Jan. 14, 2015

— A possible toxic ammonia leak inside the International Space Station on Wednesday morning (Jan. 14) caused the crew to evacuate to the Russian segment of the orbiting outpost, but further analysis by NASA flight controllers suggest it was likely a false alarm.

Update: 3:30 p.m. CST Jan. 14

- The International Space Station's crew has re-entered the U.S. side of the outpost, 11 hours after evacuating into the Russian segment due to a possible toxic ammonia leak.

NASA astronaut Terry Virts and Samantha Cristoforetti of the European Space Agency, both wearing protective face masks, were the first to re-enter the U.S. segment at 2:05 p.m. CST (2005 GMT). The two sampled the atmosphere and reported finding no indications of ammonia.

Shortly thereafter, the hatch that separates the U.S. and Russian sides of the complex was opened and the rest of the crew, including the station's commander Barry "Butch" Wilmore," was allowed to doff their masks and re-enter the U.S. segment to resume normal operations.

Astronauts Butch WiImore (front) and Terry Virts entered the U.S. segment wearing protective masks. (NASA TV)

Prior to Virts and Cristoforetti collecting air samples, flight controllers in Mission Control at NASA's Johnson Space Center in Houston studied data from a variety of the space station's systems, which also indicated no ammonia leak. The alarms that initiated the movement of the crew out of the U.S. segment are suspected to have been caused by a transient error message in one of the station's computer relay systems, called a multiplexer-demultiplexer (MDM).

Commands sent from the ground turned that relay box off and then back on again, which cleared the error message. The relay box is now reported to be in operating condition.

Flight controllers are continuing to analyze the data in an effort to learn what set today's actions in motion. Work to reactivate the space station's cooling loop B will continue throughout the night and into the day Thursday. The crew members are expected to resume their normal schedule of research activities on Thursday as well.

At around 3 a.m. CST (0900 GMT) on Wednesday, flight controllers in Mission Control at NASA's Johnson Space Center in Houston saw an increase in pressure in a water loop for the space station's thermal control system B, one of two redundant coolant loops that regulate temperatures aboard the orbital laboratory. They then saw an increase in cabin pressure that could have indicated an ammonia leak inside the station.

The cooling system uses water to collect heat from inside the space station and transfers it to ammonia outside the complex through exchangers. The ammonia is then routed through radiators to dissipate the heat.

Flight controllers were concerned that the ammonia, which is intended to stay in the station's external loop, may have leaked into the water that runs inside the lab's pressurized modules.

NASA astronaut Terry Virts helps European Space Agency (ESA) astronaut Samantha Cristoforetti with emergency training aboard the station on Dec. 1, 2014. Virts and Cristoforetti put that training into use in response to an ammonia leak alarm Jan. 14. (NASA)

To protect against that worst case scenario, the crew was directed to isolate themselves in the Russian segment of the space station while teams on the ground evaluated the situation. Non-essential equipment in the U.S. segment of the station was also powered down.

NASA astronauts Terry Virts and Barry "Butch" Wilmore, as well as Samantha Cristoforetti of the European Space Agency joined their Russian colleagues Anton Shkaplerov, Alexander Samokutyaev and Elena Serova in the station's Zvezda and Zarya modules and closed the hatch leading to the U.S., European and Japanese modules (referred to together as the U.S. Operating Segment, or USOS).

By 6 a.m. CST (1200 GMT), Mission Control told the crew that it may have been a false alarm.

"There's a possibility that this is a combination of sensor problems, [computer] partial failures, and thermal effects all thrown together in the exact wrong way to make it look like this was your classic ammonia leak," capcom James "Vegas" Kelly radioed to Wilmore, commander of the ISS Expedition 42 crew. "We have all the experts here coming in now and pouring over the data. We've got all the smart folks looking at it and are trying to figure out exactly what is going on."

"Enjoy your impromptu day off. We will keep you informed on what is going on," Kelly added.

About an hour later, after restarting some of the systems, the situation was looking even better.

"It is becoming a stronger case this is a false indication, which is great news," Kelly told the space station crew. "It is a little more positive than we thought before."

"We're working toward... getting you back in the other side [of the station]. No idea when that would happen," Kelly added. "I would still lean towards you guys won't reingress tonight but we're heading down that path and we will keep you guys updated."

"Outstanding news," Wilmore replied. "We'll be ready to do what you need us to do when the time comes."

The Russian modules have what is needed to support the six crew members, including food and its own bathroom. The Russian segment is also where the Soyuz spacecraft are docked, still protecting for an unlikely situation that the crew needed to depart the space station.

The space station, which has about the same living space as a five bedroom house, has been continuously crewed since November 2000.