WASHINGTON -- Mission controllers say the Maryland-built NEAR spacecraft is ready for a critical orbital maneuver on Sunday after losing its bearings while en route to a Jan. 10 rendezvous with an asteroid.
Scientists said yesterday that NEAR is fully capable again after a "star-tracker" -- a navigational device aboard NEAR -- became disoriented for 90 minutes on Dec. 5 and sent the spacecraft into a protective shutdown.
Controllers at the Johns Hopkins University's Applied Physics Laboratory in Laurel regained control of NEAR after several hours. They got the star-tracker working again and, over several days, gradually switched backed on NEAR's instruments.
The cause of the failure remains undetermined, but scientists don't believe it threatens the $211 million mission, or next month's attempt to put NEAR into orbit around the asteroid, called Eros.
"Even if an event like this were to occur again, it would not seriously jeopardize the mission," said Andrew F. Cheng, NEAR project scientist at APL.
NEAR (for Near Earth Asteroid Rendezvous) was designed and built at APL and launched in February 1996. Its scientists hope to learn how to maneuver around small objects. They also want to study the composition of near-Earth asteroids -- a type considered most likely to pose a collision threat to the Earth.
"This knowledge is very important if we ever have to do something about one of these asteroids," said Carl Pilcher, director of solar system exploration at NASA. Asteroids were the first solid objects to condense as the solar system formed some 4.5 billion years ago. Scientists want to see whether Eros' chemical composition matches that of the most common type of meteorite found on Earth, called "ordinary chondrites."
"If we can make that link, we will have been able to put our finger on an object that, from a geo-chemical point of view, has not changed since its formation 4 1/2 billion years ago," said Cornell University's Joseph Veverka, who heads NEAR's science team.
In June 1997, NEAR flew within 753 miles of a 41-mile-long asteroid called Mathilde. Photos and other measurements made during the fly-by revealed craters half the asteroid's width. Density measurements revealed it to be nearly light enough to float if dropped in water. The findings suggest it is a loose ball of rocky rubble rather than a solid rock.
On Sunday, NEAR is scheduled to fire its rockets in the first of four maneuvers over three weeks that will reduce its speed relative to Eros to just 19 mph. On Jan. 10, NEAR should begin orbiting Eros at an altitude of about 630 miles.
By May, APL scientists hope to have lowered NEAR's orbit to within 18 miles of Eros for detailed photo and chemical mapping.
The mission's success depends on the star-tracker. It must scan the heavens for pre-programmed sets of stars, and lock onto them in order to keep its solar panels, antennas and thrusters aimed in the right direction.
When it can't find its guide stars, the tracker shuts down the spacecraft to protect it until mission controllers can solve the problem.
Mission manager Robert W. Farquhar said controllers have contingency plans for recovering control quickly should NEAR enter a safe mode during the orbital maneuvers. NEAR has gone into safe modes "four or five" times since its launch, he said.
Pub Date: 12/17/98