(CNN) -- While many people may spend Christmas Eve doing last-minute shopping, two American astronauts have a more challenging matter to attend to Tuesday.
In orbit more than 200 miles above Earth, flight engineers Rick Mastracchio and Mike Hopkins embarked on a spacewalk Tuesday morning to repair part of the International Space Station's cooling system.
More than three hours into the job, the two astronauts had successfully bolted a replacement ammonia pump module into its location and were working to connect lines to allow the ammonia to flow, NASA said in a tweet.
The two engineers were carrying out the second in a series of expeditions needed to replace a malfunctioning pump, which circulates ammonia through loops outside the station to keep equipment cool.
The pump developed problems December 11 when an internal valve stuck in an incorrect position. The space station's life support system remains up and running, but operations were cut back as a result of the problem, NASA said.
It is the second Christmas Eve spacewalk in history, according to NASA.
The two astronauts spent about 5½ hours outside the space station Saturday working to remove the problematic pump. If everything goes according to plan, the spacewalk will last 6½ hours and the repair job will be completed Tuesday.
Mastracchio will be using a different spacesuit after a small amount of water leaked into the cooling system of the one he wore Saturday.
NASA said the issue with the suit, which happened at the end of the spacewalk, was unrelated to a problem experienced in July when water pooled in an Italian astronaut's helmet, causing a spacewalk to be cut short.
"Both Mastracchio and Hopkins reported dry conditions repeatedly throughout Saturday's activities and the two were never in danger," the agency said.
NASA had installed new safeguards, including snorkels inside the spacesuits that would allow astronauts to take breaths if water formed and they had to return to the space station.
Tuesday's spacewalk is the 176th to support the space station's assembly and maintenance, according to NASA.
The previous Christmas Eve spacewalk took place in 1999, the agency said, when Discovery astronauts Steve Smith and John Grunsfeld spent more than eight hours refitting parts of the Hubble Space Telescope.
The others on board the International Space Station at the moment include Russian cosmonauts Mikhail Tyurin, Sergey Ryazanskiy and Oleg Kotov and Japanese astronaut Koichi Wakata.
CNN's Ralph Ellis and John Zarrella contributed to this report.
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