Astronauts on the International Space Station (ISS) were woken by a smoke alarm on Thursday morning, which was ringing from the Russian part of the structure.

The crew are safe and a spacewalk, scheduled later on today, is still in the works, according to TASS, a Russian state news agency.

The alarm went off in the Zvezda module, which was added to the ISS in 2000. It sounded at 4:55 am Moscow Time (9:55 pm EST on Wednesday), per TASS.

Is it not clear what triggered the fire alarm. According to Russia-owned news agency RIA Novosti, astronaut Oleg Novitsky told mission control the crew had smelled smoke and Thomas Pesquet, the French astronaut, smelled burnt plastic.

Roscosmos, Russia's space agency, said in a tweet that the alarm went off as its batteries were being recharged.

All systems were later operating normally, Roscosmos said in the tweet.

An air-purifying system was turned on in response to the incident, Roscosmos told TASS. Officials confirmed that the air was safe to the crew, but didn't make clear whether there would have been a risk had the air not been cleansed.

The crew went back to sleep after the purification was done, Roscosmos, Russia's space agency, told TASS.

This is not the first time the Zvezda module has had this kind of issue. ISS crew also reported smoke from the module in 2014; and in 2020, astronauts tracked an air leak back to the module by releasing tea leaves in the station.

Other Russian parts of the ISS have had issues recently. Last week, cosmonauts reported finding cracks on Russia's Zarya, which was part of the ISS when it was first launched in 1998.

Earlier in the month, Russia's new ISS module Nauka mistakenly fired its thrusters and caused the station to flip 540 degrees.

The ISS has been in service for more than 20 years and is starting to show signs of its age. Is it likely to be deorbited within the decade.

This article was originally published by Business Insider.

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