An alarm went off on the International Space Station earlier this morning as cosmonauts woke up to the smell of smoke.

There’s never a dull moment on the ISS these days, whether it be mysterious air leaks, problematic small cracks, busted toilets and oxygen supply generators, or a new module stubbornly wanting to depart shortly after docking, causing the ISS to unexpectedly rotate by as much as 540 degrees.

The latest incident happened at 4:55 a.m. Moscow time, when a smoke detector was triggered in the Zvezda service module of the Russian segment, Roscosmos explained in a statement. The Zvezda module, in addition to hosting a portion of the station’s life support systems, provides living quarters for two crew members. BBC reports that the smell of smoke drifted as far as the U.S. segment.

Roscosmos says the appearance of smoke coincided with the autonomous recharging of the station’s batteries. An air filter was switched on to eliminate the “smoke pollution” and refresh the space station’s artificial atmosphere. Once the smoke was cleared, the ISS-65 crew continued their night in “rest mode,” the agency said. The main operational control group for the Russian segment said all systems are operating normally, and the air aboard the station “corresponds to the standard indicators.”

Roscosmos has not immediately responded to my request for more information, such as the cause of the smoke, the condition of the battery charger, and next possible steps.

Life on the station appears to have returned to normal. Oleg Novitsky and Pyotr Dubrov proceeded with their scheduled six-hour spacewalk today, as the cosmonauts continue to integrate the newly arrived Nauka module.

In an email, Jonathan McDowell, a researcher at Harvard-Smithsonian Center for Astrophysics, said incidents such as these are “very serious,” as they could lead to smoke inhalation or, worse, a full-blown fire (there’s lots of flammable material up there). He characterized the Russian response thusly: “So, ‘there was a burning smell, but we turned up the fans and the smell has gone away now, although we still don’t know what it was.’” This response, said McDowell, “doesn’t fill me with intense confidence.”

McDowell reminded me of a historical incident aboard the Mir space station. A fire broke out on February 24, 1997, and it took the crew of six nearly 15 minutes to put out the “searing flame,” as NASA described it, which they did with fire extinguishers. NASA astronaut Jerry Linenger was aboard Mir at the time, and he described the incident in his memoir Off the Planet (via Universe Today):

As the fire spewed with angry intensity, sparks – resembling an entire box of sparklers ignited simultaneously – extended a foot or so beyond the flame’s furthest edge. Beyond the sparks, I saw what appeared to be melting wax splattering on the bulkhead opposite the blaze. But it was not melting max. It was molten metal. The fire was so hot that it was melting metal.

So yeah, fires aboard space stations are very bad. In that case, the fire started in Mir’s solid fuel oxygen generator, and the flames were put out before they could damage the station or injure the crew. The incident led to new policies and training measures to prevent a recurrence.

So while Roscosmos is quick to downplay today’s incident, what happened is clearly no joke. Hopefully more details will emerge in the coming days to confirm everything is truly okay and that the crew is safe.

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