Spacewalking astronauts slipped outside the International Space Station on Sunday to work on the outpost's cooling system and finish outfitting Japan's newly arrived Kibo research laboratory.
The maintenance work was preventive - the station's air conditioner is functioning well but NASA wants to ensure it keeps doing so. With shuttle flights winding down, the US space agency is trying to get the space station as ready as possible to operate without shuttle servicing calls.
NASA will retire the aging shuttle fleet in two years after nine more trips to the $100 billion space station and a single flight to upgrade the Hubble Space Telescope.
Soaring 210 miles (338 km) over Earth, astronauts Michael Fossum and Ronald Garan floated outside the station's airlock at 10 a.m. EDT (1400 GMT) for their third and final spacewalk of shuttle Discovery's station assembly and maintenance call.
Their main job was to replace a 550-pound (249-kg) tank of nitrogen used to pressurize the outpost's ammonia cooling system.
Riding on the station's 58-foot (18-metre) Canadian-built robot arm, Garan is to remove the old tank and then fly over the top of the space station in a slow arc to pick up the new tank from a storage platform on the other side of the outpost.
"It's going to be a fun ride for him because the arm is going to be completely stretched out," Karen Nyberg, who will be operating the crane from inside the station's Destiny laboratory, said in a preflight interview. "He'll be clearly on top of the world at that point."
Nyberg and Japanese astronaut Akihiko Hoshide, who also will be operating the robot arm, will then reverse the maneuver so Garan can install the new tank.
Fossum, meanwhile, plans to take another look inside one of the huge paddle-wheel joints that pivot the station's solar wing panels so they can face the sun for power. One joint has been contaminated with metal shavings and engineers are determining how best to clean it up and prevent more damage.
The other side has been working well, but a quick inspection by Fossum during a spacewalk on Thursday revealed what appeared to be a thin coating of debris on the metal ring, which engineers would like to analyze.
If time allows, Fossum plans to return to the left-side rotary joint and pick up a sample, using a piece of tape, to bring back to Earth.
The spacewalkers also plan to reinstall a television camera that was outfitted with a new power supply, remove a window cover from Japan's Kibo lab and remove locks that secured Kibo's 33-foot- (10-meter-) long robot arm during launch on May 31.
Discovery, which arrived at the orbital outpost June 2, is scheduled to depart on its trip home on Wednesday.