A vital gyroscope on board the Hubble Space Telescope has sputtered and quit, forcing the high-flying observatory to shut its eye on the heavens until a crew of astronauts can get there to make repairs.
The shuttle Discovery and its crew of seven are scheduled for blastoff Dec. 6 on a much-postponed mission to replace all six of Hubble's gyroscopes and to make other repairs and improvements.
On the ground, astronomers will lose an estimated 74 observations each week until the telescope is working again. Regular observations won't resume until 10 or 12 days after the repairs. Some may be delayed for up to a year.
"They all have to be rescheduled. It's disruptive," said Michael G. Hauser, deputy director of the Space Telecope Science Institute in Baltimore. "But we do promise people they will get their observations."
The disruption of Hubble's scientific work is apparently the first since a defect in the space telescope's main mirror was corrected in 1993, although observations are routinely stopped during servicing missions and curtailed briefly during heavy meteor showers.
Hubble was launched in 1990 with a full complement of six gyroscopes. Only three working gyros are needed to reliably aim the telescope at celestial targets. The other three are spares.
But breakdowns have occurred frequently. In 1993, after a series of failures, all six original gyros were replaced by astronauts during the first servicing mission to the telescope.
Two of the replacement gyros were replaced during a 1997 servicing mission.
And by February this year, three more had quit. NASA traced the problem to fine wires inside the devices that tend to corrode and break prematurely. Engineers blamed contaminants in the fluid in which the gyros spin. The new, improved models should last longer, they said.
With no more working spares on board, and with no servicing mission planned until mid-2000, Hubble officials asked NASA last spring to schedule an emergency repair mission for October. They hoped the spacecraft's three surviving gyros would keep spinning until then.
And they would have if Discovery had launched in October. But the shuttle was grounded for nearly two months while technicians searched for suspect wiring and heat-shield tiles that had turned up on other shuttles. Then they had to replace an engine when a broken drill bit turned up in a coolant chamber.
Astronomers' luck ran out just before 8: 30 a.m. Saturday when a computer display at the Goddard Space Flight Center in Greenbelt flashed red. Electrical current in one of Hubble's last functioning gyros suddenly more than doubled.
This gyro breakdown didn't look like the others, Hauser said. Engineers could not explain it, and a review board has been formed at Goddard to finds answers.
The telescope responded as it was programmed to do, said David S. Leckrone, Goddard's senior project scientist for Hubble.
During the next 45 minutes, all of Hubble's science instruments shut down; the large door that protects the telescope's sensitive mirrors began to close, and the spacecraft rotated to point its solar panels into the sun, ensuring it would have electrical power.
For now, Leckrone said, "it's stuck where it is." Engineers at Goddard did attempt to turn the failed gyroscope on again, cycling the power on and off -- a bit like flipping the wall switch after the ceiling light burns out. But "it remained unresponsive," Leckrone said.
Discovery's pilot should have no trouble approaching the telescope and getting it into the cargo bay, he said. "We can grapple it like this, or put it into a deeper safe mode. There should be no impact at all on our mission -- just on our desire to get up there and get it fixed."
The shutdown has foiled NASA's plans to shield the telescope from this week's Leonid meteor shower. Like many other satellite operators, NASA had planned to turn Hubble to minimize its profile Thursday as the Earth plows through the meteors -- dust particles left by the comet Tempel-Tuttle.
"We will not be able to do that now," Leckrone said. "But we've done an analysis that indicates there's still a very low probability of an impact even without that precautionary measure."