WASHINGTON: Two Nasa astronauts stepped out on Tuesday on a rare Christmas Eve spacewalk to wrap up repairs to an equipment cooling system at the International Space Station.

Americans Rick Mastracchio, 53, and Mike Hopkins, 44, floated outside the orbiting lab on a second outing to replace an ammonia pump whose internal control valve failed on Dec 11.

Their task was to retrieve a spare pump module from an external stowage platform and install it. Despite recent concerns about leaking spacesuits, the spacewalk got off to a smooth start.

After about two hours outside the station, Hopkins, riding a 57-foot (15-meter) robotic arm operated from inside the station by Japanese astronaut Koichi Wakata, managed to unhook the connectors on the spare.

“Okay, my friend you have yourself a pump module,” said a Nasa official at mission control in Houston, overheard in footage broadcast live on the space agency’s online television station.

With his boots affixed to the Canadian-made arm, Hopkins’ grasped the refrigerator-sized pump module as Wakata manoeuvred him over to its installation location.

Then, Hopkins and Mastracchio pushed the module into its slot and began affixing it in place.

Hopkins and Mastracchio must complete five electrical connections and four fluid connections before the pump can be activated.

Tuesday’s spacewalk was expected to last six and a half hours, which the US space agency estimated would be enough time to complete the repair job.

Nasa said tests would be done on the new pump later on Tuesday to see how well it is working.—AFP