The task at hand this time, though, was even more crucial. Spacewalking astronauts Steve Smith, John Grunsfeld, Michael Foale and Claude Nicollier, had been sent to repair the $3 billion Hubble Space Telescope, and its future was at stake.

"People let us know that quite frequently while we were training -- 'Please be careful of my $3 billion telescope'," recalled mission-specialist John Grunsfeld, who worked on Hubble during in the shuttle mission's first and third extravehicular activities, or EVAs. He was charged with replacing some of the telescope's six gyroscopes (four of which had failed), and a radio control unit.

"For just a brief period of time, maybe two or three minutes, I forgot that I was in the spacesuit, I forgot that I was in space, I thought that I was just back training, taking the box off,"

He and crewmate Claude Nicollier, both astronomers by training, shared their experiences last week at the meeting of the American Astronomical Society in Atlanta.

Despite the near-record length of the spacewalks, the astronauts barely noticed, Grunsfeld said. "You don't carry a watch on your spacesuit, you just kind of work until somebody tells you to come in."

The crew set more than just spacewalking records. It was the most experienced space shuttle crew in history -- a statistic that reflected just how important NASA views the Hubble Space Telescope, Grunsfeld said.

Grunsfeld and mission specialist Steven Smith performed the first spacewalk. The two opened the telescope to replace a set of failed gyroscopes that had left Hubble unable to observe since mid-November.

To get at the difficult-to-reach gyroscope set number one, robotic arm operator Jean-Francois Clervoy used the mechanical arm to deliver Grunsfeld into an awkward position inside the telescope. "Jean-Francois put me on my back right into the bowels of the telescope," Grunsfeld said. "I was backed into the telescope looking up along the focal length with my head probably about one or two feet away from the prime focus of the Hubble Space Telescope."

At that point, Grunsfeld said he felt a profound kinship to Edwin Hubble sitting in the prime focus of the 200-inch Hale telescope on Palomar Mountain, in California. Hubble helped design and build the Hale, and was the first person to observe through the telescope, which, after its completion in 1949, became the world's largest.

The work space had its similarities to a mechanic's prone position beneath a car's undercarriage, but the pressure was far greater. "Any place you move your elbows or your hands too far, you hit something that folks at the Goddard Space Flight Center said, 'Well, dont hit this, or youll ruin the telescope,' " Grunsfeld said.

"If we had damaged the telescope, Claude and I would not be here at the [American Astronomical Society] meeting, we would be in Bolivia, growing big beards, and trying to hide" he joked.

That didn't happen, and even the most difficult tasks were met with success. One of those moments came for Grunsfeld during the third of the mission's three EVAs. His task was to replace a small radio transmitter on the inside of one of the spacecraft's doors.

"They never thought the folks that designed the telescope that that would be replaced. Its an extremely reliable box," Grunsfeld said. The screws that sealed the box were less than a quarter inch and the device was attached to the rest of the telescope by "sub-miniature assembly connectors," he said. "Using this very bulky glove and a pressurized hand, I had to learn to operate these tools."

He succeeded in replacing the unit, but not without intense concentration on the task.

"I was sitting there working and for just a brief period of time, maybe two or three minutes, I forgot that I was in the spacesuit, I forgot that I was in space, I thought that I was just back training, taking the box off," Grunsfeld said. "It was kind of an interesting experience then suddenly to be brought back to reality, to look up and there's the Earth and the Hubble and realize that this is the real thing."

Grunsfeld credits the training the telescope mechanics got at NASA for such ability to concentrate on getting the job done. For every hour they spend in space, the astronauts completed about 10 hours of underwater training exercises in NASA's 6.2 million-gallon Neutral Buoyancy Laboratory at the Johnson Space Center in Houston.

A successful spacewalk requires an exhaustively choreographed schedule, fellow spacewalker Claude Nicollier said. "We call them ballets, because each set of actions has to be executed precisely, according to a very accurate time line, and no deviations are tolerated."

Nicollier's toughest duty, which he performed during EVA 2, was to replace one of the satellite's fine guidance sensors, a contraption about the size of a baby-grand piano. The Hubble has three fine guidance sensors that keep the telescope pointed accurately when it is observing.

Thanks to training in NASA's virtual reality lab, Nicollier had no trouble maneuvering the oddly-weighted device. The lab uses computer-animated visuals and a metal box rigged to a set of cables, motors and counterweights to simulate the manipulation of the unit.

"For me it was no surprise when I handled the real fine-guidance sensor, although it was massive and the center of mass is quite far away from the point adjacent to me, it felt like what I had experienced in the virtual reality lab."

Nicollier's specialized training began in 1980 when he came to the United States as a European Space Agency astronaut to train with NASA. His preparation for space began much earlier, though, as a young astronomer in the Swiss Alps in the 1970s.

Nicollier spent years at the beginning of his career observing from telescopes there, and at the European Southern Observatory in Chile. "In a way, it's kind of a preparation for spaceflight," Nicollier said. "You're already at high altitude, but also, there's a form of isolation that you experience alone on a mountaintop 12,000 feet above sea level."

During winter weather, equipment failures were common. Frozen pipes, electrical-power outages, cuts in telephone communications or trouble with telescopes or detectors often occurred and the astronomers stationed at the observatory would have to repair the troubles on their own. If the telephone lines were still open, Nicollier could call down to engineers who would talk him through repairs.

"So there were, in a way, a lot of similarities to spaceflight . . If there was a problem we had to resolve it ourselves, although we could get voice or picture help from the ground, but often no more than that."

Grunsfeld is accustomed to spending long nights observing the skies from mountaintop observatories under dark skies, but he said being up in the shuttle was "absolutely the best environment I've ever worked in."

After almost four days with Hubble, it was time for the Discovery crew to send the telescope out on its own again.

"For me it was really a very touching moment," Grunsfeld said. "I felt like I'd been training for this my whole life, and we went out and did the work, it was over so quickly."

When the shuttle uncoupled from the telescope, ground control commanded the aperture door opened, and the high-gain antennas to deploy. Shuttle Commander Curtis Brown fired pulses from Discovery's jets to ease the craft away from the refurbished telescope, Grunsfeld said.

"The trajectory of Hubble was such that it went right over our heads," he said. "About 12 feet above our heads in the overhead window we watched Hubble as it finally passed overhead, the orbiter slowly rotated out from under it. And then we had these tremendous views. And it was if we were saying goodbye to a new friend, one that we had gotten very close and personal with."

Although the servicing mission had to be cut short because NASA required the shuttle to be on the ground by the end of the year as a precaution against any potential Y2K computer glitches, the important work got done, Nicollier said. The activities that would have been performed on a cancelled fourth EVA, which included wrapping the telescope in a new thermal-insulation blanket, will be performed on the next Hubble servicing mission in the spring of 2001.

Overall the mission was a stunning success, the astronauts said. Despite a decade in space, Hubble is in remarkable condition, and should enjoy another decade, at least, of making cutting-edge observations.