2001 Feb 11
NEAR Team Seeks Answers in Descent Images
As NEAR Shoemaker closes in on asteroid 433 Eros, mission scientists hope the camera will capture surface details as small as a hand-size rock before the spacecraft touches down on the boulder-strewn surface Feb. 12.Since last October, NEAR imaging team members have been puzzling over strange surface features seen in new, high-resolution images. They hope the close-ups taken in the final minutes before the spacecraft drops onto the surface will help to answer their questions about the geology of the 21-mile-long asteroid more than 196 million miles (316 million kilometers) from Earth.
"Since last October we have seen details of Eros at 1-meter resolution that we haven't seen anywhere else before and don't understand," says Cornell University astronomer Joseph Veverka, who heads the imaging team. "That's why we are so excited about getting close to the surface."
The controlled descent is a highly risky maneuver, involving four intermittent thruster firings intended to slow the rate of descent to about 5 mph from 20 mph. In the final 45 minutes, when the spacecraft is about 3.5 to 4.5 miles (about 6 to 7.5 kilometers) from its landing site at the edge of the saddle-shaped depression Himeros, the camera will begin taking a new image about every 30 seconds.
The final clear picture could be snapped at just 550 yards (500 meters) from the surface, enough to capture details as small as 4 inches (10 centimeters) across. Mission team members at the Johns Hopkins University Applied Physics Laboratory, which built the spacecraft and manages the NEAR mission for NASA, do not expect images to be transmitted from the surface because Eros's spin and topography will almost certainly prevent communication between Earth and the craft.
Veverka says his team wants a close-up look because it is puzzled by what it has seen on Eros over the past few weeks. Last October, with much of NEAR's mission accomplished, the spacecraft was sent into orbit just 4 miles (about 6 kilometers) or so from the asteroid's surface. For the first time team members saw details as small as a yard (just under a meter) across, compared with the approximately 5.5 yards (5 meters) resolution that had been captured by the camera since the spacecraft went into orbit around Eros on Feb. 14, 2000.
"Suddenly, we started seeing things we didn't expect and hadn't seen on other surfaces in the solar system," says Veverka. "It's like another door has opened."
The biggest surprise, says Cornell researcher Peter Thomas, who has been interpreting the geology of the asteroid's surface, "is that some small craters and other small depressions have flat, smooth floors, unlike most craters you see on Eros and other objects. It looks as if fine-grain material has slid down the craters' sides and ponded in the bottoms."
Apparently, he says, there is some mechanism "we hadn't anticipated" that moves fine-grain material around on the surface. Although gravity on Eros is only one one-thousandth of that on Earth - an average person would weigh only an ounce or two - it is still "very effective in gathering materials in very flat floors on the bottom of depressions."
Another surprise, says Veverka, is the discovery that some small boulders are surrounded by material that appears to have disintegrated from the boulders' surfaces. "There is some process that is very gentle that somehow disintegrates rock. We haven't seen this on the moon, and we haven't seen this before on Eros. But it seems to be very common."
On Jan. 24 the spacecraft entered a close flyby sequence, including a four-day orbit that produced images from as close as 2 miles (3.2 kilometers) above the surface. The new images have enabled imaging team members to accumulate data at a resolution of about 1.1 yards (1 meter). "The hope is that during the descent we can improve this resolution by perhaps a factor of 10 so that we can find out more about what is going on there," says Veverka.
(From a Cornell University release)
