HOUSTON (Reuters) - Spacewalking astronauts from U.S. shuttle Discovery finished outfitting Japan’s newly arrived Kibo research laboratory and carried out other tasks during 6 1/2 hours of work outside the International Space Station on Sunday.

Astronauts Michael Fossum and Ronald Garan also did preventive maintenance on the station’s air conditioner, which NASA wants in the best shape possible before the shuttle fleet’s retirement in 2010.

Sunday’s work was the latest in an ambitious schedule to complete construction of the $100 billion station over the next two years. Nine more shuttle flights to the station are planned by the U.S. space agency, along with a single flight to upgrade the Hubble Space Telescope.

Soaring 210 miles over Earth, Fossum and Garan floated back inside and closed the station’s airlock by 4:28 p.m. EDT (2028 GMT).

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 removed the old tank and picked up the new tank from a storage platform on the other side of the outpost.

“Enjoy the ride,” said Karen Nyberg, who operated the crane from inside the station’s Destiny laboratory. She maneuvered the robot arm after remarking earlier Garan would be riding “on top of the world.”

Slideshow ( 18 images )

Nyberg and Japanese astronaut Akihiko Hoshide, who also operated the robot arm, reversed the maneuver so Garan could install the new tank.

Fossum, meanwhile, took another look inside one of the huge paddle-wheel joints that pivot the station’s solar wing panels so they constantly face the sun to provide electrical 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.

The spacewalkers also reinstalled a television camera that was outfitted with a new power supply, removed a window cover from the Kibo laboratory and removed locks that secured Kibo’s 33-foot- (10-metre-) long robot arm during launch on May 31.

Discovery, which arrived at the orbital outpost on June 2, is scheduled to depart on its trip home on Wednesday.