(Updates with end of spacewalk)

By Irene Klotz

HOUSTON, Nov 18 (Reuters) - Two shuttle Endeavour astronauts on Tuesday finished the first of four spacewalks outside the International Space Station, part of a plan to eventually restore full power to the growing outpost.

The job proved messier than expected. Lead spacewalker Heidemarie Stefanyshyn-Piper discovered one of the grease guns needed to lubricate a key joint in the station's truss had leaked, soiling her glove.

Then, as she attempted to wipe the grease from her hands, the bag containing her tools floated off into space.

"Oh, great," said Stefanyshyn-Piper, who was making her third spacewalk. "You see it?"

Crewmates monitoring the spacewalk from inside the shuttle spied the bag drifting past the orbital outpost, which flies 212 miles (341 km) above Earth.

The bag contained two grease guns, scrapers and other equipment needed to begin fixing a 10-foot(3-meter)-wide rotary joint that positions the station's solar wing panels toward the sun for power.

Astronauts last year discovered the joint was contaminated with metal filings, which NASA determined was due to inadequate lubrication.

The four spacewalks by the shuttle Endeavour crew may be followed by as many as six more outings on future missions to fix the problem and make sure similar problems don't arise.

Stefanyshyn-Piper borrowed tools from her spacewalking partner Stephen Bowen to work on the joint before heading back into the station's airlock at 8:01 p.m. EST (0101 GMT) after a six-hour, 52-minute spacewalk. Earlier, the astronauts packed up an empty nitrogen tank for transport back to Earth and removed a cover from a window on Japan's Kibo laboratory.

The shuttle and seven astronauts arrived at the space station Sunday to outfit the space station for an expanded six-member crew. Astronaut Sandra Magnus will replace Greg Chamitoff, who will return home on Endeavour, after five months aboard the space station.

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.

After eight more shuttle missions, NASA plans to retire the shuttle fleet and develop a new ship called Orion that can return astronauts to the moon.

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

(Additional reporting by Chris Baltimore)

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