On Sunday, Dec. 20, the NEAR (Near Earth Asteroid Rendezvous) spacecraft will initiate a series of rocket engine firings that accelerate it toward a rendezvous with a faster-moving asteroid, 433 Eros. NEAR will reach Eros next month to begin the first close-up and comprehensive study of an asteroid in space history.
"This is the first time ever a spacecraft will orbit an asteroid," said
Professor William V. Boynton of the Lunar and Planetary Laboratory at The
University of Arizona in Tucson. "There have been flybys and snapshots, but
not much in the way of quantitative scientific data."
Boynton is on one of six science teams that will study 433 Eros on the year-
long NEAR mission that begins on Jan. 10 and is scheduled to end Feb. 6,
2000. The asteroid, which measures 24 miles in length and 10 miles in
diameter (40 kilometers x 17 kilometers) was the first near-Earth asteroid
(those whose orbits come close to or cross the orbit of Earth) spotted by
astronomers.
Boynton is a scientist on the X-Ray/Gamma Ray Spectrometer, or XGRS,
experiment. It is the primary experiment for determining the elemental
composition of the surface and layers just beneath the surface. The
instrument will begin taking data next spring while NEAR is orbiting
above the asteroid's surface, coming as close as 9 miles (15 kilometers).
The XGRS was designed and built by the Johns Hopkins University Applied
Physics Laboratory in Laurel, Md., which also built the NEAR spacecraft and
manages the NEAR mission. Boynton and his group at the UA Lunar and Planetary
Laboratory will process XGRS data and manage the data base for all XGRS data,
as well as work on the scientific interpretation of results. Boynton's UA
colleagues on the NEAR mission include Samuel (Hop) Bailey, project manager,
and software specialists Jazbir (Jesse) Bhangoo and Irina Mikheeva.
XGRS 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. Though the S-asteroids are very common in space, they do not
seem to match many of the meteorites that fall to Earth," Boynton said.
"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 added. 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, Boynton said. "This might allow us to learn
something about the processes that go on in asteroids."
NEAR is the first in NASA's Discovery Program for "faster, better, cheaper"
planetary missions. It was launched Feb. 17, 1996, from Cape Canaveral, Fl.,
9 months sooner than its 36-month schedule and $41.6 million under the $150
million budget. The other NEAR instruments include 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. Last January NEAR returned
to the Earth's vicinity for a "slingshot" gravity assist toward Eros' orbital
plane.
Next month, Boynton will travel from Cape Canaveral, site of the Jan. 3
launch of the Mars Polar Lander, to the Johns Hopkins lab to witness NEAR's
Jan. 10 arrival at Eros. On the Mars mission, Boynton heads an experiment in
the Mars Volatiles and Climate Surveyor integrated payload. That experiment,
TEGA, or the Thermal and Evolved Gas Analyzer, is to discover how much water
and carbon dioxide is in soil at the south pole landing site on Mars, and
what minerals make up that soil.
Press Note
Press packets, an 11-minute broadcast-quality video, "NEAR – The Journey Continues" (Feb 1998) and a 10-minute broadcast-quality video, "NEAR Spacecraft's Encounter with Asteroid 433 Eros" (Nov 1998) is available
to news media from Helen Worth, media contact at the Johns Hopkins University Applied Physics Laboratory, phone 240-228-5113. Lori Stiles in UA News Services, phone 520-621-1877, has copies of the press packet and Beta-format videos. NASA will televise a news conference on the NEAR mission from NASA headquarters, Washington D.C., at 1 p.m. EST Wednesday, Dec. 16. NASA-TV will also carry live briefings on NEAR held at the Johns Hopkins University at noon EST, Sunday, Jan. 10, 1999, and at 1 p.m. EST Thursday, Jan. 14, 1999.)
