Spacecraft homes in on surface of asteroid

Before NEAR ditches on Eros, scientists seek clues to mystery

January 24, 2001|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

After orbiting the asteroid Eros for nearly a year, the Maryland-built NEAR spacecraft was to fire its thrusters today to begin a final series of low-altitude photographic passes over the bleak space rock, before ditching itself on the surface next month.

If all goes well, NEAR will make five or six fly-bys over four days, the lowest, on Sunday, less than 9,000 feet above the surface.

Scientists hope to get back detailed pictures that will answer their questions about poorly understood forces that seem to be eroding Eros' surface features.

Photos to be radioed to Earth on Feb. 12, during NEAR's controlled descent to the surface, could reveal objects as small as 4 inches across.

A landing, even at the hoped-for jogging speed or slower, will almost certainly silence the spacecraft.

But the ditching would still count as the first landing on an asteroid, and add Eros to the short list of landing sites on the moon, Venus and Mars. It would also provide a dramatic end to the five-year, $224 million NEAR mission.

"I think we'll have some fun," said Cornell University astronomer Joseph Veverka, leader of the NEAR imaging team.

NEAR (for Near Earth Asteroid Rendezvous) was designed and built at the Johns Hopkins University's Applied Physics Lab. The NASA-sponsored mission has been managed by APL and controlled from its Laurel campus.

Launched in 1996, NEAR arrived at Eros last February, a year behind schedule. It is now 140 million miles from Earth. It has analyzed the 20-mile-long asteroid's chemical and physical composition, and taken more than 160,000 photos - far more than the 10,000 initially planned, Veverka said.

Mission director Robert Farquhar said today's maneuver will lower NEAR from its 22-mile-high orbit to one just a few miles over the shoe-shaped asteroid.

Scientists hope NEAR's final close-ups will give them a closer look at rocks like those glimpsed during a higher fly-by in October - rocks that appear to be falling apart where they stand. Have they been shattered by meteorites, or by something else?

"I don't think we have ever seen that on the moon. We're really puzzled," Veverka said.

They will also be looking for clues to why the interior slopes of some of Eros' craters have a lighter color than the rest of the surface.

"What seems to happen is that when stuff sits on the surface, it slightly darkens," perhaps from accumulated dust or solar radiation, Veverka said. Then, when Eros is struck by a meteorite, it shudders enough to send the surface material sliding down the craters' slopes, uncovering the unexposed, lighter material beneath.

Lab experiments and lunar data suggest it takes from a thousand to a million years for the newly exposed material to darken again, Veverka said. "Geologically, it's a fairly quick process."

NEAR's Web site is http://near.jhuapl.edu

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