Three space explorers zipping around over the Earth on the International Space Station have a bit of a mystery on their hands thanks to a small but persistent air leak. NASA is in sleuthing mode to find the source.

While an air leak in space sounds worrisome, NASA isn’t fretting it. “The leak is still within segment specifications and presents no immediate danger to the crew or the space station,” NASA said in a statement on Thursday.

A little bit of air leakage is normal, and this particular leak has been on the radar for a while. “In September 2019, NASA and its international partners first saw indications of a slight increase above the standard cabin air leak rate,” the agency said. A further increase in that rate has triggered new measures to hunt down the source so it can potentially be repaired.

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The current crew consists of NASA’s Chris Cassidy and Russian cosmonauts Ivan Vagner and Anatoly Ivanishin of Roscosmos. All three will spend Friday through Monday morning hanging out together in the Zvezda service module, a Russian segment of the ISS.

Bunking in Zvezda gives the crew a chance to close down the station hatches so NASA can monitor the air pressure in each section. “The test presents no safety concern for the crew,” NASA said, and it should help mission controllers figure out where the small leak is coming from.

Initial results are expected next week.