CAPE CANAVERAL, Fla. -- Visiting shuttle astronauts ventured outside the International Space Station late Tuesday after a computer failure almost forced an 11th-hour switch in plans to mount gas tanks on the outpost's new $164 million airlock.
Coming three months after the station's three U.S. command-and-control computers crashed during a construction mission, a similar hard drive failure on a recently installed spare temporarily rendered the outpost's $600 million robot arm inoperable.
With two spacewalkers suiting up in shuttle Atlantis, ground controllers launched a hurried bid to revive the computer, and thus, the so-called "Big Arm" -- the only crane long enough to install two of four gas tanks designed to replenish air supplies within the station airlock.
The joined shuttle-station crews, meanwhile, stood poised to carry out a back-up plan: Using the shuttle's shorter robot arm to install two other tanks within its reach.
A simple re-boot of the failed computer, however, fixed the problem, enabling spacewalkers Michael Gernhardt and James Reilly to press ahead with their spacewalk as originally planned.
And that prompted a radio call of congratulations to computer specialists who were proceeding with caution -- and keeping a watchful eye out for subsequent trouble -- on the ground."Did I mention today that you guys are really good down there?" station flight engineer Jim Voss said.
"Jim, we might be speaking too soon, but we still like to have our egos stroked," Canadian astronaut Bob Thirsk replied from NASA's Mission Control Center in Houston.
Running about an hour behind schedule, Gernhardt and Reilly started their spacewalk shortly after 11:00 p.m. EDT (0300 GMT Wednesday) as Atlantis and the station cruised some 238 miles (380 kilometers) above Earth.
The first order of business: Unlatching a 1,200-pound (540-kilogram) oxygen tank so that outpost flight engineer Susan Helms could hoist it up toward the airlock with the 57.7-foot (17.5-meter) station arm.
Atlantis mission specialist Janet Kavandi stood ready to lift Gernhardt up to the airlock with the shuttle's 50-foot (15-meter) robot arm. Reilly planned to scale up the side of the station, using bar-like handholds to climb up to the same work site.
Ultimately, Helms was to hand off the hefty tank off to the spacewalking duo so it could be mounted to the outer shell of the station's new doorway to orbit.
A nitrogen tank was to be installed on the airlock in the same manner during the second of three spacewalks planned during the shuttle's stay at the station.
Launched last Thursday from Kennedy Space Center, Atlantis rumbled into orbit with 6.5-ton airlock and four of the high-pressure gas tanks cradled in its cargo bay.
Dubbed "doghouses" due to the shape of their protective metal cases, two of the gas tanks are filled with oxygen and the other pair contains nitrogen -- the prime constituents of breathing air.
At least one set of oxygen and nitrogen tanks must be put in place for the station's new airlock to be fully functional.
The last two tanks are to be installed during a spacewalk to be staged out of the station's new airlock later this week.
Now scheduled to begin late Friday, the excursion originally had been scheduled to begin late Thursday but mission managers decided to delay the sortie 24 hours and extend the shuttle's stay at the station by a day.
The reason: An airlock water line leak and fruitless efforts to fix a failed ventilation valve put the joined crews about a half-day behind a busy airlock activation schedule.
The water line was quickly capped but the leaky valve will have to be replaced. That two-hour repair job now is scheduled to take place late Wednesday or early Thursday.
The mission extension, meanwhile, will delay by a day both the shuttle's departure from the station and landing here at Kennedy Space Center.
As it stands, Atlantis and its four-man, one-woman crew will pull out of the station Sunday, heading for an early Tuesday landing at NASA's coastal Florida spaceport.
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