Astronauts onboard the International Space Station (ISS) were temporarily evacuated to the Russian segment of the station on Wednesday after alarms were triggered that can "sometimes be indicative of an apparent ammonia leak." Although an earlier report from Russia's Federal Space Agency claimed that there were "harmful emissions," NASA later clarified that "there is no hard data to suggest that there was a real ammonia leak" and that the problem is likely "a faulty sensor or computer relay." After several hours, NASA eventually permitted the crew to return to the US segment and said an investigation remains ongoing.
Before they returned, NASA said that onboard crew — comprising two American astronauts, one Italian astronaut, and three Russian cosmonauts — followed normal safety procedures and donned gas masks, moving to the Russian half of the ISS and sealing the American segment behind them. The flight control team in Houston reports that crew members are in "excellent shape" and that all other systems onboard the ISS are functioning perfectly.
Canadian astronaut and former ISS crew member Chris Hadfield tweeted that a leaking coolant system was one of the "big three" emergencies that astronauts train for on the station. "Ammonia is used for cooling through pipes & heat exchangers on the outside of Station," said Hadfield. "We train for it & the crew and MCC [mission control center] have responded well." He added that the other big emergencies were "fire/smoke" and "contaminated atmosphere/medical."
NASA is currently updating the situation and says that the most likely cause at this point in time is "a faulty sensor or computer relay."
Update: #Exp42 crew informed by controllers that it's starting to look like a false indication, either a faulty sensor or computer relay.— NASA (@NASA) January 14, 2015
Update January 4th, 8:23AM ET: This article was amended to reflect the latest reports from NASA suggesting that the alarm was falsely triggered.