By Irene Klotz

HOUSTON, Nov 20 (Reuters) - Two shuttle Endeavour astronauts ventured outside the orbiting International Space Station on Thursday, vowing to take extra care to make sure their tool bags are in tow after losing half their gear during the mission's first spacewalk.

Lead spacewalker Heidemarie Stefanyshyn-Piper and rookie astronaut Shane Kimbrough floated through the space station's Quest airlock at shortly after 1 p.m. EST (1800 GMT) for what is expected to be a 6 1/2-hour outing.

The astronauts' main job is to work on one of the space station's two rotary joints which are needed to pivot solar wing panels to face the sun for power.

Last year, NASA discovered one joint was contaminated with metal filings and locked it in place to prevent additional damage. Stefanyshyn-Piper and astronaut Stephen Bowen began cleaning and lubricating the joint on Tuesday during the first of four spacewalks planned during Endeavour's 15-day mission.

The astronauts had to share grease guns and other tools after Stefanyshyn-Piper's tool kit, which contained $100,000 worth of gear, floated away.

For Thursday's spacewalk, she will use pre-lubricated wipes to trap metal particles so Kimbrough can use the remaining grease gun to work on the station's robot arm.

"You have to learn from your mistakes," Stefanyshyn-Piper said in an interview after the first spacewalk. "We're definitely not going to do it again. You're not going to see us lose another bag. We're going to double and triple check everything from here on out."

The shuttle arrived at the station on Sunday to work on the station's truss joints and to deliver equipment needed to prepare the station for a permanent six-person crew.

Currently, rotating groups of three astronauts live on the station, which on Thursday marked the 10th anniversary of the launch of its first module -- the U.S.-financed, Russian-built Zarya compartment.

NASA had planned to finish construction in eight years, but delays after the 2003 Columbia accident have postponed the station's completion until 2010.

"We've had a number of significant issues to deal with on the ISS, and yet always a solution to those has been with our partners pulling together," said NASA's deputy space station program manager, Kirk Shireman.

The station, which orbits about 212 miles (340 km) above the planet, is a $100 billion project of 16 nations. Since the arrival of Zarya, it has completed more than 57,300 orbits of Earth and hosted 167 people -- including six tourists -- from 15 nations.

The ongoing shuttle mission is NASA's 27th shuttle flight to the space station, with eight more remaining before the shuttle fleet is retired in 2010. NASA also plans a final servicing call to the Hubble Space Telescope next year. (Editing by Jim Loney and David Wiessler)

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