Early on Wednesday morning, US astronauts aboard the ISS were forced to evacuate the American section of the station as an alarm was triggered, warning the crew of a potential ammonia leak. During the alert, astronauts took shelter in the Russian compartments of the ISS as mission operators attempted to discern whether the alarm was indeed a toxic leak, or merely a computer malfunction.
At the time of the alarm, flight controllers at NASA's Johnson Space Center, Houston observed a pressure increase in the outpost's water loop for thermal control system B, followed by a spike in cabin pressure. Together these anomalies could have constituted early indicators of an ammonia leak. Aboard the station, ammonia is vital to the running of the facility, being utilized as a coolant and heat transfer agent for the outpost's many systems.
Whilst mission control failed to detect any direct indicators of an ammonia leak via the sensors on board the station, at 3 am CST the crew were given the order to evacuate to the Russian segment as a safety precaution, shutting down all non-essential equipment, and sealing the doors behind them as they went. ESA astronaut Samantha Cristoforetti reassured her 270,000-strong twitter following, tweeting, “Hey everybody, thanks for your concern. We're all safe & doing well in the Russian segment.“
Yesterday's alarm was not the first instance of an ammonia issue aboard the station. 2014 saw a minor crisis unfold on the station, requiring astronauts to undertake three separate spacewalks in order to repair a faulty ammonia pump module.
As threat assessment continued, evidence began to stack up in favor of a computer malfunction, with system data analyzed by flight controllers on the ground suggesting that the alarm had been caused by an error in one of the station's computer relay systems. The theory was later confirmed as the relay was switched off and on, clearing the error from the system. That's right, restarting a computer even works in orbit.
At 2:05 pm CET, in the knowledge that the station and her crew had never been in any actual danger, mission controllers directed the astronauts to don their gas masks and re-enter the US compartments of the station. Following an atmospheric sampling carried out by Samantha Cristoforetti and NASA's Terry Virts, it was confirmed that no ammonia was present. Ordinary operations are expected to resume some time today.