CAPE CANAVERAL, FLA.Space station astronauts repaired a crippled cooling system during a rare Christmas Eve spacewalk Tuesday, braving a mini blizzard of noxious ammonia as they popped in a new pump.

It was the second spacewalk in four days for U.S. astronauts Rick Mastracchio and Michael Hopkins, and only the second Christmas Eve spacewalk ever.

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 conked out Dec. 11, and many science experiments halted.

With Tuesdays success, the cooling system should be restored and all equipment back up and running by this weekend, according to NASA.

Its the best Christmas ever, Mission Control radioed as the 7 -hour spacewalk came to a close.

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

Mastracchio and Hopkins removed the faulty ammonia pump during Saturdays spacewalk. On Tuesday, they installed the fresh pump.

Standing on the end of the stations main robotic arm, Hopkins clutched the 780-pound (353.8-kilogram), refrigerator-size pump with both hands as he headed toward its installation spot, and then slid it in. An astronaut working inside, Japans Koichi Wakata, gingerly steered the arm and its precious load.

Mike Hopkins taking a special sleigh ride on this Christmas Eve, Mission Control commentator Rob Navias said as the space station soared over the Pacific.

It was slow going because of a balky ammonia fluid line that sent frozen flakes of the extremely toxic substance straight at the men a mini blizzard, as Mission Control called it. The spacewalkers reported being surrounded by big chunks of the stuff that bounced off equipment and, in all probability, their suits.

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Space station astronauts complete repairs to cooling system on Christmas Eve spacewalk