The replacement of a damaged cooling pump means a third repair spacewalk will not have to be conducted

Astronauts have completed an eventful Christmas Eve spacewalk during which they replaced a critically damaged coolant pump module aboard the International Space Station.

Nasa's mission managers are hopeful that the seemingly successful installation of the new pump has restored the orbiting outpost to full working condition ahead of schedule, negating the need for what would have been a third repair spacewalk in the coming days.

Tuesday's “extra-vehicular activity” (EVA), which was only the second time in the agency's history that astronauts had ventured outside at Christmas, ran into early problems when "a mini blizzard" of toxic ammonia flakes burst from a leaking supply line, flecking the spacesuits of the men conducting the repair. Nasa spokesman Rob Navias said that the length of the spacewalk meant that the astronauts were exposed to the sun for enough time to "bake out" the ammonia from the suits without the need for further decontamination procedures. It was, he added, "a special sleigh ride on this Christmas Eve."

The spacewalk and repair was conducted by two Americans, Rick Mastracchio and Mike Hopkins, who’d also partnered on Saturday for a five-and-a-half hour EVA during which they removed the failed pump and prepared the area for the replacement’s installation. As he returned to the space station on Tuesday, he thanked the ground crew in Houston: "Merry Christmas to everybody. It took a couple of licks to get her done, but we got it," he said.

Doug Wheelock, a former ISS astronaut who was co-ordinating the spacewalk from Houston, told Mastracchio and Hopkins that they could now enjoy a day off and a peaceful holiday with their four space station crewmates. "This is the best Christmas ever. Thanks guys," he said.

The repairs to the space station's cooling systems became necessary when one of its two coolant pumps failed on December 11, leaving only one to send ammonia through two complex cooling loops.

Although Nasa insisted that the ISS’ crew was never in danger, all non-essential equipment was turned off to prevent overheating, and the station's science experiments ground to a halt. Engineers decided that the problem with the pump – a faulty valve – could not be repaired and declared that the entire module, which is the size of a refrigerator, needed to be replaced.

Hopkins and Mastracchio conducted a number of start-up procedures on the new unit after its installation today to confirm it was in working order. The repair included hooking up four fluid lines to the pump.

"Houston, you've got yourself a new pump module," Mastracchio said as the final ammonia line was opened. He’d earlier likened the removal of the new pump from its storage bay to "Christmas morning [and] opening up a little present."

This was the 10th spacewalk from the ISS this year. Russian cosmonauts Oleg Kotov and Sergey Ryazanskiy will undertake another on Friday in order to install two hi-fidelity cameras that will provide improved imagery of Earth on the outside of the ISS.

The only other Christmas Eve EVA in spaceflight history was in 1999 when astronauts Steve Smith and John Grunsfeld conducted repairs to the Hubble space telescope during a space shuttle mission.

Tuesday's repair had been delayed a day to resize a spare spacesuit for Mastracchio. Engineers suspected water might have entered the suit he wore on Saturday, although that issue was unrelated to an incident in July in which Italian astronaut Luca Parmitano's helmet filled up with liquid during a spacewalk and almost drowned him. Navias said both spacewalkers' suits remained "bone dry" on Tuesday.

After the Parmitano episode, the suits’ helmets were modified with safety upgrades, including absorbent pads and an improvised snorkel.