This story was updated at 2:38 p.m. EDT.

The recent cooling system malfunction aboard the International Space Station has underscored the intricacies of keeping the enormous orbiting complex properly functioning. The weekend glitch, which has not endangered the crewmembers currently living aboard the station, highlights the scope and challenge of maintaining the football field-size outpost.?

"The ISS is probably the most complex engineering feat ever performed," NASA spokesman Kelly Humphries said in a telephone interview Monday.

Late Saturday, a pump module failed in one of the station's liquid ammonia cooling loops, forcing the astronaut crew aboard the ISS to power down some of the station's vital systems while engineers on Earth addressed the problem.

The malfunction affected the station's Cooling Loop A, but the backup Cooling Loop B is still operating, according to NASA officials.

NASA plans two spacewalks to repair the glitch. The first spacewalk is set to begin Thursday.

How it works

The Active Thermal Control System (ATCS) has internal and external components that are designed to maintain specific temperatures aboard the station. The system uses mechanically pumped fluid in closed-loop circuits that control heat collection, heat transportation and heat rejection.

"The internal system circulates water through the different modules on the station," Humphries explained. "Those water loops then go through a heat exchanger which connects to the ammonia loops that run outside of the station. The reason we have two different systems is that the ammonia has chemicals that we don't want near the internal cabin area of the station."

Essentially, the ATCS maintains the thermal balance of the space station.

Energy from the station's solar arrays powers the electronics and various other systems. The process creates excess heat that must be removed.

Cold plates and heat exchangers, which are cooled by the circulating water loop, help regulate the spacecraft's internal atmosphere. This liquid heat-exchange system removes the excess heat, and the energy is then sent to radiators that eject the heat into space. Before this can happen, the waste heat must be exchanged a second time through the loop that contains liquid ammonia (rather than water, which would freeze if circulated in pipes outside of the station).

Ammonia freezes at minus 107 degrees Fahrenheit (minus 77 degrees Celsius) at standard atmospheric pressure.?

The ISS has two external cooling loops, which circulate the heated ammonia through huge radiators and release the excess heat as infrared radiation, simultaneously cooling the station's equipment in the process.

First-time problem

The station's cooling system, which was largely built by Boeing, had not suffered such a malfunction before, said Mark Mulqueen, Boeing's ISS vehicle director. The pump modules for the two ammonia cooling loops were launched to the space station in 2003, and were activated and have been running continuously since December 2006.

"This type of anomaly has not happened on a pump module," Mulqueen told "This is our first pump module failure."

Still, the crew is equipped with two spare pump modules that are stored outside the station. These spare components will be retrieved in the upcoming spacewalks.

"Any time we do a spacewalk it is a serious matter, because we're taking two humans outside of a relatively safe environment and putting them in spacesuits that are like their own little spaceships," Humphries said. "We take them very seriously, and we have very detailed plans for how the spacewalks will be accomplished."

And while the intricacy of all the space station's parts can sometimes lead to technical problems, the interaction of these complex systems is also what has kept the station operating successfully for over a decade.

NASA will broadcast a space station status update live at 4 p.m. EDT (2000 GMT) on NASA TV. Click here for space station mission updates and's NASA TV feed.