CAPE CANAVERAL, Florida (AP) – Astronauts wrapped up urgent spacestation repairs during a rare Christmas Eve spacewalk Tuesday, braving a"mini blizzard" of noxious ammonia as they popped in a new pump.

Itwas the second spacewalk in four days for U.S. astronauts RickMastracchio and Michael Hopkins, and only the second Christmas Evespacewalk in NASA history.

NASA ordered up the spacewalks torevive a critical cooling loop at the International Space Station. Allnonessential equipment had to be turned off when the line conked outDec. 11, and many science experiments halted.

With Tuesday'ssuccess, the cooling system should be restored and all equipment back upand running by this weekend, according to NASA.

Mastracchio and Hopkins removed the faulty ammonia pump during Saturday's outing. On Tuesday, they installed the fresh pump.

Standingon the end of the station's main robotic arm, Hopkins clutched the780-pound, refrigerator-size pump with both hands as he headed towardits installation spot, and then slid it in. An astronaut working inside,Japan's Koichi Wakata, gingerly steered the arm and its precious load.

"MikeHopkins taking a special sleigh ride on this Christmas Eve," MissionControl commentator Rob Navias said as the space station soared over thePacific.

It was slow going because of a balky ammonia fluid linethat sent frozen flakes of the extremely toxic substance straight at themen – "a mini blizzard," as Mission Control called it. The spacewalkersreported being surrounded by big chunks of the stuff that bounced offequipment and, in all probability, their suits.

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

"Wow,"Hopkins sighed after the fourth and final fluid line was hooked to thenew pump. The electrical hookups went more smoothly, and six hours intothe spacewalk, Hopkins finally called down, "Houston, you've gotyourself a new pump module."

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

"It'slike Christmas morning opening up a little present here," Mastracchiosaid as he checked his toolkit. Later, as he worked to remove the sparepump from its storage shelf, he commented: "Now it really feels like I'munwrapping a present."

Mission Control in Houston was in afestive mood, despite the gravity of the situation. Tabletop Christmastrees, Santa dolls and red Santa caps decorated the desks.

NASA's only previous Christmas Eve spacewalk occurred in 1999 during a Hubble Space Telescope repair mission.

ButNASA's most memorable Christmas Eve was back on Dec. 24, 1968. Apollo 8astronauts read from Genesis, the first book of the Bible, as theyorbited the moon on mankind's first lunar flight.

A bad valve in the ammonia pump caused the latest breakdown.

Anotherteam of spacewalking astronauts installed that pump just three yearsago, and engineers are perplexed as to why it didn't last longer. NASAhopes to salvage it in the years ahead.

The 2010 replacementrequired three spacewalks because of the difficulty in removingpressurized ammonia fluid lines. But this time, the astronauts managedto squeeze everything into two after NASA reduced the pressure andsimplified the task.

Mission Control planned to wait until Tuesdayevening before fully activating the new pump, but initial testingshowed everything working well. The two-line cooling system uses ammoniato dispel heat generated by on-board equipment; only one loop wasdisabled by the breakdown.

The second spacewalk was supposed totake place Monday, but it was delayed a day to give Mastracchio time toswitch to another suit. He inadvertently hit a water switch in the airlock at the end of Saturday's excursion, and a bit of water encroachedon a cooling device in the backpack of his suit, making it unusable.

Otherwise,the suits remained dry during both spacewalks. Last July, an astronautalmost drowned when water from his suit's cooling system flooded hishelmet. Makeshift snorkels and absorbent pads were added to the suits asa precaution.

A Moscow-led spacewalk, meanwhile, is set forFriday. Two Russian crew members will install new cameras and freshexperiments outside.

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