NASA’s Near Earth Asteroid Rendezvous (NEAR) mission, the first to orbit an asteroid, is coming to an end.
February 7, 2001
With the spacecraft almost out of fuel, on Feb. 12 mission engineers will attempt the first-ever, controlled descent to the surface of an asteroid. NEAR has been orbiting asteroid 433 Eros since Feb. 14, 2000 and is now more than 196 million miles (316 million kilometers) from Earth
The main goal of the controlled descent is to gather close-up pictures of the surface of Eros, particularly the saddle area, a six-mile (10-km) wide depression peppered with huge boulders and cut with grooves.
"The landing will take a few hours. We’ll see the images coming in real time as the NEAR spacecraft is approaching closer and closer to the asteroid’s surface," William V. Boynton said.  Boynton, professor of planetary sciences in the UA Lunar and Planetary Laboratory, leads the UA group involved in the NEAR mission.
Boyton and the UA team will watch the descent at the Applied Physics Laboratory (APL) in Laurel, Md., which built the spacecraft and managed the mission.
NEAR Shoemaker’s 4-hour descent is scheduled to start at 10:31 a.m. EST.  A series of thruster firings will decelerate the spacecraft from about 20 mph to 5 mph.
"It is not really a landing in the sense of a spacecraft being alive once it touches down. NEAR has no legs to steady it, so it’s just going to fall over. The antenna will no longer point to the Earth so we’ll not be able to communicate with it," Boynton said.
NEAR’s camera will be taking a photo every minute. The last clear images, shot from about 1,650 feet (500 meters), could details on the surface as small as 4 inches (10 centimeters) across.
"If the instruments survive the touch down we will not be able to see whatever the camera will be looking at. NEAR will snap the last image just before it reaches the surface," Boynton added.
During its one-year orbiting mission, the NEAR Shoemaker spacecraft provided among other data X-ray, gamma ray, and infrared readings on the composition and spectral properties of the asteroid. Initial results from the X-ray Gamma Ray Spectrometer suggested that Eros might be similar in elemental composition to primitive meteorites called chondrites.
"Previously Eros was thought to be a very standard meteorite, but now it looks that it might be like a meteorite we’ve never seen before.  Its composition might be different than that of typical meteorites. There might be some very rare meteorites that resemble Eros, but right now we’re not sure. We need to wait for another few months for complete analyses results to find out, says Boynton, who is the principal investigator for NEAR’s X-ray  Gamma Ray Spectrometer.
"One thing that has been learned from this mission is how to operate the spacecraft up close with an irregularly shaped body that has very little gravity. Because of its strange shape, the gravity on Eros is not constant like it is with a spherical body, such as Earth or Mars.  When one of the asteroid’s lobes comes by, the pull of gravity is greater. It is very complicated to take all of these effects into account," Boynton explained.
Though the mission has been successful, there are still some mysteries to be solved.
"One interesting thing we still don’t understand is why there are some places which have a lot of very large boulders. They are unexpected. We don’t see them on the moon. The saddle area is the region where we see grooves looking like cracks around Eros, and we are not sure what caused them," he said.
Eros almost certainly used to be a piece of a now extinct, bigger object. At one time in the past, Eros got knocked off due to a large impact. The shock from that event might have left the cracks.
"We don’t know when it happened. The space where Eros resides is crowded enough to have such collisions today. In order to answer this question we would have to land on it, pick up a sample, and bring it back to Earth for analysis." Boynton said.
What would he recommend for future asteroid missions?
"If we had the opportunity to do this again, I’d want to land on the surface with instruments designed to measure the surface composition and bring the samples back," Boynton said.
The UA NEAR team has began archiving the spectra data collected by NEAR-Shoemaker and will make them available to other scientists for analysis. The task will take about 12 months.