A Russian satellite has broken up into more than 100 pieces of debris in orbit, forcing astronauts on the International Space Station to take shelter, U.S. space agencies said.

There were no immediate details on what caused the break-up of the RESURS-P1 Russian Earth observation satellite, which was decommissioned in 2022. U.S. Space Command said on Thursday there was no immediate threat as it tracks the debris swarm.

The event occurred around 10 a.m. Mountain Time (1600 GMT) on Wednesday, Space Command said. It occurred in an orbit near the space station, prompting U.S. astronauts on board to shelter in their spacecraft for roughly an hour, NASA’s Space Station office said.

Radars from U.S. space-tracking firm LeoLabs detected the satellite releasing several fragments up until 6 p.m. Mountain Time, the company said.

U.S. Space Command, which has its own global network of space-tracking radars, said the satellite immediately created “over 100 pieces of trackable debris.”

Large debris-generating events in orbit are rare but of increasing concern as space becomes crowded with satellite networks vital to everyday life on Earth, from broadband internet and communications to basic navigation services.

Russia sparked international fury in 2021 when it struck one of its defunct satellites in orbit with a ground-based anti-satellite missile, creating thousands of pieces of debris to test a weapon system ahead of its 2022 invasion of Ukraine.

The prospect of satellite collisions and space warfare have added urgency to calls from space advocates and lawyers to have countries establish an international mechanism of managing space traffic, which does not currently exist.