Spacewalkers resume space station repairs

HOUSTON (Reuters) - Two shuttle Endeavour astronauts slipped outside to the vacuum of space on Saturday to clean and repair a contaminated joint in the International Space Station's external frame, a key step to restoring the outpost to full power.

The spacewalk by the Heidemarie Stefanyshyn-Piper and Stephen Bowen is the third of four outings planned during Endeavour's 11-day servicing call to the orbital outpost.

The astronauts began the planned seven-hour excursion shortly after 1 p.m. EST (1800 GMT) as the station sailed about 225 miles over Earth.

Stefanyshyn-Piper and Bowen planned to work together inside a 10-foot-wide rotary joint, one of two on the station that pivot the station's sprawling solar wings so they can face the sun for power. One of the joints is contaminated with metal filings, which engineers believe stemmed from a lubrication problem.

The repair involves greasing the joint's metal ring so the debris can be collected and scraped off.

NASA had planned for both astronauts to use grease guns but a set of tools, worth about $100,000, accidentally floated off into space during the mission's first spacewalk on Tuesday.

The astronauts shared a grease gun during that outing and then revised their cleanup procedures to use grease-impregnated wipes for the second spacewalk on Thursday. Stefanyshyn-Piper said the backup plan worked well and that she would stick to that process during her third spacewalk on Saturday.

As a backup, NASA had the astronauts modify one of two caulking guns, flying as part of an emergency shuttle heat shield repair kit, so that it could dispense grease.

While astronauts worked outside the station, engineers on the ground puzzled over problems with a new water recycling system that was installed in the outpost's Destiny laboratory earlier this week. The device is designed to purify urine and other wastewater so that it can be used by the crew for drinking and for producing oxygen.

NASA needs to have the system working before the station's crew size can be expanded from three people to six next year.

A centrifuge in part of the machine that distills urine has shut down repeatedly since its initial activation on Thursday. NASA hopes to resolve the problem in time for shuttle Endeavour to bring back samples of the purified water for analysis on Earth.

The shuttle is scheduled to depart the station on Thursday and return home to the Kennedy Space Center in Florida next Saturday.

(Editing by Bill Trott)