SPACE CENTER, Houston -- Working more than 370 miles above Earth with ratchet tools, two spacewalking astronauts Wednesday fitted the Hubble Space Telescope with new instruments needed to restore its unparalleled view of the cosmos.
It was the first of three spacewalks to be conducted over three days on the $3 billion observatory and, by far, the most important. The Hubble has been out of service since mid-November.
Keenly aware of the magnitude of the job, Steven Smith and John Grunsfeld floated out of space shuttle Discovery's hatch almost an hour early and quickly began organizing their tools in the cargo bay, where the Hubble was latched down.
"You ready to go?" Smith asked Grunsfeld. "Hubble needs us."
Four hours later, the men had replaced all six of the Hubble's gyroscopes. The work was made more difficult by stubborn bolts and storage-bin lids that wouldn't close.
A quick electronic check of the new gyroscopes showed they all worked. Additional testing was needed, though, before NASA could declare 100 percent success. In any event, it will be a few weeks before the Hubble resumes making observations.
Also on the evening's agenda: equipping each of the telescope's six batteries with a voltage regulator to prevent overheating. The spacewalk was expected to last six or seven hours.
The Hubble, which was plucked from space with Discovery's robot arm on Tuesday, has been useless to astronomers since a fourth gyroscope failed on Nov. 13. A minimum of three are needed to keep the 43-foot, 25,000-pound telescope steady as it aims at stars, galaxies and other celestial targets.
NASA believes the failures were the result of corroded wires. To avoid a repeat of the problem, engineers used pressurized nitrogen rather than air to force fluid into the new gyroscopes. The oxygen in the air was thought to have caused the corrosion.
The new gyroscopes, which come in 24-pound packs of two, cost $8 million in all.
Smith and Grunsfeld took turns working inside the tight cavity that houses the gyroscopes. They had only a few inches of clearance; one wrong move and they could bump critical instruments and leave the telescope in even worse shape than it was.
Smith worked on the Hubble during NASA's last service call, in 1997. This time, he was assigned the task of replacing the two side sets of gyroscopes because of his long arms; he is 6-foot-3«.
On Thursday, astronauts Michael Foale and Claude Nicollier will replace the telescope's outdated computer and put in a new fine guidance sensor, part of the pointing system.
Smith and Grunsfeld will go back out Friday to install a new radio transmitter and data recorder, as well as steel covers to protect the telescope from damaging solar rays.
If they have time, the spacewalkers will also do some of the minor tasks that had been scheduled for a fourth spacewalk, which was canceled after Discovery lifted off several days late on Sunday. NASA wants the shuttle back on Earth on Monday, well before New Year's Eve, to avoid Y2K computer problems.
This is the third service call to the Hubble. In December 1993, astronauts fitted the telescope with corrective optics because of a defective mirror. The Hubble got a tuneup in February 1997.