How long astronauts stay in space station?

The International Space Station runs on expeditions, numbered sequentially, and the usual increment for a crew member is around six months. The figure is not arbitrary. It balances the cost and availability of crew vehicles, the certified orbital lifetime of the spacecraft left docked as a lifeboat, and the amount of physiological change a body can absorb before recovery on the ground becomes difficult.

Crews overlap rather than swapping cleanly. An arriving vehicle brings replacements who spend days handing over experiments, maintenance tasks and the unwritten knowledge of where everything is stowed, before the departing crew undocks. That overlap is why the station's population fluctuates between roughly seven and eleven people at a time.

Deliberate exceptions exist for research. NASA and Roscosmos have flown one-year missions to study how the body responds to extended exposure, and the resulting medical data feed directly into planning for journeys to Mars, where a round trip cannot be cut short. Individual stays have also stretched beyond plan for operational reasons, including vehicle problems that delayed a return.

Short visits are a separate category. Private astronaut missions and visiting crews typically spend one to two weeks docked, working a compressed schedule of experiments and outreach before undocking.

The duration drives the entire onboard health regime. Bone density and muscle mass decline without the constant load of gravity, fluids shift towards the head and vision can be affected, so crews spend around two hours a day on resistance and cardiovascular exercise. Rehabilitation continues for weeks after landing.