How long does an astronaut stay in space on average?

Permanent crews rotate roughly every six months. This duration is not arbitrary: it roughly matches the certified lifespan of a docked spacecraft, the pace of resupply cargo flights, and the useful length of a science program — long enough to carry out in-depth experiments, short enough for the body and morale to hold up.

Each rotation costs a launch and a return. Extending stays would cut costs but increase the radiation dose received and bone and muscle loss, which daily training limits without eliminating.

The average hides a very wide spread. At one end, test flights of early commercial spacecraft and private missions last days, sometimes just hours for a suborbital flight, which only brushes the edge of space.

At the other end, a few stays have exceeded a year. Frank Rubio spent 371 days in orbit in 2022-2023, not by choice but because a coolant leak had made his Soyuz unfit for return, forcing the launch of another one and delaying the whole chain of crew rotations. The absolute record for a continuous spaceflight, set aboard the Mir station in the 1990s, remains even longer.

A round trip to Mars would last far longer than an ordinary rotation, with no possibility of an emergency return. Long-duration stays therefore serve as a full-scale test bed: bone, vision, immune system, sleep, psychological balance and the reliability of recycling systems are measured on real people, year after year.