Spacewalking astronauts bolted a solar power tower to the international space station on Tuesday, completing an ambitious three-day moving process that ended with elation when the beam's giant solar panels began to unfurl.

Their joy turned to concern, however, when a rip was spotted in the second solar panel.

NASA needs to get the tower up and running to prevent malfunctioning station equipment from delaying the addition of a much-anticipated European research lab.

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A massive rotary joint is supposed to make sure the solar panel wings on the right side of the space station are facing the sun.

But the gear, which was installed in June, has been experiencing electrical current spikes for nearly two months.

The solar panels on the 17½-ton girder that was installed at its new location Tuesday were folded up like an accordion for the move, and the first one slowly was unfurled as the seven-hour spacewalk wrapped up, gleaming like gold in the sun.

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The crew kept spacewalker Scott Parazynski and Douglas Wheelock apprised of the first solar wing's unfurling as they floated back inside.

Their reaction: "Wow, that's great," and "Awesome!"

"It's a good day's work right there," Parazynski said.

The astronauts abruptly stopped the unfurling of the second panel, however, as soon as they saw the rip right next to the edge.

By then, the panel was about three-quarters of the way out. The astronauts beamed down photos of the torn and crumpled section so Mission Control could analyze them and determine the extent of the damage.

At Mission Control's request, the astronauts retracted the wing just a bit to ease the tension on it.

A spacewalking astronaut found black dust resembling metal shavings inside the motorized joint on Sunday. NASA has limited the joint's motion to prevent the debris from causing permanent damage, but that also limits the system's ability to generate power for the station.

Parazynski spent part of Tuesday inspecting the matching rotary joint that turns the space station's left set of solar wings toward the sun. NASA will examine images he gathered of the perfectly running unit to compare it to the malfunctioning one.

There were no shavings inside the joint, and Parazynski said everything looked pristine.

"It's right out of the shop, no debris whatsoever," he said.

Parazynski and Wheelock guided astronauts inside the station as they used a robotic arm to hook up the beam to the orbiting outpost's backbone.

The spacewalkers then began installing bolts to hold the beam in place and connecting wires to provide power.

"Oh, I love this job," Parazynski said as they worked 220 miles above southeast Asia. "Beautiful view."

Given the problems with the right rotary joint, NASA needs the power generated by the newly installed solar panels to proceed with the planned December launch of the European Space Agency's science lab, named Columbus.

That lab and a Japanese lab set to be delivered early next year will latch onto the new Harmony module that Discovery delivered last week.

The space agency added a day to Discovery's mission so spacewalking astronauts could conduct a detailed inspection of the troublesome joint. That work is scheduled for Thursday.

To make room for that inspection, managers canceled a shuttle thermal tile repair demonstration that was scheduled for that spacewalk. The test was added to the mission after a piece of fuel-tank foam gouged Endeavour's belly on the last shuttle flight in August.

Any repairs to the malfunctioning gear would be put off until after Discovery departs.

Discovery is now scheduled to undock from the space station on Monday and return to Earth on Nov. 7.

The shuttle crew was awakened early Tuesday with "Malaguena Salerosa" by Chignon. Astronaut George Zamka said the song was a "rousing version" of an old mariachi song that his mother liked when she was growing up in Bogota, Colombia.

"That was for you, Mom. Thank you very much," Zamka said.