NASA's Near Earth Asteroid Rendezvous mission, the first to orbit an asteroid, has met all its scientific goals in its year of orbiting asteroid Eros, and will now attempt another first: a controlled descent to the surface of the asteroid on Feb. 12.
The main goal of the controlled descent 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 Edward Weiler, associate administrator for space science at NASA headquarters in Washington, D.C. "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 five-year, 2-billion-mile journey, the NEAR Shoemaker spacecraft, which was built and is managed by APL, 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 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," said Joseph Veverka, NEAR imaging team leader from Cornell University. "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 next Monday. 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 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 four-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 is 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 4 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. Deep Space Network antenna stations in Madrid, Spain, and Goldstone, Calif., will simultaneously cover the maneuvers.
"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."
For more on the mission, log on to the NEAR Web site at http://near.jhuapl.edu.