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Alan Boyle, Science EditorNBC News

Image: Mastracchio and spacesuit

NASA astronaut Rick Mastracchio checks his spacesuit in the International Space Station‘s Quest airlock in advance of Saturday’s outing.

Imagine a movie where astronauts are one failure away from abandoning the International Space Station, and have to deal with toxic chemicals during a spacewalk while facing the risk of drowning in their spacesuits. Oh, and it’s just before Christmas.

A Hollywood producer could spin the three spacewalks that NASA is planning over the coming week as a holiday-season sequel to“Gravity,” if Sandra Bullock were available. It’s more likely that the replacement of one of the space station’s external coolant pumps will be completely routine. But NASA is taking extraordinary measures, such as providing the two spacewalkers with jury-rigged spacesuit snorkels, just in case the fix-up job really does turn into a thriller.

Several factors, including a scary spacewalk in July involving water in a spacesuit helmet, have combined to add some extra drama to the repair operation that begins Saturday.

What went wrong on the space station?

The International Space Station is a 450-ton machine that puts out a lot of heat, and there’s a complex cooling system that keeps all of the electronics aboard (not to mention the six-person crew) at their proper operating temperature. Two external ammonia-cooled plumbing loops draw the heat away from the internal water-cooled loops and radiate it out into space. Last week, one of the valves in a pump for one of the ammonia loops malfunctioned, and that caused the loop to go offline.

As a result, NASA had to shut down non-critical systems and shift some equipment, ranging from power voltage converters to the freezers that preserve scientific samples, over to the other loop. Mission managers say the station could “limp along” in that condition — but if the other coolant loop fails, urgent action would have to be taken. In a worst-case scenario, the station would have to be abandoned.

Is this a rare problem?

Not really. Repairs to the cooling system are among the “Big 14″ maintenance tasks that NASA and its partners expected to face during space station operations. Since 2007, spacewalkers have gone out to deal with coolant system issues six times. In May, NASA mounted a spacewalk to replace a leaky coolant pump control box after just two days of planning.

The astronauts who will be taking on the latest repair operation — six-time spacewalker Rick Mastracchio and Mike Hopkins, who will be taking his first spacewalk — have had on-the-ground training for coolant repair. They’ve also studied a video that was sent up a few days ago to train them for their upcoming tasks, including what to do if water pools up in their spacesuit helmets.

Water in the helmet? What’s that about?

Italian astronaut Luca Parmitano faced an unprecedented problem with that during the spacewalk in July. Water crept up onto the back of his head in zero-G, then came around and pooled up over his face. The spacewalk was cut short and Luca survived, but U.S. spacewalks were suspended while the crew and engineers on the ground tracked down the cause of the problem. They think it had to do with bits of debris clogging up the system that’s supposed to remove moisture from air inside the suit.

That system has been replaced in the suit that Parmitano wore — a suit that will now be worn by Hopkins. “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, both astronauts will be wearing absorbent pads on their necks. Every so often, they’ll bend their necks back to see if the pads feel squishy. If they do, that will be a signal to end the spacewalk and get back inside the station. They’ll also have long plastic snorkel tubes inside their helmet that extend down into the body of the suit. If water starts pooling around a spacewalker’s face, he can take that tube into his mouth and suck up some emergency air.

If the first spacewalk goes well on Saturday, that will boost the confidence level for the spacewalks that are scheduled to follow on Monday and Wednesday.

Why does it take three spacewalks to replace one coolant pump?

The operation involves moving pieces of delicate equipment that are as big as refrigerators from one part of the space station’s exterior to another. The liquid ammonia in the external cooling system is toxic. It took three gnarly spacewalks to replace a coolant pump in 2010. During those outings, astronauts had to deal with a stuck hose, leaking ammonia, spacesuit decontamination and other complications.

Each of the three spacewalks scheduled over the next week is due to begin at 7:10 a.m. ET and last about six and a half hours. Astronauts will use the first spacewalk to set up their equipment, disconnect the faulty coolant pump module and hook up a temporary jumper box. They’ll also remove layers of insulation from the replacement pump module, which is currently stashed away on a stowage platform.

NASA lead spacewalk officer Allison Bolinger previews the three repair spacewalks.

The most crucial tasks are scheduled for Monday’s spacewalk. Mastracchio and Hopkins will pull out the faulty pump module with an assist from Japanese astronaut Koichi Wakata, who will be operating the station’s robotic arm from inside the station. They’ll park that module in a temporary stowage spot, then head over to the platform to pick up the spare module and put it in its new location.

During next Wednesday’s spacewalk, the ammonia fluid lines will be hooked up to the new module, and the old module will be moved into long-term storage. If everything goes smoothly, there’s a chance that the job can be done in just two spacewalks — which means the space station’s crew might not have to work on Christmas after all.

Where can I watch what’s happening, and what should I watch for?

NASA TV’s coverage of each spacewalk begins at 6:15 a.m. ET, with the spacewalk itself due to begin at 7:10 a.m. ET. Although the spacewalks are scheduled to run six and a half hours each, they could go shorter or longer, depending on how smoothly the work goes. NBC News will air NASA coverage and additional updates online and on TODAY.

Mastracchio’s call sign is “EV1.” His spacesuit has red stripes on the arms and legs. Hopkins is “EV2.” He’ll be wearing Parmitano’s upgraded hand-me-down suit, with no stripes.

Acronyms to listen for:

  • EVA: Extravehicular activity, or spacewalk.
  • ORU: Orbital replacement unit, a generic term for spare equipment.
  • PM: Pump module.
  • S1: The truss where the pump module is installed.
  • ESP-3: External Stowage Platform 3, where the spare pump module is kept.
  • POA: Payload ORU Accommodation, the place where the old pump module will be temporarily stowed.
  • HAP: Helmet absorption pad, which would indicate whether water is pooling up in the spacewalkers’ helmets.

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.