A defunct Russian satellite shattered into nearly 200 pieces of debris, prompting astronauts on the International Space Station (ISS) to seek shelter for about an hour.

NASA, the U.S. space agency, was informed that the Russian Earth observation satellite RESURS-P1 had broken up on Wednesday. As a precautionary measure, the six American crew members on the ISS were instructed to “shelter in their respective spacecraft,” according to a post on X by the ISS.

“Mission Control continued to monitor the debris path, and after approximately an hour, the crew was cleared to exit their spacecraft, allowing the station to resume normal operations,” the post stated on Thursday.

US Space Command, responsible for coordinating space-related operations for all military branches, initially tracked “over 100 pieces of trackable debris” from the satellite, which Russia declared defunct in 2022, noting that the incident posed “no immediate threats.”

By Thursday afternoon, US space-tracking firm LeoLabs reported its radars had detected at least 180 pieces of debris. “Due to the low orbit of this debris cloud, we estimate it’ll be weeks to months before the hazard has passed,” the firm added.

Roscosmos, the Russian space agency, had decommissioned RESURS-P1 in 2021 due to onboard equipment failures. The agency did not respond to requests for comment or acknowledge the event on its social media channels.

NASA astronauts Butch Wilmore and Suni Williams took refuge in their Starliner spacecraft, a Boeing-built capsule docked at the ISS since June 6 for its first crewed test mission. Three other US astronauts and a Russian cosmonaut sheltered in SpaceX’s Crew Dragon capsule, which brought them to the station in March. The sixth US astronaut joined two other Russian cosmonauts in the Russian Soyuz capsule that had ferried them there in September last year.

Large debris-generating events in orbit are rare but increasingly concerning as space becomes crowded with satellite networks essential for everyday life on Earth, including broadband internet, communications, and navigation services.

In 2021, Russia used a ground-based anti-satellite (ASAT) missile to destroy one of its defunct satellites in orbit, creating thousands of pieces of debris. This test of a weapons system occurred ahead of Russia’s 2022 invasion of Ukraine.

There was no immediate indication from airspace or maritime alerts that Russia had launched a missile to target the RESURS-P1 satellite, according to Jonathan McDowell, a space tracker and Harvard astronomer. He speculated that the breakup might have been caused by an issue with the satellite, such as leftover fuel causing an explosion.

With approximately 25,000 pieces of debris larger than four inches (10 cm) in space, experts are concerned about the potential for a Kessler effect—a scenario in which satellite collisions generate a cascading field of hazardous debris, exponentially increasing the risk of further collisions.