The crew – three Russians, two Americans and an Italian – put on breathing equipment before fleeing the U.S. section for the Russian segment, closing the hatch to the U.S. side behind them, the BBC reported.
"The U.S. section of the ISS has been isolated, the crew are safe and in the Russian section," Maxim Matyushin, head of Russia's Mission Control Center, said in a statement. "The concentration of impurities in the atmosphere in the Russian section of the ISS is within permissible levels."
The evacuation came after Mission Control spotted changes in cabin pressure that sparked fears of an ammonia leak, NASA said.
— NASA (@NASA) January 14, 2015
The Russian Federal Space Agency reported that a leak in the cooling system was releasing ammonia, a potentially fatal gas, into the station’s atmosphere. NASA, however, described the evacuation as a precautionary move following an alarm. A leak has not been confirmed, NASA said.
"So, big-picture perspective, we're trying to figure out exactly what happened. We're not entirely convinced that this is an ammonia leak," commander Barry Wilmore told NASA Mission Control in Houston.
The crew had been awake for about two hours before the alarm sounded, and was at work unloading the SpaceX Dragon cargo carrier, which arrived days ago with more than 2.5 tons of supplies and science experiments.
Canadian astronaut and former Space Station crew member Chris Hadfield on Wednesday tweeted that astronauts practice for this sort of scenario repeatedly before starting a mission.
— Chris Hadfield (@Cmdr_Hadfield) January 14, 2015
The orbiting space laboratory, a 15-nation project which is overseen by Russia and the United States, last saw ammonia leaks in 2009 and 2013. Most recently, the crew was forced to exit the space station to perform an emergency spacewalk in search of the leak.
