Dark asteroid may be brought to light by APL satellite First pictures of primitive object could come Friday


WASHINGTON -- A mysterious black asteroid, nearly the size of Rhode Island, may be seen up close for the first time Friday as a Maryland-built spacecraft races by on a high-speed photographic mission.

NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft, built at the Johns Hopkins University Applied Physics Laboratory in Laurel, will zip past the asteroid Mathilde at 22,000 mph Friday morning. The first photographs should be released later in the day.

Scientists hope the 25-minute fly-by will yield more than 500 photographs and a wealth of new information on the carbon-rich black asteroids, thought to be some of the most primitive objects in the solar system.

Mathilde was added to NEAR's agenda when mission planners discovered it would be close by as the spacecraft, launched in February 1996, sailed toward another, smaller asteroid named Eros.

NEAR is expected to reach Eros in January 1999, where it will orbit for at least a year, gathering data on its composition and natural history.

"Mathilde is the appetizer. Eros is the main course," said Dr. Andrew F. Cheng, NEAR project scientist at APL.

Spacecraft have flown past asteroids before. Galileo, en route to study Jupiter and its moons, photographed Gaspra at a distance of 990 miles in 1991, and Ida from 1,482 miles away in 1993.

Ida and Gaspra are S-type asteroids, stony-iron objects believed to have once been part of larger objects that were partly melted. Astronomers are eager to get a close look at Mathilde because it is a "C-type" asteroid -- as black as coal and one of the darkest objects in the solar system.

Based on comparisons with meteorites that fall to Earth, scientists think C-type objects are rich in carbon and water, and little changed since the formation of the solar system 4.6 billion years ago.

Asteroids are believed to be the construction debris of the solar system. More than 4,000 asteroids have been named or numbered. But billions, ranging in size from pebbles to giants up to 600 miles in diameter, are believed to orbit the sun in the "main belt" between the orbits of Mars and Jupiter.

A few hundred more, called "near Earth" asteroids, have been found in orbits that approach or cross the Earth's orbit. That makes them capable of hitting Earth, with potentially devastating consequences.

Mathilde will be about 205 million miles from Earth -- beyond Mars' orbit -- when NEAR passes by. NEAR will cruise as close as 750 miles to the surface.

Radio signals from Laurel to Mathilde and back will need 36 minutes to make the round trip. Scientists have had to calculate all of the spacecraft's photographic maneuvers in advance, and load the commands into NEAR's computers for automatic execution.

It is something like driving down a highway at 60 mph, and trying to shoot 500 pictures of a car parked in the breakdown lane.

Worse, NEAR's camera was not designed to swivel, so APL engineers have had to program the spacecraft to fire its engines to keep its camera pointed at the spot where Mathilde has been predicted to be.

Cheng said the maneuvers would force the spacecraft to turn its stationary solar panels away from the sun for a time, depriving it of all but one-sixth of its electrical power. That will prevent NEAR from using any other in- struments to observe Mathilde.

However, Mathilde's gravity will deflect NEAR's trajectory, producing tiny changes in NEAR's radio signals. That may yield data on the asteroid's mass.

Coupled with photographic and ground radar data on its size, the mass would provide clues to Mathilde's density and composition.

If the spacecraft's power reserves drop too far, NEAR will abort its photographic work and turn back toward the sun. "That's our greatest worry," Cheng said. "All the data would be lost."

Mathilde was discovered by astronomers in Vienna in 1885 and named, it is thought, for the wife of Moritz Loewy, a vice director of the Paris Observatory. It has been little more than a spot of light ever since.

From 1885 until 1994, when NEAR scientists discovered they could fly past Mathilde en route to Eros, there had been barely 60 observations of Mathilde. There have been hundreds more since as scientists scrambled to sharpen their predictions of where Mathilde would be when NEAR got there.

The NEAR mission is the first of NASA's Discovery series of "better, cheaper, faster" science missions. It was designed and built in just 27 months and, at $109 million, cost one-third of any previous interplanetary craft.

Pub Date: 6/24/97

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