Spacewalkers Stephen Bowen and Alvin Drew wrapped up the second of two spacewalks planned for Discovery's eight-day stay at the station, the shuttle's 39th and final mission.
During the spacewalk that stretched over six hours, Bowen and Drew vented ammonia from a failed pump module that NASA plans to return to Earth on the final shuttle flight in June and removed an equipment pallet from outside Europe's Columbus module, among other tasks.
The pair got a late start due to a leak in Bowen's pressurized suit, which was caused by a damaged seal that crewmates easily fixed.
The helmet lights on Drew's spacesuit became detached late in the spacewalk, spurring ground controllers to order him back to the station airlock after Bowen was unable to reattach it.
"What have you been doing?" Bowen asked Drew. "I know we gave these a good shake test."
The six-member Discovery crew, on the 133rd mission in shuttle history, already has completed the primary goals of the 12-day flight. They attached an outside storage platform for spare parts shortly after reaching the station on Saturday. On Tuesday, they installed a combination storage room and mini-research lab to the outpost, completing assembly of the U.S. side of the station.
The space station is a $100 billion project of 16 nations that has been under construction 220 miles above Earth for the past 12 years. Russia plans to launch a final research module in May 2012.
Discovery is NASA's oldest surviving spaceship. Upon return to the Kennedy Space Center in Florida next week, Discovery will be prepared for its new mission as a museum piece.
Discovery is promised to the Smithsonian National Air and Space Museum in Washington, D.C. Retirement homes for sister ships Endeavour and Atlantis, which are scheduled to make their final flights in April and June, have not yet been announced.
NASA is ending the shuttle program due to high operating costs and to free up money to develop new spacecraft capable of flying to the moon, asteroids and other destinations in the solar system.