Astronauts leave space station for repair work

HOUSTON (Reuters) - A pair of spacewalking astronauts floated outside the International Space Station on Tuesday to begin repairs on a long-standing problem with the outpost's power system.

Lead spacewalker Heidemarie Stefanyshyn-Piper and rookie astronaut Stephen Bowen left the station's airlock about 1:15 p.m. EST (1815 GMT) for what was expected to be a 6 1/2-hour spacewalk, the first of four planned during space shuttle Endeavour's 15-day mission.

"We have a great view of you out the windows here. You're looking great," one of Endeavour's astronauts radioed to the spacewalkers as they free-floated 212 miles above Earth.

The shuttle and seven astronauts arrived at the space station on Sunday to deliver equipment needed to double the size of the live-aboard crew to six.

The station crew now includes Sandra Magnus, who replaced astronaut Greg Chamitoff as a flight engineer. Chamitoff, who has been aboard since June, will be returning home on Endeavour.

Most of the spacewalk is reserved for work on a 10-foot (3-meter) wide rotary joint needed to position the station's solar power panels so they can track the sun.

The joint has been locked in place since last year after NASA discovered it was contaminated with metal filings. Engineers believe bearings in the joint did not have enough lubrication, causing unexpected wear-and-tear to a metal ring that they brush over.

Stefanyshyn-Piper and Bower plan to clean and lubricate the joint and begin replacing its bearings. Repair work to the damaged joint and preventative maintenance on the station's second joint may take as many as 10 spacewalks, NASA officials said.

During Tuesday's outing, the astronauts also planned to remove a cover from a window on Japan's science module and move an empty nitrogen tank from a stowage platform on the station to the shuttle's cargo hold so it can be returned to Earth.

Meanwhile, inside the shuttle and space station, astronauts scrambled to unpack a cargo canister hauled into orbit by Endeavour and loaded with seven tons of gear, including a water regeneration system that recycles urine and condensation into fresh drinking water.

With six people expected to be living on the station beginning in May, NASA needs to find a way to reuse water.

Currently, the shuttle provides most of the station's water during its visits to the outpost. Water is produced as a byproduct of the shuttle's electrical system and transferred to the station.

However, NASA plans just eight more shuttle missions to the station after Endeavour's so it can retire the fleet and move ahead with development of a new ship called Orion that can return astronauts to the moon. The agency also plans a final servicing call to the Hubble Space Telescope.

The space station, a $100 billion project of 16 nations, has been under construction for 10 years.

(Editing by Jim Loney and David Wiessler)