This story was updated at 2:38 p.m. EDT.
Therecent cooling system malfunction aboard the International Space Station hasunderscored the intricacies of keeping the enormous orbiting complex properlyfunctioning. The weekend glitch, which has not endangered the crewmembers currentlyliving aboard the station, highlights the scope and challenge of maintainingthe football field-size outpost.?
"TheISS is probably the most complex engineering feat ever performed," NASA spokesman KellyHumphries said in a telephone interview Monday.
LateSaturday, 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 thestation's vital systems while engineers on Earth addressed the problem.
Themalfunctionaffected the station's Cooling Loop A, but the backup Cooling Loop B is stilloperating, according to NASA officials.
NASA plans two spacewalks to repair the glitch. The first spacewalk is set to begin Thursday.
TheActive Thermal Control System (ATCS) has internal and externalcomponents that are designed to maintain specific temperatures aboard thestation. The system uses mechanically pumped fluid in closed-loop circuits thatcontrol heat collection, heat transportation and heat rejection.
"Theinternal system circulates water through the different modules on thestation," Humphries explained. "Those water loops then go through aheat exchanger which connects to the ammonia loops that run outside of thestation. The reason we have two different systems is that the ammonia haschemicals that we don't want near the internal cabin area of the station."
Essentially,the ATCS maintains the thermal balance of the space station.
Energyfrom the station's solar arrays powers the electronics and various othersystems. The process creates excess heat that must be removed.
Coldplates and heat exchangers, which are cooled by the circulating water loop, help regulatethe spacecraft's internal atmosphere. This liquid heat-exchange system removesthe excess heat, and the energy is then sent to radiators that eject the heatinto space. Before this can happen, the waste heat must be exchanged a secondtime through the loop that contains liquid ammonia (rather than water,which would freeze if circulated in pipes outside of the station).
Ammoniafreezes at minus 107 degrees Fahrenheit (minus 77 degrees Celsius) at standardatmospheric pressure.?
TheISS has two external cooling loops, which circulate the heated ammonia through hugeradiators and release the excess heat as infrared radiation, simultaneouslycooling the station's equipment in the process.
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 SPACE.com. "This is our first pump module failure."
Still, the crew is equipped with two spare pump modules that are stored outsidethe station. These spare components will be retrieved in the upcoming spacewalks.
"Anytime we do a spacewalk it is a serious matter, because we're taking two humansoutside of a relatively safe environment and putting them in spacesuits thatare like their own little spaceships," Humphries said. "We take themvery seriously, and we have very detailed plans for how the spacewalks will beaccomplished."
Andwhile the intricacy of all the space station's parts can sometimes lead totechnical problems, the interaction of these complex systems is also what haskept the station operating successfully for over a decade.
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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 SPACE.com's NASA TV feed.