Christmas Eve spacewalk repairs broken cooling system at the International Space Station

US astronauts faced a 'mini blizzard' of ammonia during a rare spacewalk

Space station astronauts have repaired a crippled cooling system during a rare Christmas Eve spacewalk, braving a "mini blizzard" of toxic ammonia as they installed a new pump.

It was the second spacewalk in four days to be undertaken by US astronauts Rick Mastracchio and Michael Hopkins and took 7.5 hours to complete.

In a statement, Nasa said: "Following two spacewalks to replace a degraded pump module on the truss, or backbone, of the International Space Station, flight controllers in the Mission Control Center at NASA's Johnson Space Center in Houston successfully restarted the new pump Tuesday night."

Nasa ordered the spacewalk on Wednesday following the shutdown of one of the International Space Stations two ammonia cooling systems on 11 December, which forced the crew to turn off non-essential equipment and dozens of science experiments.

It has maintained that at no point have the astronauts been in danger.

Station commander cosmonaut Oleg Kotov, Mikhail Tyurin, Sergey Ryazanaskiy and Japanese astronaut Koichi Wakata are also aboard as part of Expedition 38.

Nasa said it expected the cooling system to be restored and all equipment back up and running by the weekend following yesterday's success.

"It's the best Christmas ever," Mission Control radioed as the spacewalk came to a close.

"Merry Christmas to everybody," replied Mr Hopkins. "It took a couple weeks to get her done, but we got it."

The astronauts removed the faulty ammonia pump during Saturday's spacewalk, before installing a new pump yesterday.

Standing on the end of the station's main robotic arm, Mr Hopkins clutched the 780lb, refrigerator-size pump with both hands as he headed toward its installation spot, and then slid it in.

An astronaut working inside, Japan's Koichi Wakata, steered the arm. The operation took longer to complete because of a faulty ammonia fluid line that sent frozen flakes of the extremely toxic substance straight at the men, Mission Control said.

The spacewalkers reported being surrounded by the flakes which bounced off their equipment.

The ammonia needed to dissipate from their suits before the pair returned inside, to avoid further contamination.

Christmas references filled the radio waves as the action unfolded 260 miles above the planet.

"It's like Christmas morning opening up a little present here," Mr Mastracchio said as he checked his toolkit.

Spacewalks were temporarily suspended in July when Italian spacewalker Luca Parmitano almost died after water began pouring into his helmet. The suits worn by Mastracchio and Hopkins have been modified with snorkels in case of another leak and absorbent pads to pick up the first signs of excess moisture inside their helmets.

Additional reporting by Associated Press

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