Published by the American Geological Institute
and Trends in the Geosciences
|In December of 1998, the first of NASA’s “faster, better, cheaper”
Discovery missions was in trouble. Having already collected important data
from asteroid 253 Mathilde, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft
was intended to begin its orbit of asteroid 433 Eros. Instead, NEAR sailed
past its target at a distance of more than 3,827 kilometers and at a speed
of over 965 meters per second.
On Feb. 14 this year, NEAR had a second chance. On that day, the probe — recently renamed NEAR Shoemaker in honor of planetary science pioneer Eugene Shoemaker — successfully entered into orbit around Eros. The first high-orbit data, taken from a distance of about 325 kilometers, were only a taste of what the researchers expected as the spacecraft approached its closest orbit 50 kilometers from the asteroid’s center.
Visual images collected by the probe’s Multispectral Imager showed a surface covered with craters. But many other bombarded bodies in the solar system are marked with craters, making Eros’ surface “special, but not unique,” said NEAR team member Clark Chapman of the Southwest Research Institute. Chapman and colleagues announced initial NEAR findings at the 31st Lunar and Planetary Science Conference held March 13-17 in Houston.
Usually, crater densities can be used to estimate the duration of an orbiting body’s exposure to space debris. However, Eros has been in near-Earth orbit for an unknown length of time and far from the densely inhabited asteroid belt where its craters were likely produced. As a result, the asteroid’s age will probably remain a mystery even as image resolutions improve later this year.
Of particular interest to the NEAR researchers is the successful performance of the probe’s suite of detection equipment. All of the instruments that were designed to collect data from high orbit are online and working correctly.
One instrument, the X-ray/gamma-ray spectrometer, was meant to collect substantial data only at low orbit. However, a 300-second solar flare on March 2 provided an unusual illumination of Eros’ surface, allowing the instrument to detect magnesium, aluminum, silicon, calcium and iron.
The gamma-ray spectrometer has also proven to be a valuable detector of the most energetic events that occur in our universe, gamma-ray bursts, said NEAR team member Jacob Trombka of NASA’s Goddard Space Flight Center. A software patch applied to the instrument after launch enables NEAR Shoemaker to detect the enigmatic blasts as part of the Interplanetary Gamma-Ray Burst Timing Network.
The network, comprised of a fleet of spacecraft that include Ulysses and Mir, can pinpoint the bursts with far greater accuracy since NEAR Shoemaker joined the search. Burst sources are now being discovered at a rate of about one per week.