The International Space Station, an orbiting laboratory the size of a football field, is not perfectly sealed and always leaking a bit of air.
But now that rate of air loss has "slightly increased," NASA said, and the three men living aboard the station have to hunker down while mission controllers on Earth figure out where the problem is — and how to fix it.
In a blog post, NASA said the American and Russian space agencies were "working a plan to isolate, identify, and potentially repair the source." NASA added: "The leak is still within segment specifications and presents no immediate danger to the crew or the space station."
NASA astronaut Chris Cassidy and his Roscosmos crew mates Ivan Vagner and Anatoly Ivanishin will spend the weekend huddled inside the station's Zvezda service module.
The Soyuz MS-16 spaceship, which the three-person crew rode to the space station in April and can whisk them back to Earth in the event of an emergency, is attached to a small laboratory near the Russian module.