WASHINGTON (AFP) - Two United States astronauts on Saturday stepped outside the International Space Station for the first of three spacewalks to fix a broken equipment cooling system, Nasa said.
"Today's spacewalk has officially begun," said a Nasa commentator as the space agency broadcast live images of Japanese astronaut Koichi Wakata leading the operation from inside the space station.
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 career spacewalk, accompanied by rookie Mike Hopkins, who is making his first venture outside the global research lab.
Mr Wakata is operating the station's 15m robotic arm, hoisting Mr Mastracchio and hefty equipment from one section of the lab to another.
The men's first task is to disconnect the failed ammonia pump, which is about the size of a refrigerator.
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 "stepped outside the Skylab space station to retrieve film from a telescope and photograph Comet Kohoutek", the US space agency said.
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 repair 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 Jan 7.
Nasa also had to rig up some last-minute contingency gear inside the American-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, just in case.
The investigation into the cause of the helmet leak is ongoing. Mr Hopkins will be wearing the suit that had the problem, though its inner water pump has been replaced.
Ms Dina Contella, International Space Station flight director, told reporters on Wednesday the space agency is "confident that this suit is a very clean suit and ready to go."