- At roughly 4 a.m. ET Wednesday morning, an alarm inside the International Space Station went off, forcing all six crew members to don masks and take shelter in the Russian portion of the station.
- The alarm indicated elevated water pressure in the ISS coolant system, which may have been a sign of a toxic ammonia leak, but NASA believes it was likely a faulty sensor.
- All six crew members —including two Americans, one Italian, and three Russians — are now safe, and NASA has directed its astronauts to return to the American portion by evening.
- Ammonia is used as the space station's main coolant, and ammonia leaks are considered one of the main emergencies that astronauts train for, according to former crew member Chris Hadfield.
Update: This post has been edited to reflect ongoing developments.