LAUREL -- Scientists poring over nearly 300 pictures of the asteroid Mathilde said yesterday that the heavily cratered rock seems to be only a bit more dense than water ice.
The pictures were snapped Friday during a 25-minute flyby by NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft, built at the Johns Hopkins University Applied Physics Laboratory in Laurel.
Coupled with other clues from the photos, scientists said, the low density suggests that C-type asteroids like Mathilde are not the remnants of much larger, planet-like objects that have undergone heating and melting.
Instead, they appear to be loose, carbon-rich accumulations of the solar system's oldest building blocks.
Among the other NEAR findings announced yesterday at APL:
Mathilde is a bit smaller and less elongated than expected -- about 35 miles long, 33 miles wide and 31 miles thick. It is bigger than both Gaspra and Ida, the only other asteroids ever photographed by passing spacecraft.
There is no sign yet that Mathilde has a moon. A moon, however, remains the best explanation scientists have for the asteroid's extraordinarily slow spin -- about once in 17 days.
Mathilde reflects just 3 percent to 4 percent of the sunlight that strikes it, making it twice as black as charcoal.
Thanks to Mathilde's weak gravity, an astronaut could jump 1,000 feet or more off the surface.
The high number of craters suggests that Mathilde has been taking hits for several billion years.
The success of the photo reconnaissance of Mathilde came as a relief to NEAR team members. The APL's Dr. Robert W. Farquhar, NEAR mission director, said, "Not everybody was that enthusiastic."
NEAR's real goal is another asteroid called Eros. A Mathilde flyby cost the mission 22 pounds of precious fuel.
Getting the pictures required aiming the $108 million spacecraft where its navigators thought Mathilde would be, and panning as NEAR passed by that spot at 22,000 mph.
"If you don't know exactly where it is, you don't see anything," Yeomans said.
The maneuvers depended on precise optical navigation -- course adjustments based on photo images snapped just hours before the encounter.
But the asteroid's coal-black surface and the position of the sun behind it made the job even more difficult.
Yeomans confessed that he feared having to explain "530 images of just blank sky."
Farquhar called it "one of the most difficult flybys ever performed." Nevertheless, he said, "we kinda nailed it."
The spacecraft acquired 294 pictures of Mathilde. Three hundred photos were blank, the expected result of APL's photo strategy, which called for exposures across the entire region where the asteroid might be, given the navigators' uncertainties.
Billions of asteroids orbit the sun in a belt between the orbits of Mars and Jupiter. Mathilde resides in the inner part of that belt. It was about 205 million miles from Earth when NEAR zipped past.
NEAR was launched in February 1996. It is expected to reach Eros in January 1999 and orbit there for a year.
On Thursday, engineers at APL will command NEAR's main engine to fire for a critical course change. If it works, the 1,700-pound spacecraft would swing past Earth in January 1998, gathering enough gravitational energy to fling it out again toward Eros.
NEAR will fly within 330 miles of Earth, "right over Baghdad [Iraq], over the no-fly zone," joked Cornell University scientist Dr. Joseph Veverka, the NEAR imaging team leader.
Scientists were astonished that Mathilde, an object barely 37 miles long, could survive impacts that left at least five craters more than 12 miles across. The largest, about 18 miles wide and 3.7 miles deep, could hold the District of Columbia.
"There are essentially as many large craters as you could put there," said Veverka.
The crater walls show no variation in color, "a very significant discovery," he said.
It suggests that Mathilde is made of the same stuff all the way through.
And that supports the notion that it is a simple accumulation of pristine material from the solar system's beginnings.
Pub Date: 7/01/97