The words ammonia and astronaut have been linked recently by more than their proximity in the encyclopedia. Last December, astronauts aboard the International Space Station embarked on a pair of dangerous spacewalks to replace a failed ammonia pump module outside the orbital laboratory.

It was the second time in three years that spacewalks were required for an ammonia-related issue. In 2010, astronauts replaced a depleted ammonia tank in a spacewalk that lasted nearly eight hours. Last year’s project was completed in two spacewalks by NASA astronauts Rick Mastracchio and Mike Hopkins.

The space station’s External Thermal Control System consists of two separate cooling loops – with a 600-pound ammonia charge – that work in tandem to transport heat away from electronic equipment and toward the radiators, where it then dissipates into space. The ammonia is circulated around each of the loops by the pump module.

Last year’s failure occurred in the flow control valve inside the pump package of one of the loops. The flow valve controls the temperature of the ammonia in cooling loops by mixing cool ammonia that is exiting the radiators with warm ammonia that has bypassed the radiators. Without thermal controls protecting the computers and electronic systems, the temperature of the orbiting space station would fluctuate wildly, from 250 degrees on the sun-facing side to minus-250 on the dark side. Ammonia, with its high thermal capacity and wide range of operating temperatures, is the ideal refrigerant for such a volatile environment.

The failure of one of the two ammonia pump packages meant that several items of electronic equipment had to be shut down because one cooling loop is not sufficient to handle all of the space station’s needs.

The pump module and valves are encapsulated in a box the size of a refrigerator, with hoses attached to the back of the module. Initially, astronauts attempted to adjust the electronics from inside the space station in hopes of resolving the issue, but they quickly realized that a spacewalk was necessary to replace the entire unit.

The most difficult challenge was disconnecting the four ammonia lines from the failed pump module, which involved manually pulling a lever to close the bail and shutting a valve to prevent ammonia loss into space via an open line, then pulling the jumpers away from their connecting ports. This exercise required significant physical strength and manipulation from the astronauts.

The 780-pound pump module was then removed from the space station as Mastracchio, riding on a robotic arm, pulled the bulky pump partly out of the space station’s truss so Hopkins could install a robotic arm grapple fixture to its side. The new pump module was installed on a second spacewalk taken on Christmas Eve.

Although working with ammonia can be somewhat easier in space, where there is no fear of exposing nearby neighbors, there are still significant challenges. Mastracchio reported being surrounded by ammonia “snowflakes” as he clipped the four fluid lines into place onto the new pump. Although NASA said the ammonia was residual fluid and not another leak, some flakes did strike the astronauts’ spacesuits, and there was fear the flakes might attach to the suits in solid form and then turn to vapor inside the space station, thus contaminating the air lock. The spacewalkers took extra precautions to clean and brush their suits so that ammonia would not find its way back into the space station.

While working with ammonia may present significantly different challenges in space than on earth, safety, it seems, always comes down to the same thing: good engineering practices, and even better planning.