The space station’s two American astronauts went out on a riskier-than-usual spacewalk Wednesday to fix one of two equipment failures that have crippled their power system and threatened to stall construction.

Commander Peggy Whitson and Daniel Tani floated outside well before dawn, 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.

It was a hazardous job because the astronauts risked being shocked. For safety, they waited until the international space station was on the dark side of Earth, then carefully undid fasteners and disconnected cables, and pulled out the old electric motor. Tani noted that one of the connectors was manufactured on his birthday, Feb. 1. “I’m reading the manufacture date,” he said.

A few minutes later, the spacewalkers popped in the new 200-pound-plus motor, a spare that had been stored on board. “We’re all breathing down here. Thanks a lot,” Mission Control said.

The tilting mechanism stopped working in early December, exasperating a power problem that arose three months earlier when a solar wing rotating joint jammed up and had to be shut down.

To avoid being shocked, Whitson and Tani had to do the replacement job in the darkness of night, pausing during the daytime swings around Earth when 160 volts of electricity would course through the cables. As an added precaution, the spacewalkers were advised not to point any nonessential lights at the solar wing in question to prevent power generation.

Because the motor serves as the structural backbone for the solar wing, the spacewalkers had to make sure the wing didn’t come off and fly away.

Earlier in the morning, the spacewalk almost ended up being aborted when a radio-relay problem prevented Whitson and Tani from hearing Mission Control. Flight controllers restored communication through a backup channel within 20 minutes.

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 next week with the European science lab, Columbus. However, 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.

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.

Wednesday’s spacewalk fell on the eve of the 50th anniversary of the launch of NASA’s first satellite, Explorer 1. The very next day, Friday, will mark the fifth anniversary of the Columbia disaster.

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