Astronauts returned to the U.S. side of the International Space Station on Wednesday, hours after an apparent false alarm raised concerns about an ammonia leak and forced a partial evacuation.
The six crew members shut down the U.S. segment and hung out on the station's Russian side for much of the day while ground controllers went about troubleshooting the problem. Eleven hours after the alarm arose, mask-wearing astronauts went back in and sampled the air.
"No ammonia indication," NASA reported in a tweet.
The likeliest cause of the alarm was a malfunctioning card in a signal-processing box, NASA space station project manager Mike Suffredini said during a televised update.
"At this point, the team does not believe we leaked ammonia. ... What we are dealing with is a failure, probably of a card inside a multiplexer-demultiplexer," he said. The space station has a number of multiplexer-demultiplexer boxes that process readings from components aboard the orbital outpost, which has as much livable space as a six-bedroom house.
NASA said turning the box off and then back on cleared up the worrisome readings.
The alarm was raised at 4 a.m. ET, when Mission Control saw pressure changes that could have been caused by an internal leak in the station's coolant system, which uses water on an inside loop and toxic ammonia on an outside loop. "If this is possible, then we immediately 'safe' the vehicle and get the crew in a safe place," Suffredini said.
Crew members put on emergency masks, powered down the systems on the station's U.S.-built segment, moved into the Russian segment and closed a connecting hatch. The ammonia coolant system services only the U.S. side of the station, so controllers said there was no risk to the crew.
"The safety of the team was preserved thanks to swift actions of the cosmonauts and astronauts themselves and the team on the ground in Moscow and Houston," said Maksim Matyushin, the chief of Russia's Mission Control.
Follow-up readings indicated no sign of an actual ammonia leak into the station, although fluctuations in cabin pressure continued to cause concern. Suffredini said those fluctuations were probably the system's "normal reaction to the events that started to unfold."
The systems that had been shut down during the emergency were powered back up, and no research was lost due to the interruption, Suffredini said.
Two American astronauts, space station commander Barry Wilmore and Terry Virts, are among the six crew members aboard the space station. The others are Italian astronaut Samantha Cristoforetti and Russian cosmonauts Elena Serova, Alexander Samokutyaev and Anton Shkaplerov.
Suffredini said a leak of toxic chemicals inside the station's cabin was one of the top three anomalies that the space station's crew has been trained to handle. The other potential anomalies are rapid depressurization and an onboard fire or smoke event. "We actually practice [to deal with] them once we're in orbit," he said.
In a worst-case scenario, the crew could be forced to board the Soyuz capsules that are attached to the station and fly back to Earth, leaving the orbital outpost empty. "These kinds of emergencies start with protecting the crew, then trying to deal with the issue, and then if in the end we can't deal with the issue, some of them can result in evacuating the station," Suffredini said.
Space station crews have had to cope with external leaks of ammonia from the coolant system, most recently in 2013, but never with an actual leak inside the cabin.
Suffredini quoted Wilmore as saying "everything went really well" during Wednesday's alarm.
"Really, there was never any risk to the crew," Suffredini said. "It seemed like the team, as you would expect, reacted appropriately to the event."