Dec. 11, 2013 -- One of the cooling systems on the International Space Station has been shut down because of temperature fluctuations, cutting the artificial satellite's cooling ability in half.

Shutting down the modules -- Kibo, Columbus and Harmony -- means that the six astronauts will have to stay in the other modules for now, NASA said.

Officials don't know yet whether the problem resulted from a software glitch or involves hardware, so it's unclear whether a space walk will be needed to resolve the issue.

There are two cooling loops on the space station, A and B, and if one of them goes down, engineers push loads to the second loop. The loads were too much for this plan to work in this situation.

"Earlier Wednesday, the pump module on one of the space station's two external cooling loops automatically shut down when it reached pre-set temperature limits," NASA said in a statement posted on its website. "These loops circulate ammonia outside the station to keep both internal and external equipment cool. The flight control teams worked to get the cooling loop back up and running, and they suspect a flow control valve actually inside the pump module itself might not be functioning correctly."

If the station is experiencing a hardware problem, astronauts will have to go out on a spacewalk to replace the pump. If it is a software problem, engineers hope to upload a patch, but don't yet know what prompted this failure, hence the shutdown of some systems.

Any spacewalk would be complicated by Italian astronaut Luca Parmitano's near-drowning in August while doing routine maintenance on the space station, according to a source.

Engineers have to determine what caused that mishap before NASA would commission a spacewalk to fix the cooling problem, according to a source.

While Russian cosmonauts had a flashy Olympic torch spacewalk, the cooling systems are a U.S. problem so its astronauts are the ones with the training and skills to replace the pump.

Astronauts Tracy Dyson and Doug Wheelock installed this very same pump on a spacewalk in the summer of 2010.

The astronauts at the space station are asleep and engineers on the Mission Management Team are working around the clock through the scenario to fix the problem.

NASA emphasized that there was never any imminent threat.

"At no time was the crew or the station itself in any danger, but the ground teams did work to move certain electrical systems over to the second loop," NASA said. "Some non-critical systems have been powered down inside the Harmony node, the Kibo laboratory and the Columbus laboratory while the teams work to figure out what caused the valve to not function correctly and how to fix it.

"The crew is safe and preparing to begin a normal sleep shift while experts on the ground collect more data and consider what troubleshooting activities may be necessary," NASA said.