Christer Fuglesang of Sweden and mission lead spacewalker Danny Olivas installed a liquid ammonia tank to keep the space station cool and completed several other tasks, including bolting portable foot restraints on the orbiter's truss.

The pair returned to the decompression chamber after six hours and 39 minutes in space, NASA said, after bolting the 1,760-pound (800-kilogram) tank into place and linking up cooling lines.

During this second of three spacewalks for the space shuttle Discovery's mission at the ISS, the pair also moved a spent ammonia tank, which was removed Wednesday, into the shuttle's cargo bay to be taken back to Earth.

The ammonia helps move excess heat from inside the ISS to the radiators outside the station.

Astronauts Kevin Ford and Nicole Stott operated the robotic arm carrying Fuglesang and the tank to the installation site on an ISS truss segment, live footage from NASA TV showed.

Fuglesang and Olivas spent the night in the decompression chamber 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. But the US space agency said "the other is providing enough light to continue work."

After his outing, Fuglesang -- who was the first Swede in space in December 2006 -- became the first astronaut outside the United States or Russia to participate in more than three spacewalks.

Earlier, 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 feed on micro-blogging website Twitter.

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.

The duo also fetched US and European equipment from the orbiting station's Columbus laboratory that will be brought back to scientists on Earth.

Olivas and Fuglesang will set out for the third and final spacewalk of the mission on Saturday.

Discovery's mission is the fourth of five planned for the shuttle program this year. The last is scheduled for November.

The shuttle will remain docked at the ISS for nine days 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.

earlier related report
Astronauts start second spacewalk of Discovery mission
A Swedish astronaut and his US colleague floated out of shuttle Discovery on Thursday to conduct the mission's second spacewalk at the International Space Station, NASA said.

The pair exited the shuttle over China at 1012 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 Danny Olivas were to install a new liquid ammonia tank used to keep the ISS cool.

Olivas -- the mission's lead spacewalker -- and Fuglesang had overnighted in a decompression chamber in order to acclimate their bodies for their outing in space, were delayed by 53 minutes due to a problem with Olivas's communications gear.

Astronauts Kevin Ford and Nicole Stott operated the robotic arm carrying Fuglesang and both the old and new ammonia tanks to the installation site on an ISS truss segment, live footage from NASA TV showed.

Meanwhile the team removed the new tank assembly from its perch in the shuttle payload bay and maneuvered it towards the truss.

Minutes later, Fuglesang reported to Mission Control that his helmet spotlight died. NASA urged him to turn on a backup in order to continue his work.

After installing the massive, 800-kilogram (1,760-pound) tank, Olivas and Fuglesang were to place the old tank in the shuttle bay with the help of the robotic arm.

With Thursday's outing, Fuglesang -- who was the first Swede in space in December 2006 -- became the first astronaut outside the United States or Russia to participate in more than three spacewalks.

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 experiment equipment 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 Discovery mission is complete, just six more shuttle flights remain before NASA's three shuttles are retired in September next year.

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