Space station astronauts repaired a crippled cooling system during a rare Christmas Eve spacewalk, braving a "mini blizzard" of noxious ammonia as they put in a new pump.
It was the second spacewalk in four days for US astronauts Rick Mastracchio and Michael Hopkins, and only the second Christmas Eve spacewalk ever.
NASA ordered the spacewalks to revive a critical cooling loop at the International Space Station. All non-essential equipment had to be turned off when the line failed on December 11, and many science experiments were halted.
With yesterday's success, the cooling system should be restored and all equipment back up and running by the weekend, according to NASA.
"It's the best Christmas ever," Mission Control radioed as the spacewalk came to a close after more than seven hours.
"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. Yesterday, they installed the fresh pump.
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, 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 faulty 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.
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.
Later, as he worked to remove the spare pump from its storage shelf, he commented: "Now it really feels like I'm unwrapping a present."
Mission Control in Houston was in a festive mood, despite the gravity of the situation. Tabletop Christmas trees, Santa dolls and red Santa caps decorated the desks.
A bad valve in the ammonia pump caused the latest breakdown.
Another team of spacewalking astronauts installed the pump just three years ago, and engineers are perplexed as to why it did not last longer. NASA hopes to salvage it in the years ahead.
The 2010 replacement required three spacewalks because of the difficulty in removing pressurised ammonia fluid lines.
But this time, the astronauts managed to squeeze everything into two after NASA reduced the pressure and simplified the task.
Mission Control successfully activated the pump last night. The two-line external cooling system uses ammonia to dispel heat generated by on-board equipment, and only one loop was disabled by the breakdown.
NASA's only previous Christmas Eve spacewalk occurred in 1999 during a Hubble Space Telescope repair mission.
But NASA's most memorable Christmas Eve was back in 1968. Apollo 8 astronauts read from Genesis, the first book of the Bible, as they orbited the moon on mankind's first lunar flight.