American crew members aboard the International Space Station were evacuated after a sensor indicated a potential ammonia leak. NASA says that leak appears to be a false alarm.
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CAPE CANAVERAL, Fla. — Crew members are safe in the Russian segment of the International Space Station after an alarm prompted the evacuation from the U.S. section, according to NASA.

An alarm signaled at 4 a.m. that sometimes can be indicative of an ammonia leak forced Expedition 42 crew members to wear masks and seal themselves in the Russian segment. They were then allowed to remove their masks.

NASA confirmed that there is no hard data suggesting there was an ammonia leak.

“The space station crew is safe,” NASA spokesman Bob Jacobs said.

Flight controllers are unsure if the alarm was triggered by a pressure spike in a water loop for thermal control systems, a faulty sensor or a problem in a computer relay box.

“We saw an increase in water loop pressure, then later saw a cabin-pressure increase that could be indicative of an ammonia leak in the worst case scenario, so we protected for the worst case scenario and isolated the crew is the Russian segment of the space station while the teams are evaluating the situation,” Jacobs said.

As late as 6:02 a.m., communications between the Johnson Space Center in Houston and the ISS indicate that they are “not entirely convinced this is an ammonia leak.”

Flight controllers at the Johnson Space Center in Houston are gathering more and more evidence that leads them to believe this could be related to a faulty sensor.

The crew will most likely spend the rest of the day and night in the Russian segment, where up to a week of provisions are available.

Russia’s space agency Roscosmos said in a statement that a “leak of harmful substances from the cooling system” prompted the crew to isolate the American module. “The crew is safe and is in the Russian segment now,” it said in a statement.

Mission control experts in Moscow and Houston quickly and efficiently cooperated to ensure the crew’s safety, Roscosmos said. It added that Houston specialists are now analyzing the situation with the U.S. segment.

The space outpost is manned by NASA astronauts Barry Wilmore and Terry Virts, Russians Elena Serova, Alexander Samoukutyaev and Anton Shkaplerov and European Space Agency astronaut Samantha Cristoforetti.

Contributing: The Associated Press