How long astronauts stay in space?

The station runs on expeditions of roughly half a year, a compromise rather than a physiological limit. Six months is long enough to justify the cost of a seat and to run experiments that need months of continuous exposure, and short enough to keep radiation dose, bone loss and crew fatigue inside margins flight surgeons are comfortable with. Rotations also have to fit the launch cadence of the vehicles that carry crews up and down, so an increment can stretch or contract by weeks for reasons that have nothing to do with the human body.

The Mir programme pushed much further. Valery Polyakov spent 437 days in orbit between 1994 and 1995 and walked away from the landing capsule under his own power, deliberately, to demonstrate that a Mars-length transit was survivable. No single flight has exceeded it since. One NASA astronaut passed a year aboard the ISS in 2022-23 after the spacecraft meant to bring him home was disabled by a coolant leak, and the highest career totals, all Russian, now run to years of accumulated time off the planet.

Three constraints bind. Radiation dose accumulates and cannot be undone. Bone and muscle waste steadily, a process that exercise and diet slow without stopping. And fluid shifts towards the head produce changes in the eye that remain the least understood of the three, and the most worrying for any mission that cannot turn back. On top of the medicine sit hard engineering limits: consumables, and the certified on-orbit lifetime of the docked spacecraft that serves as the crew's ride home.