Two US astronauts stepped outside the International Space Station on Saturday for the first of three spacewalks to fix a broken equipment cooling system, NASA said.
The spacewalk began at 7:01 am (1201 GMT), a few minutes earlier than planned. It is expected to last six and a half hours.
Veteran spacewalker Rick Mastracchio is making his seventh such walk, accompanied by Mike Hopkins, who is making his first venture outside the global research lab.
Japanese astronaut Koichi Wakata is operating the station's 50-foot (15-meter) robotic arm, hoisting Mastracchio and hefty equipment from one section of the lab to another.
The men's first task is to disconnect a failed ammonia pump, which is about the size of a refrigerator.
With each astronaut sporting a helmet camera, NASA TV showed the operations in real-time from the perspective of the spacewalkers.
On the second spacewalk, set for Monday, the astronauts are to remove the pump so it can be replaced with a spare that was already stowed at the ISS.
A third spacewalk is planned for Christmas Day, when the failed pump will be shuttled away and final installations made on its replacement.
However, there is a chance the astronauts will be able to complete all their work in two spacewalks, NASA has said.
If not, the Christmas Day outing would be the first since 1974, when a pair of NASA astronauts did work outside the Skylab space station.
The urgent spacewalks were called for this week due to a faulty valve that caused a partial shutdown in the system that regulates equipment temperature at the space station.
Engineers tried to fix the problem from the ground, but eventually decided they needed to replace the ammonia pump.
The six-man crew was never in danger, but NASA wanted to fix the problem sooner rather than later, agency officials said.
The spacewalks meant the first regular commercial cargo supply mission by Orbital Sciences' Cygnus craft, which had been planned for earlier in the week, was postponed. It will now fly no earlier than January 7.
NASA also had to rig up some last-minute contingency gear inside the US-made spacesuits, which have not been used since a helmet water leak nearly drowned a European astronaut in July.
Now the helmets carry an extra absorption pad and a snorkel inside, in case there is a malfunction in the water pump system in the 35-year-old suits.
The investigation into the cause of the helmet leak is ongoing. Hopkins is wearing the suit that had the problem, though its inner water pump has been replaced.
NASA said it would check with the astronauts throughout the day to make sure they were not feeling any "squishy" sensation behind their necks, which might indicate a water buildup and would cut short the spacewalk, space agency commentators said.
Early checks were clear, with astronauts reporting dry conditions and no problems in the spacesuits or gloves, NASA said.
The astronauts also trained for the emergency spacewalks by watching video of a previous outing to replace an ammonia pump in 2010.