The Maryland-built NEAR spacecraft is back on track for a rendezvous with the asteroid Eros.
But scientists at mission control in Laurel say it will arrive a year behind schedule and short of fuel after the unplanned shutdown of its main engine on Dec. 20.
The fuel shortage may shorten the time available for scientists to study Eros.
The engine failure has been blamed on a computer programming glitch, which scientists at the Johns Hopkins University Applied Physics Lab in Laurel say has been fixed with new computer instructions radioed to the spacecraft.
A 24-minute firing of the same engine Sunday went flawlessly, and NEAR is now expected to arrive at Eros in mid-February 2000 -- 13 months behind schedule.
The engine shutdown also led to the unexplained loss of 30 kilograms of the hydrazine fuel that powers the spacecraft's smaller maneuvering thrusters, APL officials said.
The fuel losses could cut three months from the spacecraft's planned 13-month operational life after it finally gets to Eros.
NEAR mission manager Robert W. Farquhar said the loss is not critical.
"I don't think it makes much difference," he said. "Everybody should be very happy, because it looked like they had nothing."
APL space department head Tom Krimigis said an independent review board, with experts from APL and around the country, will conduct a "very intensive, quick investigation" to make sure similar software failures are avoided in the future.
Ed Weiler, NASA associate administrator for space science, was expected at APL today for a briefing on the $211 million mission.
NEAR was designed and built at APL.
It was launched in February 1996 as the first of NASA's "better, cheaper, faster" Discovery series of space science missions.
It has already provided hundreds of close-up photos of a 41-mile-long asteroid called Mathilde, snapped during a fly-by in June 1997.
It had been scheduled to reach Eros this Sunday.
But on Dec. 20, a planned 20-minute firing of NEAR's main engine -- needed to catch up with the faster Eros -- ended as soon as it began.
Radio contact with the spacecraft was lost for several hours.
The 30 kilograms of lost hydrazine fuel represents "about 70 percent of the fuel we planned to have when we went into orbit" around Eros, Farquhar said.
The thruster fuel is needed to adjust NEAR's trajectory as it approaches Eros, and later to ease it into progressively lower orbits.
NEAR's planners anticipated such problems by packing lots of extra fuel.
Still more could be set aside by using NEAR's rocket engine for two future mid-course maneuvers. The engine relies on other fuel than the hydrazine.
But the Dec. 20 failure left Farquhar wary of waking the big engine again.
"It would probably work fine. But it doesn't buy you that much," he said.
"If we don't need to fire it again, we won't."
The mission delay adds a year of mission control costs to the venture. NASA officials have not yet calculated the sum.
Scientists used the delay to snap 1,100 photographs of Eros as the asteroid flew past the stalled spacecraft Dec. 23, giving them a head start on calculating Eros' spin axis and mass -- data critical to the APL navigators who must place NEAR in orbit around Eros.
For more information, see http: //near.jhuapl.edu