U.S. astronauts Robert Curbeam, who has taken two previous spacewalks this week, and Sunita "Suni" Williams, a spacewalking rookie, began the spacewalk around 2:25 p.m. EST as the coastline of Australia passed by 220 miles below. The spacewalk was scheduled to last six hours.
"It's going to be a blast!" Williams said after Mission Control instructed her to have fun.
Williams joined an elite group of eight other female spacewalkers. Only seven other U.S. women and a single Russian woman have participated in the 281 spacewalks taken since 1965.
"Welcome to the club, Suni," Curbeam told her.
As part of the spacewalk Saturday, "flight controllers today successfully activated a powerful pump on the space station's main solar power truss, pushing ammonia coolant through cold plates and into big radiators to keep critical electrical gear from overheating, reports CBS News space analyst William Harwood. "The pump activation was the final major step in a complex two-spacewalk re-wiring job to route solar array electricity through four big main bus switching units, transformers and other components making up the lab's permanent electrical system."
About half of the lights, a fire alarm, some ventilation ducts and some communication on the U.S. section of the space station were powered down as a safety precaution to the spacewalking electricians who unhooked and plugged back in connecting hoses to half of the lab's electrical system. The astronauts reconnected the space station's electrical system to a permanent power source from a temporary one.
Less than two hours into the spacewalk, the rewired electrical system was powered back up, and Mission Control reported that it was operating without problems.
"It's great to have some good, on orbit electricians working for us,'' said astronaut Stephen Robinson in Mission Control.
NASA had to race to get the space station's ammonia cooling system operating before the equipment overheated. The ammonia flowed without trouble, earning a sigh of relief from Curbeam.
"Excellent. That is awesome news," said Curbeam, whose spacesuit was inadvertently contaminated by a leaking coolant line during a similar spacewalk in 2001.
The spacewalk tasks were almost repeats of those done Thursday by Curbeam and Swedish astronaut Christer Fuglesang, who finished their spacewalk an hour ahead of schedule. If Curbeam and Williams worked as quickly, NASA planned to have them inspect the halfway retracted solar wing, and possibly pat the box in which it's folding up in an attempt to loosen wire tension.
Click here to read CBS News space consultant Bill Harwood's full report on the spacewalks.
The obstinate array has refused to fold properly despite repeated attempts by remote control since last Wednesday.
Other spacewalk tasks included rewiring power feeds to the Russian side of the space station and relocating protective panels which stacked together are known as "the Christmas tree."
The half-retracted solar wing was part of the space station's temporary power system. A primary goal of Discovery's visit to the station was to rewire the lab and hook a new set of solar wings delivered in September onto the permanent electricity grid. To do that, NASA needed to retract the old solar panel so that the new ones had room to rotate with the movement of the sun to maximize the amount of electricity generated.
The old solar panel retracted enough to give the new ones clearance, but it did not fold all the way as NASA wanted.
NASA managers were expected to decide late Saturday whether astronauts will conduct a fourth spacewalk Monday. NASA can keep the array in its current position until April, leaving open the possibility that the next shuttle crew in March or even the current station crew could fix the problem during a spacewalk. In a worst-case scenario, the array could be jettisoned.
Space shuttle Discovery's astronauts were heading into the home stretch of their 12-day mission, which includes seven days at the space station. Discovery delivered a 2-ton, $11-million space station addition which was installed during the first spacewalk and planned to rotate out a station crew member upon undocking Monday.