Snorkel

12 minutes ago

Two NASA astronauts began a high-stakes series of spacewalks to repair the International Space Station's cooling system on Saturday, outfitted for the first time with snorkels in their helmets to keep them from drowning.

'Beautiful day,' astronaut Rick Mastracchio said after the spacewalk's start at 7:01 a.m. ET.

Mastracchio and fellow spacewalker Mike Hopkins are taking on three spacewalks through Christmas Day to replace a coolant pump module with a faulty valve. Last week's valve malfunction cut the station's cooling capability in half, forcing mission managers to shut down non-critical systems on the $100 billion-plus orbital outpost.

Ground controllers couldn't get the valve back in service by remote control, which led to the current plan for three spacewalks running through Christmas Day. Replacing the refrigerator-sized module is one of the station's expected maintenance tasks - but the job has been complicated by the fact that an Italian spacewalker, Luca Parmitano, almost drowned in July when water pooled up in the helmet of his U.S.-made suit.

That incident led NASA to suspend U.S. spacewalks while the station's crew and controllers worked to track down the source of the problem. They think they've identified the cause: previously undetected contamination that clogged up the system designed to remove moisture from the suit. And they've given a clean bill of health to the spacesuits Mastracchio and Hopkins are using.

'I would be surprised if we have a problem with the suits,' Mike Suffredini, NASA's space station program manager, told reporters this week.

But just in case, the astronauts are wearing absorbent pads on their necks that should pick up the first signs of excess moisture in the helmet - and are primed to stop the spacewalk if those pads start feeling squishy. They also have jury-rigged snorkel tubes within easy reach of their mouths in the event that water starts covering their faces.

Mastracchio has done six previous spacewalks, while Hopkins - who is wearing the suit that Parmitano wore in July - is a first-time spacewalker. During Saturday's outing, they're due to set up the tools for their worksite on the station's S1 truss. They'll also disconnect cables and fluid lines attached to the faulty pump module, hook up a jumper box and unwrap the insulation covering a spare module that's been stowed away on the station's exterior.

As they work, the spacewalkers will have to watch out for toxic ammonia coolant that may leak out from the coolant system's plumbing. Japanese astronaut Koichi Wakata will help out from inside the station by operating the 50-foot-long robotic arm.

During follow-up spacewalks on Monday and Christmas Day, Hopkins and Mastracchio are scheduled to replace the old pump module with the spare. Each of the spacewalks is scheduled to last six and a half hours.

The cooling system problem poses no immediate danger to the crew, but it does reduce the safety margin on the station. If a second coolant loop fails, urgent action would be required - and in a worst-case scenario, the station would have to be abandoned. The situation already has forced NASA to delay a cargo resupply mission until Jan. 7 at the earliest. NASA sources say they are studying whether last week's glitch resulted in damage to Europe's Columbus laboratory.

And as if the six-man crew didn't have enough to worry about, NASA reported in an advisory that one of the toilets on the station has broken down.

In addition to Hopkins, Mastracchio and Wakata, the space station is home to three Russian cosmonauts: station commander Oleg Kotov, Mikhail Tyurin and Sergey Ryazanskiy. The 450-ton structure has been continuously occupied since 2000.

More about the spacewalks: Tune in to TODAY on Saturday for the latest on the spacewalk to repair the International Space Station. Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by 'liking' the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. You can also check out 'The Case for Pluto,' my book about the controversial dwarf planet and the search for new worlds.