Four astronauts are now en route home from the International Space Station, marking the first time NASA has brought a crew home prematurely amid a health concern.

Axar.az reports, citing CNN, only three people remain on the orbiting laboratory: Roscosmos cosmonauts Sergey Kud-Sverchkov and Sergei Mikaev of Russia and NASA astronaut Chris Williams.

Fincke and Cardman, along with astronaut Kimiya Yui of the Japan Aerospace Exploration Agency, or JAXA, and Roscosmos cosmonaut Oleg Platonov, are part of the Crew-11 mission that’s leaving the space station. NASA did not say which crew member is experiencing a medical concern — nor did the space agency provide any details about the nature of the condition, citing privacy concerns. However, NASA has said the affected astronaut is in stable condition.