HOUSTON The mission to repair the Hubble Space Telescope has come down to the wire.
NASA scientists say electrical wires inside the six gyroscopes aboard the orbiting telescope are corroding and then breaking. So far, three of the six gyros aboard Hubble have failed.
The telescope needs three gyros to continue its science mission.
Even if another gyro fails before space shuttle Discoverys crew can replace them, the space agency says the system will go into a "safe" mode and ground controllers will operate the craft.
When another gyro failed this year, leaving only three operating, NASA pushed up the schedule for a servicing mission originally scheduled for 2000. Discovery should have flown in October, but shuttle fleet wiring problems and other delays kept it grounded.
NASA has set a tentative launch date of December 6 for Discoverys mission to Hubble.
"This is not a routine servicing mission. Its not an emergency mission. Its somewhere in between," said Dr. Edward Weiler, associate administrator for space science at NASA.
He added the scientific community hasnt displayed any anxiety about the potential loss of scientific research time, but the sooner the gyros are replaced the better.
The gyros are a critical element in the pointing system to keep the telescope on target and stabilized to get clear images. The devices have a four to six year life when operating and the agency changes the gyros at regular intervals. NASA plans to replace the gyros again in 2003.
In 1993, astronauts replaced four gyros on the telescope. Since then, a gyro has failed in 1997, 1998 and one this year. Astronauts returned to Hubble in 1997 but none of the gyros were replaced at the time.
"These are the most accurate gyros that have been produced," said Dr. John Campbell, the Hubble project manager. "Without those gyros those stars would be just fuzzy points of light."
Campbell said the gyros and pointing system are so accurate the telescope has observed the same target for 10 days.
Each gyro is custom made and filled with fluid the thickness of motor oil. Scientists believe pressurized air used to force the fluid into the gyro is the likely wire corrosion culprit. Oxygen may have reacted with the fluid, corroding the silver/copper alloy wires that are about as thick as a human hair.
A new process using nitrogen instead of air has been used on five of the six gyros going aloft.
Debra Henretty, a NASA engineer working on the gyros, said the agency did not have enough time to produce a sixth gyro with the new process, so one built with the old way will go aloft.
The gyros are housed in pairs in three rate sensing units located near the telescopes middle.
Discoverys astronauts will replace the three sensing units as one of their initial tasks in the first of four planned spacewalks.
Astronauts John Grunsfeld and Steve Smith will enter Hubble and replace three of the units. Each unit weighs about 24 pounds on Earth.
Grunsfeld said it will be a delicate operation as the boxes are located near critical elements of Hubbles scientific package.
"Weve been told that once were in the telescope to be a statue and not touch anything other than the gyroscopes," Grunsfeld said.
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