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.
Nasa spacewalks in pictures
Nasa spacewalks in pictures
1/11 Italian astronaut Luca Parmitano participates in a spacewalk on 9 July. In official jargon spacewalks are known as extravehicular activity (EVA). Parmitano is seen here anchored to the Canadarm2 mobile foot restraint.
2/11 Nasa astronaut Chris Cassidy exits the Quest airlock during the six-hour spacewalk alongside Parmitano on the 9th.
3/11 Always time for a quick selfie. Parmitano grabs one for Facebook on the 9th.
4/11 Cassidy is seen dwarfed by some of the complex structures of the ISS during a later spacewalk on 16 July.
5/11 Parmitanto uses a digital camera on the 16th. A little more than an hour into the mission Parmitano reported water floating inside his helmet
6/11 Cassidy is seen outside during the 16 July spacewalk. Although the water in Parmitano's helmet posed no threat, Nasa decided to end the spacewalk early.
7/11 The view across the way. An image captured from 26 June of the waning gibbous moon. The picture was taken from a position 225 miles about the Eqautor near the northern coast of Africa.
8/11 Luca Parmitano (left) and Chris Cassidy (right) are seen out of their suits on board the ISS later in July. Juggling seems decidedly easier in zero-gravity conditions.
9/11 Parmitano is seen installing a new Life Support Module on the Biolab of the ISS. This will be used to study the effects of microgravity on different sorts of organisms. Growing plants in space will be one of the key steps towards permanent habitation.
10/11 It's not all astronauts that hang about outside the ISS. This pictures shows the Japanese HTV-4 unmanned cargo spacecraft. It delivered equipment to the space station and was jettisoned in August, burning up in the Earth's atmosphere during reentry.
11/11 Russian cosmonaut Alexander Misurkin is seen outside the iSS in August 2013. Misurkin is wearing the Russian-made Orlan spacesuit rather than the standard Nasa issue attire.
Additional reporting by Associated Press