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The Near-Earth Asteroid Rendezvous (NEAR) spacecraft, which was supposed to have begun orbiting the asteroid Eros next week, will meet up with it in February 2000 instead. The delay, which project managers say will not reduce the mission's scientific return, was caused by a software problem when the spacecraft's main engine was fired on 20 December.

NEAR — operated for the US space agency NASA by the Johns Hopkins University Applied Physics Laboratory (APL) — was designed to orbit the asteroid, photographing its surface, mapping minerals, and returning gravity data.

The manoeuvre on 20 December was intended to position the spacecraft to begin orbiting Eros. But when the engine fired, the acceleration triggered software designed to protect the spacecraft from uncontrolled motion. NEAR automatically aborted its engine burn and waited for a signal from the ground, ruining chances of a rendezvous with Eros this year.

Engineers switched to a contingency plan for an engine firing on 3 January and a rendezvous 13 months later. Another option, for a meeting with Eros in July, was scrapped because the command sequences had to be revised too quickly.

Tom Coughlin, the NEAR project manager at APL, says the software that triggers the abort modes will be reworked to avoid a similar mishap in the future. The spacecraft wasted about 30 per cent of its fuel during the aborted manoeuvre, but Coughlin says it still has enough to execute the full year-long study of Eros.

Managers want to keep open the option to land the spacecraft briefly on Eros after the main mission ends. Such an extension to the mission, which would have to be approved and funded by NASA, is still possible with the amount of fuel remaining.

The delay is likely to cost NASA millions of dollars, but has at least one benefit. Because the spacecraft was able to return images and take data on 23 December, scientists will have a better understanding of the asteroid's shape and mass when close manoeuvres begin next year.