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A problem with one of the International Space Station's cooling systems may require a repair spacewalk, NASA told NBC News on Wednesday.

The situation doesn't represent a life-threatening emergency, but it has required a cutback in normal operations on the orbiting outpost, NASA spokesman Josh Byerly said.

"The crew was never in any danger," he said. "They're fine for the near future."

The problem is one of the "Big 14" maintenance issues that NASA and its partners expect to face on an occasional basis during space station operations. "This is the kind of thing that they expected to happen, at about the expected rate," NBC News space analyst James Oberg said.

Shutdown mode

One of the station's two external cooling loops, known as Loop-A, shut down when it reached a pre-set temperature limit on Wednesday, Byerly told NBC News. That forced NASA to reroute coolant into Loop-B. As a result, the station's six-person crew had to prioritize life support systems, electrical systems and science experiments — including the freezers that preserve scientific samples. Some non-critical systems were turned off in NASA's Harmony node, Japan's Kibo lab and Europe's Columbus lab.



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The crew "worked to keep the freezers going," Byerly said.

Engineers think the problem was caused by a malfunctioning flow control valve for the station's ammonia coolant. Mission managers are trying to determine whether a software fix can get the valve working again, or whether a spacewalk will be required. Sorting through all the issues might take a couple of days, or as much as a couple of weeks.

Routine maintenance

It generally takes two weeks to plan for a "Big 14" repair operation, but that depends on the resources at hand and the urgency of the problem. In May, NASA astronauts mounted a spacewalk to replace a leaky coolant pump control box after just two days of planning. Spacewalkers also dealt with coolant system maintenance issues in 2012, 2011, 2010, 2009 and 2007.



U.S. spacewalks were temporarily suspended after Italian astronaut Luca Parmitano's spacesuit filled with water during a July outing, but Russian spacewalks have proceeded without problems. And since July, NASA has made significant progress on resolving the spacesuit problem that Parmitano faced.

"We are cleared for a contingency spacewalk if we need to do one," NASA spokesman Kelly Humphries told NBC News.

Two Americans, three Russians and a Japanese astronaut are living on the space station, which has been continuously occupied since 2000 by spacefliers who spend tours of duty generally lasting four to six months. The current plan calls for the space station to be maintained through 2020, although the United States and the 14 other countries supporting the $100 billion-plus station project are already considering extending its operating life.

More about the 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. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.