Updated at 5:00 p.m. EST.

WASHINGTON — The crew of the International Space Station evacuated to the station’s Russian segment early Jan. 14 after an alarm indicated a potential ammonia leak in the U.S. segment, but were allowed to return later the same day after mission controllers concluded it was a computer glitch and not a real leak.

An alarm sounded at approximately 4 a.m. EST in the station’s U.S. segment “that sometimes can be indicative of an apparent ammonia leak,” NASA spokesman Rob Navias said in an status update broadcast on NASA Television at 8 a.m. EST. The station’s crew then moved into the Russian segment and closed a hatch to seal off the U.S. modules from the rest of the station.

ISS Expedition 42 Commander Barry Wilmore Credit: NASA
ISS Expedition 42 Commander Barry Wilmore Credit: NASA

Navias said that an increase in water pressure in one of two thermal control loops in the U.S. segment triggered the alarm. While that increase could be caused by a leak of ammonia coolant, Navias said it could also be caused by a faulty sensor or data relay box. “There is no data at the moment suggesting that there was in fact a real ammonia leak,” he said.

Space station controllers decided “out of an abundance of caution” to seal the crew in the Russian segment and to power down equipment in the U.S. segment, although Navias said some of those systems were being turned back on.

In a later interview on NASA Television, NASA ISS program manager Michael Suffredini said flight controllers did not believe any ammonia leaked into the station. “What we’re dealing with is a failure of probably a card inside of one of our multiplexer/de-multiplexers,” he said, an electronics unit that relays data to and from the station.

At a meeting of the NASA Advisory Council at the Stennis Space Center in Mississippi later Jan. 14, William Gerstenmaier, NASA associate administrator for human exploration and operations, confirmed that it was a failure of that electronics unit that triggered the alarm, and that there was no ammonia leak.

“It decided to have a unique failure signature such that it output data that just happened to match the indication of an accumulator pressure increase,” Gerstenmaier said of the electronics unit, triggering the alarm. Those readings returned to normal after recycling the power on the unit, he said.

Astronauts reentered the U.S. segment of the space station at about 3 p.m. EST, 11 hours after the alarm sounded. Tests confirmed no traces of ammonia within the station.

The disruption to station operations caused by the alarm should have no long-term effect on science experiments planned there, Suffredini said. “We didn’t lose any research,” he said. “It’s just that we’ll have to replan tomorrow and subsequent days.”


Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...