Second Chance for Date With Asteroid

4 January 1999 7:20 pm

After a brush with death last month, the Near Earth Asteroid Rendezvous (NEAR) spacecraft is back on track for an encounter with a hurtling hunk of space rock. Mission control successfully gunned the craft's engines yesterday, raising its speed to a rate that will bring it to the asteroid 433 Eros in February 2000.

NEAR was scheduled to begin orbiting Eros in late January for a 9-month study of the asteroid's shape and composition. But on 20 December its first attempt to catch up with Eros ended with NEAR tumbling out of control and losing radio contact. The sudden acceleration apparently exceeded certain tolerance levels in the guidance system software, causing it to shut down. A few hours later, however, on-board safety systems turned the spacecraft toward Earth and re-established contact with mission control at Goddard Space Flight Center in Greenbelt, Maryland.

Engineers trace the failure to a change in programmed tolerance levels made to improve NEAR's performance after the rocket had worked perfectly in a July 1997 simulation. "In hindsight, we should have left it alone," laments Andrew Cheng, NEAR project scientist at the Applied Physics Laboratory in Laurel, Maryland.

The temporary loss of control caused Eros to zoom past the comatose satellite on 23 December. Scientists still managed to obtain a few grainy pictures that will help them plan for the upcoming rendezvous. "Now we know where not to point the camera," says Cheng, referring to large shadowed regions of the asteroid's surface visible in the new images.

At present, NEAR is trailing the asteroid by 904,000 kilometers as both objects orbit the sun. When NEAR arrives next February, ground controllers will again fire the thrusters in an attempt to make NEAR the first spacecraft to orbit an asteroid.

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