After one of the softest planetary landings in history, ground controllers extended the mission of the Near Earth Asteroid Rendezvous (NEAR) to allow the NEAR Shoemaker spacecraft's instruments to continue to gather data from the surface of the asteroid Eros. The NEAR Shoemaker, the first spacecraft to touch down and operate on the surface of an asteroid, was guided to a 4-mi/hr touchdown on February 12, ending a full year in orbit around the large space rock (see Laser Focus World, December 2000, p. 22).
The spacecraft snapped 69 detailed images during the final 3 miles of its descent, the highest resolution images ever obtained of an asteroid. The camera delivered clear pictures from as close as 394 ft. (120 m), showing features as small as one centimeter across (see figure). The images revealed bizarre and surprising aspects of surface structures on Eros, including unexplained collapse features, and flat, sharply delineated areas in the bottom of craters.
"There are remarkably fewer small, fresh craters on Eros than we would expect compared to the moon, and an amazing profusion of boulders," said NEAR project scientist Andrew Cheng. "We do not know yet what covers or obliterates craters while making boulders. Part of the story may be mass motion such as loose material sliding downhill. We also believe that at least some of the bouldery debris is comprised of ejecta from impacts on Eros; some of these ejecta do not escape but fall back to the surface."
The extra days will enable the spacecraft's x-ray/gamma-ray spectrometer (XGRS) to observe the elemental composition on and below the surface of Eros, and the NEAR team at least two more opportunities to download this information through NASA's heavily used Deep Space Network of antennas. The XGRS can measure elemental composition down to a depth of about 4 in., and is much more sensitive on the surface than it was in orbit. Mission engineers fine-tuned the device last week to account for a higher operating temperature near the surface, because it can no longer radiate heat into space.
The orbital data from the x-ray spectrometer showed that the abundance of the volatile element sulfur is less than expected from an ordinary undifferentiated chondrite. The x-ray spectra tell us only about the uppermost hundred microns of the surface, however, and it is not known whether the sulfur depletion occurs only in a thin surface layer or throughout the bulk of the asteroid. The data from the gamma-ray spectrometer measures the composition to a depth of about 10 cm, which may help answer the question. Scientists will try to determine the abundances of potassium and iron to compare the bulk depletion of volatiles on Eros and ordinary chondrites.
The NEAR Shoemaker landing on Eros has turned out to be a mission planner's dream. "We put the first priority on getting high-resolution images of the surface, and the second on putting the spacecraft down safelyand we got both," said NEAR mission director Robert Farquhar of the Johns Hopkins University Applied Physics Laboratory, which manages the NEAR mission for NASA. "This could not have worked out better."
The laser rangefinder instrument completed its mission objectives of mapping the surface from orbit, and will not be used on the surface (see Laser Focus World, September 1996, p. 77). The magnetometer found no evidence of magnetism at the landing site, and has been turned off. The lack of magnetic field tells scientists that there is no dynamo effect inside the meteor, as expected, confirming that the interior is not molten. The gamma-ray measurement will be the last from NEAR. "Its job is almost done," said Cheng, "but ours is just beginning."