Touch down

 作者:荆橼     |      日期:2019-03-07 03:11:06
By Adrian Cho A NASA spacecraft crash-landed on the rocky surface of asteroid 433 Eros late on Monday – and remarkably lived to tell the tale. The Near Earth Asteroid Rendezvous craft had orbited Eros since Valentine’s Day 2000, collecting 160,000 images. But it had nearly run out of fuel for its boosters, so researchers decided to end the mission with a bang and drop it onto the 33-kilometre-long rock in a “controlled descent”. Their aim was to capture high-resolution images of the asteroid on the way down. But NEAR was never designed to land and the team gave it only a one in 100 chance of surviving the collision with the surface. In the event, the little machine proved far tougher than expected. It fared far better than NASA’s Mars Polar Lander, which was designed to touchdown, but was lost without trace in December 1999. “I’m happy to report that the NEAR spacecraft has touched down on the surface of Eros,” said Robert Farquhar, mission director at Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “We’re still getting some signals, so evidently it is still transmitting from the surface itself.” NEAR researchers surprised themselves with their good fortune. “When the word came down that the spacecraft was down and that it was still talking to us, it was the thrill of my life,” says Andrew Cheng, a planetary scientist at the Applied Physics Laboratory. NEAR landed at 2005 GMT. The probe took pictures from as low as 120 metres above the surface. These immediately helped answer questions that had troubled researchers, such as why did Eros show no craters smaller than a few metres across? The new pictures reveal that such craters are mostly filled up with a layer of fine soil, says Mark Robinson, a geologist at Northwestern University, in Evanston, Illinois. “They’re there – they’ve just been covered up,” he said. Now that it is on the surface, NEAR won’t be able to take more pictures or use its other instruments. But it may still provide useful data, Cheng says. NEAR is currently transmitting a signal giving its precise location on the asteroid. It also may be able to transmit stored data that was taken just as it landed. Combined, that information should tell researchers whether the spacecraft bounced off the surface and how far it may have rolled, Cheng says. However, researchers have precious little time to retrieve the data. After 14 February they will no longer be able to use radio telescopes in the United States, Spain, and Australia to receive signals from the probe. “NEAR will probably continue broadcasting for months,” Cheng says, “but after those two days, no one will be listening.” More at: