American astronauts Butch Wilmore and Terry Virts wearing gas masks when returning to the US segment of the ISS

Computer glitch behind space station alarm

International Space Station astronauts have been allowed back into the station’s US segment after a computer glitch was identified as having triggered an evacuation alarm.

The alarm, indicating a possible leak of highly toxic ammonia from the station’s cooling system into the interior, prompted the six-strong crew to scrap their activities yesterday and isolate themselves inside the station’s Russian section.

However, Nasa’s ground-based engineers – assisted by their European and Japanese counterparts – analysed available data and didn’t find any indication of an actual leak.

The engineers concluded a computer glitch had triggered a series of false readings causing the station’s software, which monitors the cabin atmosphere, to ring the false alarm.

“The alarms this morning that initiated the movement of the crew out of the US 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,” Nasa said in a blogpost on its website. 

“A subsequent action to turn that relay box off and back on cleared the error message and the relay box is reported by flight controllers to be in good operating condition.”

Following the alarm, the station’s protection software shut down one of the two redundant cooling loops, which the engineers are still working on to reactivate.

The astronauts were given the all clear to return about 11 hours after the original alarm.

Before opening the hatch between the Russian and US modules, American astronauts Butch Wilmore and Terry Virts together with European Samantha Cristoforetti were instructed to don protective masks as a precaution. Upon entering, the crew conducted additional measurements that ruled out any presence of ammonia in the atmosphere.

"No indication of ammonia," said Nasa mission commentator Rob Navias. "Crew doffed their masks and are getting about their evening business."

Nasa maintained that the crew’s lives were never at risk during the incident and that the evacuation followed strictly defined emergency procedures.

The crew was about two hours into their workday when they retreated to the Russian section. They had planned to spend Wednesday unpacking a Space Exploration Technologies (SpaceX) Dragon cargo capsule, which arrived on Monday, and setting up new scientific experiments.

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