By Irene Klotz
HOUSTON (Reuters) - A pair of spacewalking astronauts floated outside the International Space Station on Tuesday to pack up a broken cooling pump and tackle other tasks before NASA's last space shuttle heads back to Earth.
Space station flight engineers Mike Fossum and Ron Garan ventured through the space station's airlock for a spacewalk that lasted six hours and 31 minutes.
Riding the station's robotic arm, Garan grappled the refrigerator-sized coolant pump and wrestled it into a cargo rack at the back of shuttle Atlantis' payload bay, a delicate task considering the unit's 1,400-pound (635 kg) bulk.
"I love it when a plan comes together," Fossum said after Garan bolted down the pump for its return to Earth.
Engineers are eager to take apart the pump, which shut down last year, wiping out cooling to half the station, a $100 billion project of 16 nations that took more than a decade to build.
NASA suspended most science experiments until space station astronauts could make three unplanned spacewalks to install a spare pump.
"Understanding what caused that failure is going to help us in the long run. It may change how we operate the pumps on board, maybe lead to some design changes in the end," said station flight director Jerry Jason.
Normally, shuttle astronauts handle spacewalks while they are visiting the station, but Atlantis, which arrived at the station on Sunday for an extended nine-day stay, is short-handed.
With the spacewalk complete, the main task of Atlantis' crew will be to unpack 9,403 pounds (4,265 kg) of spare parts, equipment and supplies that are meant to hold over the space station crew until private companies can establish cargo flights.
NASA cut the crew size to just four instead of the typical six or seven to accommodate the smaller Russian Soyuz capsules that would be used as rescue vehicles if Atlantis were too damaged to safely return to Earth.
Since the 2003 Columbia accident, shuttle crews have had a second shuttle on standby to mount a rescue mission if needed. But there is no backup shuttle for Atlantis, which is closing out the 30-year-old program with NASA's 135th and final flight.
During their spacewalk, Fossum and Garan also installed a $22.6 million technology experiment outside the station that will be used by the station's Dextre robot to demonstrate tools and techniques for refueling satellites in orbit.
NASA hopes the project, known as the Robotic Refueling Mission, will lead to a new U.S. industry servicing satellites that have run out of maneuvering fuel or which need minor repairs.
Tuesday's spacewalk, the 160th devoted to station assembly and maintenance, was the last planned outing by U.S. astronauts for about a year and the final one supported by space shuttle crews.
Atlantis, which blasted off on July 8, is due back at the Kennedy Space Center on July 21.
(Additional reporting by Chris Baltimore; editing by Jane Sutton, Vicki Allen and Mohammad Zargham)