Spacewalk 3: Station Crew Tackles More Repair Work

Two space station astronauts tackled another big spacewalk repair job Monday to install a vital new pump in an attempt to restore their ailing cooling system back to full strength for the first time in more than two weeks.

It is the third of four repair spacewalks dedicated for NASA astronauts Douglas Wheelock and Tracy Caldwell Dyson, who plan to spend about 6 1/2 hours installing the new ammonia coolant pump. The astronauts removed the faulty pump during an earlier spacewalk on Aug. 11.

The space station's cooling system has been running at half strength since July 31, when an electrical short shut down one of two pumps that move liquid ammonia through the system. The failure forced astronauts to turn off some experiments and systems, as well as leave others without backups, in order to prevent the station from overheating.

Monday's spacewalk began at 6:20 a.m. EDT (1020 GMT), more than 30 minutes ahead of time, NASA spokesperson Rob Navias said during televised commentary.

Space station managers hope Wheelock and Caldwell Dyson will be able to completely hook up the new pump during Monday's spacewalk. The astronauts removed the faulty ammonia pump over two previous spacewalks (on Aug. 7 and Aug. 11) and are expected to install to perform a fourth spacewalk to move the disabled pump to its final storage location.

Mike Suffredini, NASA's space station program manager, said Monday's spacewalk – or extravehicular activity in NASA parlance – would go as long as possible to complete as much work as possible.

"The EVA will be as long as the consumables will allow," Suffredini said last week. "We'll take all the time we can get because we won't be done."

There are two main cooling system loops – Loop A and Loop B. The failed pump is in Loop A, while the other cooling loop remains operational. It was delivered to the space station in 2002, but wasn't activated until 2006.

NASA station managers have said the ammonia pump failure has been one of the most challenging repairs for the International Space Station ever attempted. The cooling system is so critical to station operations that a pump repair is one of 14 major failures for which NASA engineers have prepared emergency plans for in advance, they added.

There are four spare ammonia pumps on the space station, one of which will be used for this repair. Each pump weighs 780 pounds (353 kg) and is 5 1/2 feet long (1.6 meters) by 4 feet wide (1.2 meters).