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 The spacewalkers began their
seven-hour jaunt nearly an hour early, eager to begin the daunting construction work.
"Another great day at the office," spacewalker Scott Parazynski told Douglas Wheelock as they floated out of the shuttle's hatch.
Parazynski will conduct a brief inspection to help engineers understand what's wrong with a gear that controls the station's solar wings.
The rotary joint, which was installed in June, makes sure the huge solar panel wings on the right side of the space station are facing the sun. It has been experiencing electrical current spikes for nearly two months.
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.
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.
Parazynski will inspect the matching rotary joint that turns the space station's left set of solar wings toward the sun. NASA wants to see what a perfectly running unit looks like to compare it to the malfunctioning one.
But Parazynski and Wheelock will mainly focus on finishing the grueling task of moving the massive beam to its new home on the far left end of the station. The 17 1/2-ton girder began its 145-foot journey on Sunday.
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 The spacewalkers also plan to help astronauts inside the station use a robotic arm to hook up the beam to the orbiting outpost's backbone. Then they will install bolts to hold the beam in place, connect wires to provide power and remove protective covers.
The girder's huge solar panels, which are folded up like an accordion, will be unfurled later Tuesday. The panels, which extend 240 feet from tip to tip when outstretched, will be controlled by the normally functioning left rotary joint.
Given the problems with the right rotary joint, NASA needs the newly installed solar panels up and running 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 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.
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On the Net:
NASA: http://spaceflight.nasa.gov/
[The Associated Press;
By LIZ AUSTIN PETERSON]
Copyright 2007 The Associated Press. All rights reserved. This
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