NEAR Spacecraft Set For Valentine's Day Rendezvous With Asteroid Eros

Their Eros is not the Greek god of love, however. Their Eros is an asteroid 240 million miles from Earth.

"This is the first time ever a spacecraft will orbit an asteroid," said William V. Boynton of the Lunar and Planetary Laboratory of the University of Arizona in Tucson. "There have been flybys and snapshots of asteroids, but not much in the way of quantitative scientific data."

Boynton is a scientist for an experiment on the NEAR (Near Earth Asteroid Rendezvous) spacecraft. It is a spacecraft that in December 1998 "almost had been given up for dead" after a problem with an attempted rocket firing aborted the initial try at flying into 433 Eros' orbit, noted a news release sent yesterday from Cornell University, which has scientists with experiments on the mission.

Launched in 1996, NEAR is to spend almost a year orbiting as close as 9 miles (15 kilometers) above Eros' surface. 433 Eros is the first-discovered of the near-Earth asteroids whose orbits come close to or cross the orbit of Earth. The asteroid, shaped like an Idaho potato, measures 21 x 8 x 8 miles (33 x 13 x 13 km).

Boynton is a scientist on the X-Ray/Gamma Ray Spectrometer, or XRGS, experiment. It is the primary experiment for determining the elemental composition of the surface and layers just beneath the surface. The XRGS will begin taking data several weeks after arrival at Eros, when NEAR descends to within 120 miles (200 kilometers)of the asteroid's surface. The best quality XRGS data will be collected around April 30, as the spacecraft orbits 30 miles (50 kilometers) from the surface, Boynton said.

The XRGS was designed and built by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which manages the NEAR mission for NASA.

Boynton and his group at the UA Lunar and Planetary Laboratory will process XRGS data and manage the data base for all XRGS data, as well as work on the scientific interpretation of results. Samuel (Hop) Bailey, project manager, and software specialists Jasbir (Jesse) Bhangoo and Irina Mikheeva work with Boynton in the research.

XRGS results are basic to solving such mysteries as the source of meteorites and their relationship to asteroids.

"Eros is a very important asteroid because it is a member of a class called 'S' asteroids, which appear to be similar to a rare type of meteorite on Earth called 'stony-irons,' which have 50 percent metal and 50 percent silicate," Boynton said. "Though the S-asteroids are very common in space, they do not seem to match many of the meteorites that fall to Earth," he added.

"The other half of this problem is that the most common meteorites found on Earth, called ordinary chondrites, are very common on Earth but appear to be rare in space. Some people think that ordinary chondrites might come from S-asteroids and that S-asteroids actually might have a lower metal content than ground-based astronomical data suggest. This mission should really answer this question."

It's possible that the composition of Eros might turn out to be different from any of the known meteorites, Boynton said. It's also possible that Eros, a highly irregularly shaped object, is "possibly a chip off some larger, pre-existing asteroid that was smashed up," richer in silicates on one side and richer in metal on another, he said. "This might allow us to learn something about the processes that go on in asteroids."

NEAR, which was launched Feb. 17, 1996, from Cape Canaveral, Fl., is the first in NASA's Discovery Program for "faster, better, cheaper" planetary missions. It was launched 9 months under the 36-month schedule and $41.6 million under the $150 million budget. The other NEAR instruments are a multispectral imager, a laser rangefinder, a near-infrared spectrometer, a magnetometer and a radio science package.

NEAR is the first spacecraft powered by solar cells to operate beyond the orbit of Mars. It returned 500 images of asteroid 253 Mathilde when it flew within 750 miles of that object on June 27, 1997. In January 1998, NEAR returned to the Earth's vicinity for a "slingshot" gravity assist toward Eros' orbital plane.