Astronauts on  board the International Space Station have been allowed to return to the U.S. section after Wednesday’s evacuation.

It was feared that the alarm was caused by a leak of toxic ammonia into the cabin, but NASA now reports that it has found no evidence of a leak. The U.S. side includes European, Japanese and U.S. station modules, which NASA’s Terry Virts and Barry Wilmore re-entered along with European Space Agency astronaut Samantha Cristoforetti. The trio were wearing oxygen masks as they took air samples, which tested negative for ammonia.

Suspected ISS Leak Proves To Be False Alarm

ISS toxic ammonia leak: No danger

“The crew is in good condition, was never in any danger and no ammonia leak has been detected on the orbital laboratory,” NASA officials wrote.

Ammonia is used in the ISS cooling system in order to regulate temperatures inside the modules, and an alarm sounded because the gas was detected leaking into the cabins. After the alarm sounded the aforementioned trio joined Russian cosmonauts Elena Serova, Alexander Samokutyaev and Anton Shkaplerov in their module.

Following the negative test results they will now be able to remove their masks and safely move through all areas of the station. It is thought that the false alarm was due to an error in the multiplexer-demultiplexer computer used to send and receive information on-board the spacecraft.

Back to work

NASA claims that the age-old technology fix known as “turn it off and back on again” was used to good effect and the computer is now back up and running. Mission Controllers still need to work out exactly what caused the false alarm, but the astronauts can now continue working as normal.

It is not thought that the evacuation had any adverse effects on research projects being carried out on the station, and NASA will now re-activate the cooling system.

This is not the first time that a false ammonia alarm has caused the evacuation of crew members to the Russian side of the craft, with two previous incidents recorded over the 15 years that astronauts have inhabited the space station, which has approximately the same living space as a 5-bedroom house.