Two NASA astronauts, their spacesuits newly modified with snorkels in case of another water leak, floated outside the International Space Station on Saturday to begin a marathon three-day task to fix the outpost's cooling system.
The spacewalk, which is being broadcast live on NASA Television, is 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.
Saturday's spacewalk was prompted by the Dec. 11 shutdown of one of the station's two 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 U.S. 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 Center 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.
The work is expected to take station flight engineers Rick Mastracchio and Mike Hopkins three spacewalks to complete, the first of which began at 7:01 a.m. ET on Saturday as the station sailed 418 kilometres over the southern Atlantic Ocean.
"Beautiful day. Awesome view," Mastracchio, a veteran of six spacewalks, said as opened the airlock's hatch to begin the planned 6.5-hour outing.
He and Hopkins wore spacesuits that were modified to protect them from another possible water leak. The problem in July was traced to contamination in piece of equipment called a fan pump separator, which 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, but NASA managers say they are confident the problem will not reoccur during Saturday's spacewalk.
Hopkins, who is making his first spacewalk, is wearing Parmitano's spacesuit, but it has been outfitted with a new fan pump separator.
In addition, both Hopkins and Mastracchio rigged their helmets with homemade snorkels, fabricated out of pieces of plastic tubing and Velcro, which they can use for breathing in case of another water leak.
The helmets also now include water-absorbent pads that can hold up to 800 millilitres of water, said NASA's lead spacewalk officer Allison Bolinger.
The pads are attached to the back of the astronauts' helmets. During the spacewalk, Mastracchio and Hopkins periodically will test if the pads are squishy by leaning their heads back.
"This is our first line of defense," Bolinger told reporters during a press conference on Wednesday.
"As soon as the crewmember senses squishiness ... that's the sign that there is a problem in the (spacesuit) and it's time to come inside," she said.
During Saturday's spacewalk, Mastracchio and Hopkins are expected to prepare the 354-kilogram, 1.5-metre wide cooling system pump for removal. A spare will be installed during two more spacewalks, scheduled for Monday and Wednesday.
The failed pump, which is located on a pallet on the right side of the station's external truss, will be stored outside 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 pressurized and they contain liquid ammonia, so that's a hazard for us if it were to come in contact with us or our suits," Caldwell Tyson said in an interview with a NASA TV mission commentator.
Maintaining focus also can be a challenge, she added. "When you're on one of those pallets, you really have that sensation that you are sticking out on the edge of a skyscraper. Especially when you look down, you see your feet and then you see the Earth going [28,164 km/h] beneath you, it really does get your attention," Caldwell Dyson said.
"You tend to slow down and be a lot more careful when you have a backdrop like that," she said.
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