Two NASA astronauts, their spacesuits rigged with snorkels in case of a water leak, have floated outside the International Space Station for more than five hours, successfully completing the first steps to fix the outpost's cooling system.
The spacewalk, which was broadcast live on NASA Television, was the first for NASA since July when the spacesuit helmet worn by Italian astronaut Luca Parmitano began filling with water, a situation that could have caused him to drown.
No such leaks were detected in Saturday's spacewalk, the first of up to three that will be needed to complete the cooling system repair.
The operation was prompted by the December 11 shutdown of one of the station's two US ammonia cooling systems, which forced the crew to turn off non-essential equipment and shut down dozens of science experiments.
While the six-member crew is not in danger, the remaining cooling system cannot support the three laboratories and other modules on the US side of the $100 billion station, a project of 15 nations. The Russian side of the station has a separate cooling system.
Engineers at the Johnson Space Centre in Houston tried devising ways to bypass a suspected faulty pump valve, but with time running short, managers decided to have astronauts replace the pump, located outside the station, with a spare.
'Beautiful day. Awesome view'
The work went smoothly, with station flight engineers Rick Mastracchio and Mike Hopkins finishing up an hour earlier than expected.
They were able to not only disconnect the old pump, but also remove it from its pallet on the station's exterior truss, a task originally slated for a second spacewalk on Monday. A third spacewalk, if needed, is scheduled for Wednesday.
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"Beautiful day. Awesome view," Mr Mastracchio, a veteran of six previous spacewalks, said as he opened the airlock's hatch and saw the view from 418 kilometres above the southern Atlantic Ocean.
The water leak problem in July was traced to contamination in a piece of equipment called a fan pump separator that circulates water and air in the spacesuit and removes moisture from air.
How the water-separator portion of the device became clogged remains under investigation.
Mr Hopkins, who was making his first spacewalk, wore Mr Parmitano's spacesuit, but it had been outfitted with a new fan pump separator.
In addition, both Mr Hopkins and Mr Mastracchio rigged their helmets with homemade snorkels, fabricated out of pieces of plastic tubing and Velcro, which they could have used for breathing in case of another water leak.
The helmets also included water-absorbent pads.
Working 'on the edge of a skyscraper'
During Saturday's spacewalk, Mr Mastracchio and Mr Hopkins disconnected electrical and fluid lines and removed the 354 kilogram, 1.5 metre wide cooling system pump.
The failed pump, which was then anchored in a temporary storage site, will remain on the station for possible future repair and reuse.
It was installed in 2010 during an unexpectedly difficult series of spacewalks by astronauts Doug Wheelock and Tracy Caldwell Dyson.
"What makes this pump very difficult (to work on) are (the) fluid disconnects because they are so large and they are pressurised and they contain liquid ammonia, so that's a hazard for us if it were to come in contact with us or our suits," Ms Caldwell Dyson said in an interview with a NASA TV mission commentator.
When you look down, you see your feet and then you see the Earth going 17,500 mph (28,164 kph) beneath you, it really does get your attention.
Astronaut Tracy Caldwell Dyson
She said maintaining focus can also be a challenge.
"When you're on one of those pallets, you really have that sensation that you are sticking out on the edge of a skyscraper," she said.
"Especially when you look down, you see your feet and then you see the Earth going 17,500 mph (28,164 kilometres per hour) beneath you, it really does get your attention," she said.
Mr Mastrocchio and Mr Hopkins encountered no major problems during Saturday's outing.
"These guys really went out there and were so efficient," Ms Caldwell Dyson said.
"It may be more difficult to remove the pump because you don't know exactly what to expect.
"You tend to slow down and be a lot more careful when you have a backdrop like that."