CAPE CANAVERAL, Fla. Forget the dangerous commute to work. What really makes Discoverys astronauts anxious is the prospect of doing irreparable damage to NASAs already disabled Hubble Space Telescope.

A world of antsy scientists will be watching this week as the astronauts attempt to resuscitate the crippled observatory, which was temporarily knocked out of commission when its pointing control system broke down in November.

The astronauts, meanwhile, know full well that even a small slip-up during an intimidating array of spacewalking repair work could permanently put the delicate telescope out of business, forever halting a stream of stunning discovery that has shed extraordinary light on the nature of the universe.

With the shuttle and Hubble both cruising along at 17,500 mph, Discovery skipper Curt Brown will pilot his winged spaceship to within 35 feet of the 13-ton telescope. Then its up to French astronaut Jean-Francois Clervoy to snatch the 40-foot-tall observatory from orbit with the shuttles 50-foot robot arm and place it in the rear of Discoverys expansive cargo bay.

Doing so will set the stage for a three spacewalks, the first of which is vital to fixing Hubble. The critical nature of the work, meanwhile, is not lost on Discoverys crew, which includes four professionally trained astronomers.

"I do feel a lot of pressure from my scientific background to get the spacecraft back in operation in orbit doing astronomy," said Grunsfeld.

"Its obvious that astronomers are eager to use this incredible space and time exploration machine," added Discovery mission specialist Claude Nicollier, a Swiss astronaut and astrophysicist. "Obviously, we are eager to bring the telescope back into operation for our fellow astrophysicists all over the world."

That, however, will be no easy task.

First off, the spacewalking repairmen will be clad in cumbersome spacesuits equipped with gloves that are about as nimble as catchers mitts, and their work environment will be both extreme and dangerous.

With Discovery circling Earth 16 times a day, temperatures on the sunlit and dark sides of the planet will range from 250 degrees Fahrenheit to minus 250 degrees Fahrenheit. In the airless vacuum of space, a serious rip in one of the $10 million suits could expose an astronaut to instant death.

The actual repairs, meanwhile, range from the type of blue collar grunt work done by earthbound construction crews to the kind of meticulousness required of watch-makers and surgeons.

Take a look at what the astronauts will be doing during the three spacewalks, which will take place on consecutive days. Each is expected to take about six hours to complete.

First and foremost are repairs to Hubbles failed pointing control system, which is designed to steady the observatory so it can precisely point at stars, planets, galaxies and other celestial objects.

Crucial to the system is a set of six gyroscopes, three of which are needed to operate the telescope. Four now are broken, and a new $8 million set of six gyroscopes are to be installed on Hubble by Smith and Grunsfeld.

The job itself is not fundamentally difficult. But like a golf fairway surrounded by sand traps and water, there is plenty of potential trouble.

Smith, for instance, will have to open a door and slide into the heart of the telescope, easing himself within inches of a trio of crucial star trackers, which also play a key role in precision pointing. Consequently, Smith will have to make sure he doesnt bump into them while hes inside the observatory.

Smith said former astronaut Story Musgrave, who replaced four gyroscopes during a 1993 Hubble repair mission, gave him the following advice: Be a statue.

The star trackers "are kind of fragile, and theyre about four inches away from my chest when Im in there," he said. "And the best advice Story had was, `Once youre in there, dont move."

Complicating matters is the fact that Smith also will be perilously close to internal insulation that could flake away and contaminate Hubbles ultra-sensitive science instruments.

"We certainly dont want to rub up against it because it would create little particles (that could damage the instruments,)" Smith said.

The gyroscope swap is just one of three chores Smith and Grunsfeld will carry out during the first spacewalk. The astronauts also will service a valve on an instrument cooling system and install six voltage regular kits designed to keep the telescopes aging batteries from overcharging and overheating.

Astronauts Nicollier and Michael Foale will perform what amounts to brain surgery on the second spacewalk of the mission, replacing Hubbles main flight computer with an upgraded version.

Like the gyroscope replacement work on the first spacewalk, the job itself is not considered intrinsically difficult. Nicollier and Smith will have to remove and replace some bolts, and there are several computer-like connectors that must be unhooked from the old unit and then reconnected to the new one.

The criticality of doing it correctly, however, is readily apparent. Without an operational flight computer, Hubble simply wont work.

The job "only involves six bolts and eight or nine connectors, so in that respect, its kind of a standard swap," said Smith. "I think the pressure has to do with the fact that the computer is the brains of Hubble, and thats why you want to be very careful on that task."

The $7 million computer will replace a 1970s-era unit that was upgraded with a now outdated Intel 386 co-processor in 1991. By no means, however, is it state-of-the-art. In fact, the new computer is based on Intel 486 microchip now considered virtually obsolete on Earth.

NASA officials are quick to point out that it takes four years to develop a computer that will operate properly in a high-radiation environment. And while it wont have the speed of a Pentium or G3 chip, the replacement still will be 20 times faster and have six times the memory of the old Hubble computer.

"Its not quite like going from a slide rule to a Macintosh or IBM, but its close," said NASA space science chief Ed Weiler.

Nicollier and Foale also will replace one of Hubbles three Fine Guidance Sensors with a refurbished unit. Shaped like a baby grand piano, the $13 million sensor gives Hubble the ability to remain locked onto an astronomical target for days at a time.

The resulting stability can be compared to holding a laser beam on a dime that is 200 miles away, or the distance between Washington, D.C. and New York City.

"We are eager to bring the telescope back into operation for our fellow astrophysicists all over the world."

Smith and Grunsfeld will be back in the shuttles cargo bay for the third spacewalk, during which the astronauts will replace a failed radio transmitter and install new solid state data recorder.

Neither job is considered "high priority" for the mission, but the radio transmitter replacement promises to be "one of the most difficult tasks on the mission," said Calvin Seaman, NASAs lead spacewalk integration engineer for the mission.

Unlike most Hubble parts, the transmitter which beams science data via satellite to project scientists on the ground is not designed to be swapped out in orbit.

Smith and Grunsfeld, as a result, will have to work with tiny bolts that will be difficult to keep hold of with stiff spacesuit gloves a chore the latter likens to repairing a watch with fingerless winter mittens.

In addition, the astronauts also will have to disconnect and then reconnect a coaxial cable similar to those on standard cable TVs another tough job for someone wearing gloves that are hard to manipulate.

About the size of a motorcycle battery, the failed transmitter is one of two designed to operate for 15 years. It inexplicably failed after eight. The other, however, still is operating as advertised and had been shouldering the burden of beaming back all science data until the telescope shut itself down last month.

Both the old and new recorders are capable of storing science data until it can be beamed back to Earth via satellite. But unlike the old mechanical device, the new recorder is considered less prone to failure. It has no reels, no tape and no moving parts. It also can store 10 times as much data as the old mechanical recorder.

- Outfitting parts of the telescope with a new metallic skin.

A decade in space has taken a harsh toll on Hubbles skin multi-layer insulation that protects the telescope and its instruments from extreme and rapid temperature changes the observatory is exposed to as it circles around the sunlit and dark sides of Earth.

Some of the insulation is torn and cracked particularly in areas exposed to the sun. So Smith and Grunsfeld will equip parts of Hubble with a new covers made of a specially coated stainless steel foil. Like oversized cookie sheets, the covers have been trimmed to fit over the doors of bays that house telescope electronics.

A fourth and final spacewalk was dropped from the flight in order to get Discoverys crew airborne before a Dec. 19 launch deadline.

The reason: NASA had to shorten a planned 10-day mission by two days and cancel the last spacewalk - to make certain Discovery would be back on the ground, with its orbiter and associated ground computers powered down, before the highly anticipated Y2K rollover.

Postponed until a spring 2001 Hubble servicing mission as a result was a job that amounts to an orbital version of hanging wallpaper.

Armed with seven rolls of specially designed fabric insulation, spacewalkers on the 2001 mission will cover much of the sunlit side of the tubular telescope. The idea is to better protect Hubble from the harsh space environment until the observatory is retired in 2010.

Outfitting the observatory with the new insulation was considered a low-priority job. Hubble project scientists, consequently, were willing to put it off in order to get Discoverys mission launched and the telescope fixed as quickly as possible.

After all, some 100 astronomy projects are on hold until the telescope can be repaired, and only at that point will Hubble scientists start breathing a sigh of relief.

Up until then, theyll spend several nail-biting days watching Discoverys spacewalkers crawl around the fragile telescope, knowing that any misstep can put Hubble into premature retirement.

Anne Kinney, a senior astronomer at NASA Headquarters in Washington, D.C., admitted the three-day ordeal promises to be "terribly nerve-wracking."

"You watch with you heart in your mouth, basically, because people live and breath this stuff, and it really means a lot to them," she said.

NASA Hubble program manager John Campbell agreed.

"Thats true," he said. "But you have to suppress that because you have to keep thinking ahead to what might go wrong, and how you would react to that. So its necessary to stay as cool as possible."