Scientists are excited but puzzled by features seen in
pictures of a crater-ridden asteroid called
Mathilde, which was
visited last month by the
Near Earth Asteroid Rendezvous spacecraft.
NEAR, designed and built at the Applied Physics Laboratory, flew to within 750 miles of the asteroid on June 27, when it was about 200 million miles from Earth. The spacecraft took more than 500 images as it whipped past Mathilde, revealing a charcoal-black world covered with a surprising number of craters, one of them large enough to swallow up Washington, D.C.
Upon seeing the images for the first time, one scientist exclaimed: "Hey, it's all craters. There's no asteroid," Cornell University astronomer Joseph Veverka said during a news conference on June 30 at APL.
"On the surface of Mathilde there are essentially as many large craters as you could put there," Veverka said. "This is very, very unusual and the first time we have seen something like that."
But Mathilde itself wasn't the only unusual thing about the mission.
"This was not just an ordinary flyby," said Robert Farquhar, an APL scientist and NEAR mission director. "This was one of the most difficult flybys ever performed."
The spacecraft took the images as it sped past Mathilde at 22,000 miles per hour, a feat for which it was never designed. NEAR's real purpose is to orbit an asteroid known as Eros in January 1999, eventually landing on the space rock.
Because NEAR was not designed to take pictures in a fast flyby, the camera is stationary, instead of being mounted on a swiveling platform.
"So we had to turn the entire spacecraft to point the imager at the asteroid as we flew by," Farquhar said.
"Just imagine trying to videotape a billboard as you ride in the back seat of a car going down a highway at 55 miles an hour," said Donald Yeomans, an astronomer from the Jet Propulsion Laboratory in Pasadena, Calif. "You just sort of pan, and catch the billboard as it goes by. Now imagine stepping on the gas and going up to 22,000 miles per hour ... roughly 10 times the speed of a high-speed bullet, and then try the same stunt.
"If you don't know exactly where the asteroid is with respect to the spacecraft, you don't see anything."
Further complicating matters was the sun, which was close to Mathilde, almost blinding NEAR's view and making it difficult to see the stars near the asteroid. Astronomers must be able to see those stars to accurately navigate the spacecraft, Farquhar said.
Still another problem was Mathilde's sheer darkness; it reflects only about 3 percent of the sun's light, making it twice as black as charcoal. Consequently, it was difficult to spot; scientists didn't see it until the spacecraft was 30 hours away, giving them little time to navigate NEAR close enough to take good-quality images.
Nevertheless, the team of scientists guided the spacecraft with nearly pinpoint accuracy.
"In plain English, we kind of nailed it," Farquhar said.
It was the first time a solar-powered spacecraft had returned images from so far away. NEAR operated at only about 15 percent of its maximum power because Mathilde was twice as far from the sun as is the Earth, and turning the spacecraft to take images of the asteroid faced the solar panels away from the sun.
"This meant we had a big power shortage," Farquhar said. "We had to turn things off in order to keep the imager going."
The strategy worked. NEAR sent back a string of detailed images of the largest asteroid ever to be visited by a spacecraft, a battered chunk of rock with a diameter of 35 miles (57 kilometers) at its widest point.
Scientists ended up seeing only about 60 percent of the asteroid's surface, but it was enough to convince them that they had something unusual.
The largest crater in the images was nearly 19 miles across and possibly close to 4 miles deep.
"What is unusual is that for the first time we have seen a large abundance of very large craters" on an asteroid, Veverka said. The astronomers counted at least five craters that were 12 miles across. And the surface was riddled with craters of all sizes.
Usually, when objects collide with asteroids, forming new craters, debris from the impact covers up the older craters. But that hasn't happened on Mathilde, Veverka said.
"Most of the excavated material apparently has gone somewhere," he noted. The large number of craters suggests that Mathilde is at least a few billion years old.
Asteroids are believed to be left over from the formation of the solar system, about 4.6 billion years ago. A swirling cloud of gas and dust initially produced a rocky material, which coagulated into larger and larger bodies that merged to form the planets. But not all the material went into building the planets. Some of it remained in small bodies--the asteroids--which inhabit a vast belt located between the orbits of Mars and Jupiter.
Certain asteroids, such as Mathilde, are made up mostly of carbon-rich rock, and they are believed to have changed little since the solar system's birth. Therefore, learning more about their composition could yield new insights into the formation of the sun and planets.
NEAR's encounter marked the first time a spacecraft has visited a so-called carbonaceous asteroid.
Astronomers had been intrigued by Mathilde well before NEAR. They knew by studying variations of the object's brightness that it rotates much more slowly than other asteroids: one day on Mathilde takes as long as about 17 Earth days.
"That's very unusual for anything, especially unusual for asteroids, which are subject to collisions," Veverka said.
What is causing the ultra-slow rotation?
"One suggestion early on was that the reason Mathilde was rotating so slowly is because its spin has been slowed down by an interaction with a close satellite," he said. The NEAR images, however, have not yet shown the presence of any moons around the asteroid. But not all of the images have been studied.
"We do have about 200 frames, where we will be closely looking for small satellites in the vicinity," Veverka said.
Another interesting revelation was that the asteroid does not vary in color; it is the same gray shade throughout, indicating that the object is made of a uniform mixture of like materials.
"Even if you look at it in detail, there are no places on the asteroid where the color varies very much," Veverka said. "This is different from the experience we had with some previous asteroids."
As NEAR sped away from Mathilde July 3, its main engine was fired to send the spacecraft back toward Earth, where it will receive a gravity boost needed for its journey to Eros.
It will make its closest approach to Earth on Jan. 23, passing 300 miles above Baghdad, "right over the no-fly zone," Farquhar said.
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