How long do astronauts stay on the ISS?

Since the first permanent crew arrived in November 2000, the Station has never been unoccupied. A typical astronaut's stay follows the lifespan of the vehicle that brought them and the pace of relief flights: about six months, sometimes a bit longer when the launch schedule slips. Crews are numbered through successive Expeditions, and a handover never happens in a vacuum: incoming crew members spend a few days with those departing to transfer responsibilities, temporarily bringing the station's population to ten or eleven people.

This duration is not arbitrary. It reflects a compromise between the cost of a crewed flight, the physiological toll on the crew, and the on-orbit certification of the Soyuz and Crew Dragon vehicles, which remain docked throughout the stay and serve as a lifeboat.

Shorter missions exist: visiting flights, particularly those of commercial partners or guest agencies, are sometimes limited to about ten days. At the other extreme, a few stays have stretched well beyond the standard. Valeri Polyakov holds the absolute record for a single flight with 437 consecutive days, but aboard the Mir station, between January 1994 and March 1995. On the ISS, Frank Rubio spent 371 days in orbit between September 2022 and September 2023, an American record caused by the replacement of a Soyuz damaged by a coolant leak.

This duration also serves as a laboratory. Loss of bone and muscle mass, fluid redistribution toward the head, changes in vision: six months is the window on which daily exercise protocols, about two hours a day, have been calibrated. Long stays are used to extrapolate toward a future voyage to Mars.