With the help of the station's robotic arm and guided by shuttle Commander Rick Sturckow and spacewalk choreographer Pat Forrester, the duo removed the bulky -- albeit weightless in space -- liquid ammonia coolant tank from the International Space Station's (ISS) truss.

A new, 800-kilogram (1,760-pound) replacement will be installed during the second spacewalk late Thursday while the old tank will return to Earth with Discovery.

Astronaut Nicole Stott and Mission Specialist Danny Olivas returned to the orbiter's decompression chamber at 0424 GMT after a spacewalk that lasted six hours and 35 minutes, NASA said.

Stott and Olivas also fetched European experiment equipment from the orbiting station's Columbus laboratory that will be brought back to scientists on Earth. The astronauts eventually won out over some stubborn bolts and pins.

The spacewalk did, however, hit a few snags.

Shortly before the halfway point of their spacewalk, the pair lost communication for about 30 minutes with Mission Control in Houston, Texas as a storm interfered with communications equipment at a satellite relay station on the ground.

US space agency officials played down the incident, saying it had not affected the safety of the flight.

"That's not incredibly unusual. We try to plan around that kind of situation," NASA spokeswoman Brandi Dean told AFP from the Johnson Space Center in Houston.

"These spacewalks are very carefully choreographed, and the crew knows exactly what to do. Mission Control just looks over their shoulder... They don't need communications input every step along the way."

Another concern emerged when Olivas alerted officials on the ground that he had some fraying on his glove.

As it turned out, he had only popped a stitch, which did not effect the outermost layer that keeps the air inside the spacesuit.

"It was pretty much a non-issue," Dean said.

Stott meanwhile noticed a high reading of her carbon dioxide levels. But teams on the ground later determined that it was a sensor problem that had not affected the function of her spacesuit.

Astronauts are scheduled to conduct two more spacewalks during the mission, the fourth of five planned for the shuttle program this year. The last is scheduled for November.

On Tuesday, the Discovery's seven-member crew -- including Swede Christer Fuglesang -- and the six ISS residents began unloading tons of equipment to the orbiting station, including popular US talkshow comedian Stephen Colbert's namesake treadmill.

The items were unloaded from the Leonardo Multi-Purpose Logistics Module, a huge pressurized chamber carrying 7.5 tons of supplies, including new station crew quarters, a freezer, food, two research racks and the Combined Operational Load Bearing External Resistance Treadmill (COLBERT).

COLBERT will be the second treadmill aboard the ISS. Exercise is key for astronauts spending long periods of time in space, where zero-gravity can result in muscle atrophy.

The freezer being delivered will store samples of blood, urine and other materials that will eventually be taken back to Earth for study on the effects of zero-gravity.

Stott is taking over at the ISS from engineer Tim Kopra, who has been aboard the orbiting laboratory since July and is slated to return to Earth with the shuttle.

Discovery blasted off from Cape Canaveral, Florida, Friday after its launch was delayed three times by bad weather and a valve problem. It is due to return to Earth on September 10.

The mission is the 128th for the space shuttle program, and the 30th mission to the ISS.

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

earlier related report
ISS Spacewalks Begin
Two US astronauts gingerly stepped out of the shuttle Discovery on Tuesday and began the first spacewalk of the mission's nine-day stay at the International Space Station (ISS).

Astronaut Nicole Stott emerged from the orbiter's decompression chamber at 2149 GMT some 350 kilometers (255 miles) above the Pacific ocean and was joined in the blackness of space by Mission Specialist Danny Olivas for what is expected to be a walk of 6.5 hours, according to US space agency NASA.

With the help of the station's robotic arm and guided by shuttle Commander Rick Sturckow and spacewalk choreographer Pat Forrester, the duo will be removing an old liquid ammonia coolant tank from the ISS truss.

A new, 800-kilogram (1,760-pound) replacement will be installed during the second spacewalk.

They will also be retrieving experiment equipment from the outside of the ISS, from the European Columbus module, and returning it to Earth for processing.

Astronauts are scheduled to conduct two more spacewalks during the mission, the fourth of five planned for the shuttle program this year. The last is scheduled for November.

Earlier Tuesday the Discovery's seven-member crew -- including Swede Christer Fuglesang -- and the six ISS residents began unloading tons of equipment to the orbiting station after opening the hatch to the Leonardo Multi-Purpose Logistics Module built by the Italian space agency, NASA said.

The huge pressurized chamber is carrying 7.5 tons of supplies, including new station crew quarters, a freezer, food, two research racks and a Combined Operational Load Bearing External Resistance Treadmill, or COLBERT, named in honor of popular US talkshow comedian Stephen Colbert.

The freezer being delivered will store samples of blood, urine and other materials that will eventually be taken back to Earth for study on the effects of zero-gravity.

The COLBERT will be the second treadmill aboard the ISS. Exercise is key for astronauts spending long periods of time in space, where zero-gravity can result in muscle atrophy.

Stott is taking over at the ISS from engineer Tim Kopra, who has been aboard the orbiting laboratory since July and is to return to Earth with the shuttle.

Discovery blasted off from Cape Canaveral, Florida, Friday after its launch was delayed three times by bad weather and a valve problem.

The mission is the 128th for the space shuttle program, and the 30th mission to the ISS.

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

earlier related report
The US space shuttle Discovery's crew awoke Tuesday to begin unloading tons of equipment to the International Space Station, with two astronauts preparing to set out on the mission's first spacewalk.

A total of 13 astronauts at the linked shuttle and orbiting station were to help transfer cargo after Discovery's crew opened the hatch into the Leonardo Multi-Purpose Logistics Module, built by the Italian space agency, NASA said.

The huge pressurized chamber is carrying 7.5 tons (6.8 metric tonnes) of supplies, including new station crew quarters, a freezer, food, two research racks and a treadmill named after popular US talkshow comedian Stephen Colbert.

Astronaut Nicole Stott, the newest ISS resident, will conduct the mission's first spacewalk at 2149 GMT with Mission Specialist Danny Olivas, said the US space agency. They are expected to be on the walk for 6-1/2 hours.

Stott is taking over at the ISS from engineer Tim Kopra, who has been aboard the orbiting laboratory since July and is returning to Earth with the shuttle.

Discovery docked with the ISS on Sunday, on the eve of launching eight days of joint operations between the combined shuttle and station's crews.

The shuttle blasted off from Cape Canaveral, Florida, Friday after its launch was delayed three times by bad weather and a valve problem.

Astronauts are scheduled to conduct three spacewalks of six-and-a-half hours each during the 13-day mission, the fourth of five planned for the shuttle program this year. The last is scheduled for November.

One of the key goals of the spacewalks is to replace an old liquid ammonia coolant tank with a new 1,760-pound (800-kilogram) unit.

The astronauts on Tuesday's walk also will be retrieving experiment equipment from the exterior of the ISS, from the European Columbus module, and returning it to Earth for processing.

The freezer being delivered will store samples of blood, urine and other materials that will eventually be taken back to Earth for study on the effects of zero-gravity.

The COLBERT treadmill will be the second aboard the ISS. Exercise is key for astronauts spending long periods of time in space, where zero-gravity can result in muscle atrophy.

The mission will be the 128th for the space shuttle program, and the 30th mission to the ISS.

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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