Expedition 42 patch.
There is a very good reason why astronauts spend a lot of their time on maintenance procedures. SPacecraft and space stations like the ISS are very complex machines, with thousands and thousands of parts, some of which wear out and break. These machines also use a lot of important but dangerous chemicals, gases, and fluids to maintain the life-support and propulsion (or station-keeping) systems. On January 14, one of those systems gave NASA a problem.
Mission Control engineers confer on how best to handle the situation.
At the beginning of the astronaut's workday, alarms sounded in the US modules indicating a possible ammonia leak. Ammonia is used to regulate the temperature of the ISS. Leaking ammonia gas could kill the astronauts. Earlier expeditions had replaced one of the ammonia units located outside the station. It's still possible for ammonia to get inside the station, because some of the cooling loops traverse the interior.
InterFace Heat Exchanger (IFHX). Image from NASAspaceflight.com.
While it was possible that there could have been a false indicator signal, safety procedures were followed and the astronauts immediately put the US modules into a safe mode while they donned oxygen masks. Then the crew evacuated to the Russian segments of the station while ground engineers worked with the astronauts to analyze the fault. It was thought that perhaps ammonia was leaking into the station from one of 10 IFHX units. The InterFace HEat Exchange units route ammonia through loops into and out of the station in order to transfer heat to colder areas and then transfer the coolant back into the station. The heat exchangers themselves can be replaced during a spacewalk, but the coolant loops could be more difficult.
In the end, it turns out that the fault WAS a bad indicator, and after a day of investigation Mission Control gave the green light for astronauts to re-enter the US modules and begin resetting for continued work. NASAspaceflight.com reported that the astronauts had already been ahead of schedule on removing items from the Dragon supply ship, so no important operations were delayed.