NEAR Shoemaker's mission extended by ten days
BY JEFF FOUST
Posted: February 14, 2001
Operations of the spacecraft were due to end on Wednesday, one year to the day after the spacecraft entered orbit around the near-Earth asteroid and two days after it performed a controlled descent to the asteroid's surface. However, with the spacecraft in good condition on the surface, the space agency decided this was too good an opportunity to pass up.
"This is successful beyond our highest expectations and NASA is taking advantage," said Jay Bergstralh, NASA's acting science program director for solar system exploration, at a press conference Wednesday at the Applied Physics Laboratory of The Johns Hopkins University, mission control for NEAR. Bergstralh said the ten-day extension would be funded out of the project's budget reserves.
The primary purpose of the extended mission will be to collect data on the composition of the asteroid's surface using a x-ray and gamma-ray spectrometer on the spacecraft. Mission officials said they were in the process of turning the instrument on this afternoon and expected to get some data back from it, using NEAR's low-gain antenna, later in the week.
"Now that we have landed, collection and recovery of critical gamma-ray data is our primary objective," said NEAR operations manager Robert Nelson. The ten-day extension is believed to be long enough for the spacecraft to return enough data to improve the accuracy of past measurements of the asteroid's surface and subsurface composition by a factor of ten.
Other instruments on NEAR, including a camera and magnetometer, will likely not be used during the mission extension. With the spacecraft sitting on the surface the camera is not in a position to take images, and even if it were the low data rate provided by the low-gain antenna would make it difficult to return other images. Project scientists also decided against turning on the magnetometer after measurements made by the spacecraft during its descent to the surface Monday failed to detect any residual magnetic field at altitudes as low as 120 meters (400 feet.)
Project officials have also all but ruled out plans for NEAR to lift off the surface again and "hop" to another region of the asteroid. Low levels of propellant -- no more than 8 kg and possibly as little as zero -- would make it difficult to perform such a flight. In addition, the science that would be returned by such a flight would be limited, since it would be difficult to both take images and return them to Earth with the high-gain antenna before landing. "This is still a scientific mission," said mission director Robert Farquhar of the decision to keep NEAR on the surface and collecting spectral data.
Project officials and scientists also recounted Wednesday the successful landing and the data returned by the spacecraft during its descent. The spacecraft landed only about 200 meters (660 feet) from its projected landing site, traveling no more than 1.8 meters per second (4 mph). Analysis of the spacecraft's telemetry shows the spacecraft may have jiggled, but did not bounce significantly: engineers believe the spacecraft's thrusters turned off the instant the spacecraft initially made contact with the asteroid, keeping any bouncing to a minimum.
"This was not a crash, this was a soft landing," said Farquhar. He noted that NEAR may have made "the softest landing of all time", as its landing speed was considerably less than past missions that landed on the Moon, Venus, and Mars.
NASA had been downplaying the chances of a successful landing, preferring to call it instead a "controlled descent" to avoid raising expectations. Yet, Farquhar said, he scheduled the descent for the 12th, two days before the mission's end, just in case things worked better than expected. "I was a little more optimistic than I let on," he said.
The successful landing also showed that asteroid landings are feasible, even by spacecraft not designed to land. "It essentially confirmed that all the mathematical models we proposed for a controlled descent would work," said Bobby Williams, NEAR navigation team leader. "The spacecraft did what we expected it to do, and everyone's real happy about that."
Scientists are already pouring over the high-resolution images returned by NEAR during its descent, some of which show unexpected flat areas and depressions that defy easy explanation. "These spectacular images have started to answer the many questions we had about Eros," said Joseph Veverka, a Cornell University professor and NEAR imaging team leader. "They also revealed new mysteries that we will explore for years to come."
"Scientifically, NEAR was a tremendous success," said Veverka. "We are getting to know Eros."
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Images from Monday's descent:
Last view before landing
Extreme close up!
View from 2.5km
Closest view yet
Views of Eros: 1 2 3 4
View of the horizon
A preview of landing:
The landing site on Eros
Illustration showing NEAR's drop from orbit
Graph shows descent in altitude vs. time
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