an orbit that crosses that of Mars—and the second largest. After months of examining Eros with the array of instruments and sensors on the NEAR Shoemaker spacecraft, NEAR team members now have solid data on the history, makeup, and topography of the complex, potato-shaped space rock.

On Oct. 26, after more than eight months in orbit around asteroid Eros, the NEAR Shoemaker spacecraft swooped down to just four miles above the surface of the asteroid, taking images and collecting data from a distance closer than any spacecraft has ever come to an asteroid (see figure).

"The proximity to Eros was equivalent to the cruising altitude of a commuter airplane on earth," says Robert W. Farquhar, NEAR mission director at The Johns Hopkins University Applied Physics Laboratory (APL; Laurel, MD), which manages the mission for NASA.

"We've never seen the surface of an asteroid or planetary satellite at this high resolution without actually landing," noted Scott Murchie, NEAR imaging team member.

Chip off the old block

The goal of the mission, which is to glean information about the structure and chemical composition of Eros, has been a success. Data continues to confirm that Eros is a fractured chip off a larger body, consisting 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 suggest that 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, loose soil and dust pulverized by meteorite impacts. 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.

Scientists have drawn the first detailed maps and a three-dimensional model of an asteroid by combining digital images, Doppler tracking, and data from NEAR Shoemaker's laser altimeter, which measures Eros' surface height with laser pulses. They have also used the laser altimeter 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 raised questions, especially the square craters likely shaped by the underground fabric of cracks and grooves of Eros. Scientists on the NEAR mission 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.

The spacecraft's x-ray/gamma ray spectrometer, which reads the unique fluorescent spectra elements emit when hit with the sun's x-rays, has detected low levels of aluminum relative to magnesium and silicon, indicating an undifferentiated composition. This finding links Eros to some of the most primitive meteorites in the solar system, the ordinary chondrites. The most common type of meteorite, 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. The NEAR Shoemaker imager and infrared spectrometer also found spectral properties consistent with a primitive, chondritic composition.

After the close flyby, the NEAR Shoemaker spacecraft fired its thrusters to achieve a more stable, higher orbit. The car-sized spacecraft will continue to orbit at altitudes from 22 to 125 miles until the mission ends in February 2001.