The Hubble Space Telescope went into hibernation over the weekend, its astronomical observations halted, when one of its pointing units failed, NASA said yesterday.

The telescope will remain in this so-called safe mode, which is akin to being asleep, until space shuttle astronauts install new pointing devices, or gyroscopes, and other crucial replacement parts. The mission is set for December.

The gyroscope, needed for aiming and holding the telescope steady, "just stopped in its tracks" on Saturday morning, said John Campbell, NASA's Hubble program director. The unit had been acting up for the previous 12 hours, becoming less and less accurate in its guidance, he said.

It was the fourth of six gyroscopes to fail on Hubble. A minimum of three are needed for the telescope to conduct observations.

When the gyroscope failed, Hubble's computer sensed the problem and ordered the aperture door to close, Campbell said.

"It's quite safe," he said. "We're not doing science, so the power load has been reduced."

Campbell said "it could have been a lot worse," considering that the next Hubble servicing mission originally was set for June 2000. The flight was split into two missions, with the first one scheduled for this fall, when the third gyroscope failed in February.

Hubble was launched in 1990. Four of the six gyroscopes were replaced during the telescope's first repair mission in 1993. One gyroscope failed in 1997 and a second in 1998. Corroded wires were believed to be the problem.

"We all hate sitting here not doing science, but it's better than it could have been," Campbell said.

Space shuttle Discovery and its crew will take up six new gyroscopes, as well as another data recorder, radio transmitter and improved computer. Liftoff is targeted for Dec. 6, almost two months late because of shuttle wiring repairs and an engine replacement.

"We're certainly more anxious for [the mission], that's for sure," Campbell said.

NASA never built the computer software that would have allowed Hubble to work with only two gyroscopes.

"We had never seen a way that we could do science" with two gyroscopes, Campbell said. "So we designed it that if we were going to have less than three, that the easiest thing to do was to build one safe mode that required none."

The two remaining gyroscopes are working fine, Campbell said.