A Swedish astronaut and his US colleague floated out of shuttle Discovery to conduct the mission's second spacewalk at the International Space Station, NASA said.
The pair exited the shuttle over China on Thursday at 2212 GMT for an excursion expected to last 6.5-hours, after the US space agency determined that space debris drifting towards the shuttle and ISS posed no threat.
Swede Christer Fuglesang and mission lead spacewalker Danny Olivas installed a new liquid ammonia tank used to keep the ISS cool, bolting it into place and mating a series of cooling lines to the new assembly.
Astronauts Kevin Ford and Nicole Stott operated the robotic arm carrying Fuglesang and the new, 800-kilogram (1,760 pound) tank to the installation site on an ISS truss segment, live footage from NASA TV showed.
"Now they'll move an empty tank to the shuttle's cargo bay," NASA said in a Twitter message, referring to the spent ammonia tank that will return to Earth with Discovery.
Fuglesang and Olivas had overnighted in a decompression chamber in order to acclimate their bodies for their outing in space, but were delayed by 53 minutes due to a problem with Olivas's communications gear.
An hour into the spacewalk, Fuglesang reported to Mission Control that one of his helmet spotlights died. "The other is providing enough light to continue work," NASA said in a message.
Earlier Thursday a large piece of space debris drifted toward the ISS, but NASA said it would not affect the second of mission's three planned spacewalks.
"Mission Control evaluated a piece of space junk and decided it's not necessary to move the space shuttle/space station to avoid it," NASA said on its Twitter feed.
Mission Control was keeping a close watch on the remains of the three-year-old Ariane 5, a European space rocket, that were moving in an oval-shaped orbit.
The piece, which is some 200 square feet (19 square meters) in size, was expected to pass almost two miles (three kilometers) from the outpost on Friday, NASA said.
Officials were developing a contingency plan to potentially "reboost" the station-shuttle complex that currently holds 13 astronauts if the space junk posed a threat.
The linked spacecraft are currently orbiting 220 miles (354 kilometers) above the Earth.
On Wednesday Olivas and Stott performed the first spacewalk of Discovery's nine-day stay at the ISS, removing the old tank that will return to Earth with Discovery.
The duo also fetched US and European equipments from the orbiting station's Columbus laboratory that will be brought back to scientists on Earth.
The third and final spacewalk is scheduled for Saturday.
Discovery's mission is the fourth of five planned for the shuttle program this year. The last is scheduled for November.
The two crews continued unloading equipment from the Leonardo Multi-Purpose Logistics Module, a huge pressurized chamber carrying 7.5 tons of supplies, including new station crew quarters, a freezer, two research racks and a treadmill named after popular US talkshow comedian Stephen Colbert.
The freezer will store samples of blood, urine and other materials that will eventually be brought back to Earth for study on the effects of zero-gravity.
Discovery will remain docked at the ISS until Tuesday and is due to return to Earth on September 10.
Once the mission is complete, just six more shuttle flights remain before NASA's three shuttles are retired in September 2010.
Source: AFP American Edition