Space is Freezing. Why Does the Space Station Need an Air Conditioner?

Over the weekend, the International Space Station's air conditioner broke. To be exact, one of its two redundant ammonia-fed cooling loops failed when a circuit breaker for its pump tripped. NASA said that the six occupants were in no danger, and that the orbiting complex was stable – for now. An emergency spacewalk has been scheduled for Friday to replace the broken pump.

But why on Earth would the Space Station need an air conditioner to begin with? (Okay, not "on Earth"; it's the space station.)

On Earth, air is warmed and cooled by convection and conduction, or collisions between individual air molecules and the circulation of air. In space, heating and cooling occur only by way of radiation – objects are warmed by solar rays warm and cool off by emitting infrared energy. Without thermal controls, the Space Station's Sun-facing side would heat to 250 degrees F (121 C), while thermometers on the dark side would plunge to minus 250 degrees F (-157 C).

The ISS addresses this extreme heating and cooling effect with special Mylar insulation, which keeps solar radiation out and keeps the bitter cold of space from penetrating the Station's metal skin.

Hot computers

But here's why the A/C is necessary: the mansion-sized station is packed with computers, gadgets and experiments that threaten to really hot things up.

Imagine that "your house was really, really well insulated and you closed it up and shut off the air-conditioning," says Gene Ungar, a thermal fluid analysis specialist at NASA's Johnson Space Center. "Almost every watt of power that came through the electric wires would end up as heat."

Forget the extremes of space: the Station's many systems all produce heat, and something has to be done to dispose of the excess.

How the Space Station's Air Conditioner Works

That's why NASA engineers created the Active Thermal Control System, or ATCS for short, an air conditioner that's not exactly light years ahead of the one you've got at home.

To keep the astronauts comfortable in short sleeves, the system removes waste heat in two ways: Air and water heat exchangers cool and dehumidify the spacecraft's internal atmosphere, while high heat generators are attached to custom-built cold plates. To cool both of these devices, cold water — circulated by a 17,000-rpm impeller the size of a quarter — courses through these devices to cool them off. a

From there, heat is carried outside the space station via a grid of coiled pipes – essentially a giant radiator – that circulates outside of the space station. But these pipes don't contain water: that would freeze in the deep cold of space. Instead, they're full of ammonia, which freezes at -107 degrees F (-77 C) at standard atmospheric pressure. The ammonia carries the heat outside and deposits it into space.

Astronauts practice replacing the cooling pump underwater in Houston.

The A/C broke

It was a pump in this system that failed, and set off the kind of alarm that no astronaut wants to awake to. While the station is not in danger now, the failure of one part of the system leaves the station without a backup.

For some, the episode highlights the risks that come with space living, and the importance of the Space Shuttle, which for years has been the Station's only cargo truck. But now that the ISS is basically built, the Shuttle is set to retire early next year. If the station needed extra equipment, Russia's small Soyuz vehicle wouldn't be of much help. (At least we'll be sending up a robot soon; hopefully he knows something about air conditioners.)

The cooling pump failure also triggered the shutdown of several pieces of equipment, including two of the station's four gyroscopes, which keep the whole thing orientated in orbit, one of two communications systems, one of two Global Positioning System receivers, power converters and routers.

This might also explain why the Tweeps on board the station haven't been Tweeting much either.