Proving that no matter how expensive your air conditioner the cooling pump will still break, NASA is planning a pair of emergency spacewalks on the $100 billion International Space Station this week to replace a cooling system component that unexpectedly failed Saturday.

The emergency spacewalks don’t just punctuate a cooling system problem, but a breakdown in the Earth-to-orbit ISS maintenance supply chain. The broken cooling pump module weighs an unwieldy 780 pounds and can only be transported to the ISS aboard the Space Shuttle.

There are already two spare pumps aboard the station, but after this week’s planned emergency repairs the ISS will have just one spare pump module — a necessary component for long-term function of the station — to last until 2020. Needless to say, that leaves astronauts aboard the ISS in a somewhat precarious position.

What went wrong in the first place? The ISS relies on a cooling system that pumps super-cold liquid ammonia throughout the station to ferry excess heat away from the research labs and other systems and otherwise keeps the station from overheating. Late Saturday, a circuit breaker tripped, shutting down the Loop A cooling pump module, leaving Loop B (the only other cooling pump) to cool the entire station.

This triggered alarms, awakening the six astronauts aboard who promptly began shutting down non-necessary systems to reduce the burden on the system, which was at that point operating at half capacity. The station is now stable and the astronauts safe, but the cooling pump module for Loop A needs to be replaced before the ISS can power back up to full cooling — and research — capacity.

That’s likely going to entail two separate spacewalks, one to retrieve an extra cooling pump module from a spare parts platform tethered to the ISS’s exterior and move it into position, and another to run the electrical and ammonia fluid connections to the new pump. The operation should restore the ISS but will leave it with just one spare pump with which to finish out the decade. The extra-vehicular activities could begin as early as Thursday, replacing a spacewalk that was already scheduled for that day.

Congress and the White House are still hashing out a budget for NASA that could add one additional shuttle mission if certain interests get their way. Whether or not the need for another spare cooling pump will influence where the money flows in Congress is debatable, but it does highlight the perils of ISS habitation in a post-shuttle era and might lead some to re-evaluate the maintenance of one program without the other.

[Space, Discovery]