Commander Peggy Whitson and Daniel Tani floated outside well before dawn in the United States, hauling a new motor that NASA hoped would enable a solar wing to tilt toward the sun again and draw more power for the orbiting complex.
The spacewalk had barely begun when a radio-relay problem cropped up, preventing Whitson and Tani from hearing Mission Control. Flight controllers managed to restore communication through a backup channel within 20 minutes, allowing the spacewalk to proceed.
"Welcome back," a relieved Tani told Mission Control.
The tilting mechanism stopped working in early December, exacerbating a power problem that arose three months earlier when a solar wing rotating joint jammed up and had to be shut down.
Engineers traced the most recent trouble to a suspect motor.
Luckily, the international space station had a spare on board.
To avoid being shocked, Whitson and Tani had to do the replacement job on the dark side of Earth, pausing during daytime passes when 160 volts of electricity would course through the cables, which had to be disconnected. As an added precaution, the spacewalkers were advised not to point any nonessential lights at the solar wing in question to prevent power generation.
The motor weighs close to 113 kilograms and is enclosed in a 75-centimetre can with two handles on the end, looking somewhat like a garbage can. It serves as the structural backbone for the solar wing, and the spacewalkers had to be careful that the wing didn't come off and fly away.
NASA is still uncertain what to do about the clogged joint, which is supposed to continuously rotate 360 degrees to keep the solar wings pointing toward the sun. As many as four spacewalks will be required later this year to remove metal shavings from the joint and get it working again.
Even with both failures, NASA could still launch Atlantis to the space station with the European science lab, Columbus. But unless the tilting mechanism is fixed, any further shuttle missions would be in jeopardy. The joint problem alone, if left unresolved, could delay shuttle flights starting in the fall.
It was the first spacewalk for Tani since his 90-year-old mother was killed in a car accident outside Chicago just before Christmas. Flight director Holly Ridings said Tani has been coping extremely well, and that his work has not been affected.
Tani was supposed to return to Earth in December aboard Atlantis, but his trip home was delayed because of problems with the fuel gauges in the shuttle's external tank. NASA is now aiming for a Feb. 7 liftoff after replacing a bad connector at the bottom of the tank.
The spacewalk fell on the eve of the 50th anniversary of the launch of NASA's first satellite, Explorer 1. The very next day, February 1, will mark the fifth anniversary of the Columbia disaster.