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 Tuesday’s success, the cooling system should be restored and all equipment up and running by this weekend, according to NASA.“It’s the best Christmas ever,” Mission Control radioed as the 71/2-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 Saturday’s spacewalk. On Tuesday, they installed the fresh pump.Standing on the end of the station’s main robotic arm, Hopkins clutched the 780-pound, 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, 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.