The failure of one of the telescope's last three gyroscopes suspends all scientific research involving Hubble, although it remains in a safe orbit over Earth. The breakdown also turns the Dec. 6 repair mission into an emergency.

"It's definitely more important now," said Steve Maran, an assistant director of space science at NASA's Goddard Space Flight Center in Greenbelt, Md., on Monday.

Hubble can safely remain in its hibernated state for months, if necessary.

Hubble needs at least three of its six Allied Signal gyroscopes to aim at celestial targets. Three gyroscopes had already failed when NASA scheduled the servicing call earlier than originally planned. The fourth gyroscope shut down about 8:30 a.m. EST Saturday.

Flight controllers got a hint about 12 hours earlier that something was going wrong with one of the three remaining gyroscopes. The telescope's Fine Guidance Sensors were unable to lock onto targets, as one gyroscopes drifted. Hubble was performing a calibration exercise when gyroscope No.1 shut down.

With only two gyros operating, the telescope goes into sun-point mode, said Ray Villard, a spokesman for the Space Telescope Science Institute, the organization that oversees Hubble's scientific operations.

"It just keeps itself oriented so the solar panels are getting the power they need to keep the spacecraft alive, really," he said.

Until astronauts on the repair mission get to the telescope and replace the gyros, Hubble cannot be aimed at specific targets in the sky and observing with it is impossible.

The telescope is operating as designed, given this kind of gyro failure, Villard said.

"While we would like to be doing science, it was planned and expected that the telescope would go into this level of hibernation given an on-board problem," he said.

Discovery's astronauts anticipate no problems once they are in space when they attempt to snatch the 13-ton spacecraft from orbit and tow it into a cargo bay for repairs.

"It's very stable. It's just not doing any science. That's the only difference," said Kyle Herring, a spokesman for NASA's Johnson Space Center in Houston, Tx.

Speeding around Earth at 17,500 mph, Discovery is to make its final approach to Hubble at 3:44 a.m. EST on Dec. 8, ending a two-day chase that is set to begin with the shuttle's 2:37 a.m. Dec. 6 liftoff from Kennedy Space Center.

Veteran mission commander Curt Brown will ease up to Hubble from directly below the bus-sized telescope. Wielding the shuttle's 50-foot robot arm, European Space Agency astronaut Jean-Francois Clervoy then will pluck Hubble from orbit and place it on a Lazy Susan-type device in Discovery's 60-foot-long cargo bay.

Herring said plans for the high-flying rendezvous will change little due to the science-stopping gyro failure.

"From a rendezvous, approach and grapple standing, there may be a few subtle changes. But the spacecraft is not tumbling, so we don't expect to have any major problems," he said.

Once Hubble is inside Discovery's cargo bay, astronauts will perform a quartet of space walks to repair and service the telescope, which was launched in April 1990.

Cape Canaveral Bureau Chief Todd Halvorson and Staff Writer Greg Clark contributed to this report.