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NEAR Mission Completes Main Task, Now Will Go
Where No Spacecraft Has Gone Before
NASA's Near Earth Asteroid Rendezvous (NEAR) Shoemaker
spacecraft, the first to orbit an asteroid, has met all its
scientific goals in its year of orbiting the asteroid Eros,
and will now attempt another first: a controlled descent to
the surface of the asteroid on Feb. 12.
The chief goal of the controlled descent to the surface is to
gather close-up pictures of the boulder-strewn surface of 433
Eros, more than 196 million miles from Earth.
"NEAR Shoemaker has set a high standard for low-cost
planetary exploration," said Dr. Edward Weiler, Associate
Administrator for Space Science, NASA Headquarters,
Washington, DC. "This mission has provided answers to a range
of fundamental science questions, and it has excited the
public with its exploration and great images. The team at
Johns Hopkins University Applied Physics Laboratory and its
many partner institutions are to be congratulated for
achieving this historic first in space exploration."
During its 5-year, 2-billion-mile journey, the NEAR Shoemaker
mission, which was built and is managed by The Applied
Physics Laboratory (APL) in Laurel, MD, provided the most
detailed profile yet of a small celestial body. It began a
yearlong orbit of Eros on Feb. 14, 2000, and has collected 10
times more data than originally planned.
The data include a detailed shape-model culled from more than
11 million laser pulses; radar and laser data on Eros' weak
gravity and solid but cracked interior; X-ray, gamma-ray and
infrared readings on its composition and spectral properties;
and about 160,000 images covering all of the 21-mile-long
asteroid's bouldered, cratered, dusty terrain.
"We have answered the questions we had when the orbit began.
We now know that Eros is a solid body of uniform composition,
made of material probably older than the Earth," said Dr.
Andrew Cheng of APL, Project Scientist for NEAR. "But we also
found many other things
we didn't expect to see and have questions we didn't know to
ask at the start of the mission. Scientists will be looking
at these data for years."
"On the tiny fraction of the surface we've seen at high
resolution, we noticed strange processes we haven't seen on
the moon or anywhere else," added Dr. Joseph Veverka, NEAR
imaging team leader from Cornell University in Ithaca, NY.
"For example, some boulders seem to have just disintegrated
on the surface. We've also seen that some of the fine surface
material moves downhill, filling low areas and creating flat
surfaces in craters, even with Eros' low gravity. These are
big puzzles and we need to get a better look."
That look should come Feb. 12. The primary goal of the
controlled descent is to get the closest images yet of Eros,
particularly its "saddle" area, a 6-mile wide depression that
has intrigued scientists with its boulder patches, relatively
craterless surface and patterns of grooves and ridges. The
secondary aim is to practice the maneuvers that would lead to
a landing, creating a flight plan for future missions to land
on a small body.
"With the spacecraft just about out of fuel and our science
objectives met, this is a great way to end a successful
mission," said NEAR Mission Director Dr. Robert Farquhar of
APL. "It's all bonus science. It's never been tried before
and it certainly is a complicated set of maneuvers, but at
this point the only real risk is not taking one."
NEAR Shoemaker's 4-hour descent is scheduled to start at
10:31 a.m. EST with a maneuver moving it out of its current
orbit 22 miles from the center of Eros. On the way down it
will take images that will help determine its exact location
and altitude, and set the timing for the final thruster
firings. This series of thruster firings are designed to
decelerate the spacecraft from about 20 mph to 5 mph.
NEAR Shoemaker will approach the surface on its side, its
outward-facing camera pointed down, snapping a photo every
minute. The last clear pictures from the telescopic camera,
taken from approximately 1,650 feet could show surface
features as small as four inches across. After that, NEAR
mission operators will use the blurring photos, altitude data
from NEAR Shoemaker's laser range-finder, Doppler tracking
and the eventual loss of signal to learn when the spacecraft
touches down, predicted for just after 3 p.m. EST.
"The whole sequence of engine burns has to go right, or it
might not be a very soft touchdown," Farquhar said. " The
unknown nature of the surface makes it hard to predict what
will happen to the spacecraft, especially since it wasn't
designed to land. The most we can hope for is a beacon from
NEAR Shoemaker that says it's still operating."
Images and information on end-of-mission media activities can
be found at:
http://near.jhuapl.edu/media/index.html
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