Two space station astronauts ventured out on the second Christmas Eve spacewalk in NASA history, hoping to wrap up urgent repairs to a cooling system.

It was the second spacewalk in four days for US astronauts Rick Mastracchio and Michael Hopkins.

NASA ordered up the spacewalks to revive a critical cooling loop at the International Space Station. All nonessential equipment had to be turned off when the line stopped working on December 11, and many science experiments halted. Mastracchio and Hopkins removed a faulty ammonia pump during the previous outing on Saturday. On Tuesday, they worked to install a new pump 418 kilmoteres above the Atlantic Ocean.