FAQ: International Space Station Cooling System Pump Failure
The International Space Station is featured in this image photographed by an STS-130 crew member on space shuttle Endeavour after the station and shuttle began their post-undocking relative separation.
Credit: NASA

NASA is forging ahead with a plan to send astronauts on two emergency spacewalks outside the International Space Station to fix a major malfunction in the orbiting laboratory's cooling system.

American astronauts Doug Wheelock and Tracy Caldwell Dyson will replace a faulty ammonia pump on the space station's right, or starboard, side during the repair spacewalks set for Saturday and Wednesday, Aug. 11.?

Here are some questions and answers about the situation:

Exactly what went wrong on the space station?

The cooling system problem occurred Saturday, July 31 at about 8 p.m. EDT (0000 Aug. 1 GMT), when what NASA engineers suspect was a power spike tripped a circuit breaker for the liquid ammonia pump for Loop A of the International Space Station's cooling system. The pump, one of two (the other is for Loop B) shut down as planned, knocking out half of the station's cooling system. ?It cannot be opened to fix its internal components, so astronauts must replace the whole pump during spacewalks. [Graphic: Space Station's Cooling System Problem Explained]

How does the space station's cooling system work?

The space station's 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.

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.

How bad is the problem for the space station?

NASA space station program manager Mike Suffredini said Monday that this malfunction is among the top potential failures for the International Space Station. NASA has a list of 14 different malfunctions (called the "Big 14") that are considered critical to the continued operation of the space station. This cooling system pump failure is on that list, but it is the first such failure ever to occur, Suffredini said.

Did the failure endanger the space station crew?

No. NASA has repeatedly stressed that at no point were the six astronauts living aboard the space station in any danger or at risk of having to evacuate the International Space Station. The space station is currently home to three American astronauts and three cosmonauts from Russia's Federal Space Agency.

After the initial failure on July 31, the station crew shut down several systems and backup systems to reduce the heat load on onboard equipment. With those systems powered down, the station's second cooling loop ? Loop B ? is able to maintain cooling of the active equipment.

The space station's Russian segment also has its own cooling system independent of the U.S. system.

How can astronauts replace the station's ammonia pump?

Spacewalking astronauts will replace the station's stricken ammonia pump in two stages. During a Saturday, Aug. 7 spacewalk, they will physically remove the big faulty pump module and then bolt down a spare pump module in its place.

One astronaut (Wheelock) will ride the space station's robotic arm, while the other (Caldwell Dyson) will be floating free and tethered to the station's truss structure. Their main worksite is on the station's Starboard? 1 (S1) truss segment, which is where the faulty pump is located. The spare pump is on a storage platform about 30 feet (9 meters) away.

During the second spacewalk, the astronauts are expected to hook up the ammonia plumping and electrical lines to fully activate the new pump. There are five electrical connections and five liquid ammonia connections to be made.

The astronauts must be careful when handling liquid ammonia plumbing lines in order to avoid any residual liquid ammonia which can contaminate their spacesuits and require extra decontamination procedures.

Doesn't it take weeks to plan a spacewalk? How was this response so fast?

Typically, for a repair like the upcoming ammonia pump replacement, NASA requires around two weeks to fully plan, rehearse and prepare for such a spacewalk.

By chance, Wheelock and Caldwell Dyson were already scheduled to perform a different spacewalk this week, so had already prepared their spacesuits and general tools for an outdoors procedure. Since the pump replacement is one of NASA's Big 14 space station repairs, the space agency also had a spacewalk repair plan in hand in case of an emergency.

Together, those two facts have helped speed the spacewalk planning along, though engineers at NASA's Johnson Space Center in Houston have also been working round-the-clock to get things ready.

How big are the station's ammonia pumps and how much do they cost?

The space station crew has said the size of the pumps is similar to a laundry dryer.

Each pump weighs 780 pounds (353 kg) and is 5 1/2 feet long (69 inches) by 4 feet wide (50 inches). They are about 3 feet tall (36 inches).

NASA doesn't have an official cost for each of the ammonia pumps on the space station. But a spare pump assembled on the ground in 2009 cost about $3.9 million, NASA officials told SPACE.com.

What space station systems were affected by the pump's failure?

Once the pump shut down, station astronauts turned off two of four control moment gyroscopes (used to maintain the station's orientation in space without thrusters), some station power converters and command-routing equipment, as well as backup systems for the station's S-band communications antenna and Global Positioning System. Some station systems were running without backups in place if they fail.

The following day, Mission Control reactivated one of the two offline gyroscopes and astronauts installed jumper cables between some station systems and the power system on the outpost's U.S. Destiny laboratory to preserve redundancy.

What happens if both U.S. cooling loops on the space station fail?

If both U.S. cooling loops fail (an unlikely event according to NASA) the space station's U.S. systems could be powered down and the crew moved into the Russian segment for a time, NASA officials have said. Since the Russian side of the space station has an independent cooling system, it could support a six-person crew for a short while, perhaps a few days until a repair or response is made, station managers said.

How large is the International Space Station?

The International Space Station is the largest structure ever built in space by humans and has a backbone-like main truss as long as a football field. On a clear night, the space station can easily be seen by the unaided eye as it flies overhead, at times even outshining the planet Venus.

The space station is built in two major segments: A U.S. segment and a Russian segment. The Russian segment currently consists of the Zarya and Zvezda modules, and several smaller docking and research nodes. The U.S. segment includes NASA's Destiny lab, connecting nodes (Tranquility, Unity and Harmony) and international laboratories from Europe and Japan. Canada built the station's robotic arm and mobile railcar system.

Altogether the station has 13 different habitable compartments and features enough living space as 1.5 Boeing 747 jumbo jets (about equivalent to a three-bedroom house).