How much younger are you after a year in space?
Time passes at a different rate depending on your velocity and altitude. This is not science fiction — it is what GPS satellites have to correct for every day.
Einstein's theory of relativity predicts two effects that alter the rate at which time passes. Special relativity says that moving clocks run slow — the faster you travel, the more time dilates. General relativity says that clocks in weaker gravitational fields run fast — the higher your altitude, the faster your clock ticks relative to someone on the ground. On the International Space Station, both effects apply simultaneously: the ISS moves at 7,660 m/s (slowing time) but orbits at 408 km altitude (speeding time). The velocity effect is stronger, so ISS crew members age approximately 7 milliseconds less per year than their counterparts on Earth.
After 6 months on the ISS, you would be 5.812 milliseconds younger than your twin on Earth.
Special Relativity Effect
5.151 milliseconds
Moving clocks run slow
General Relativity Effect
661.14 microseconds
Higher clocks run fast
Net Effect
5.812 milliseconds
Velocity dominates
The ISS travels at 7,660 m/s — fast enough that special relativity slows your clock measurably. But the reduced gravity at 408 km altitude means your clock also ticks slightly faster than on the surface. The velocity effect wins: ISS crew members return to Earth fractionally younger than they would have been. Over a six-month mission this amounts to approximately 3.5 milliseconds. NASA and ESA account for this in long-duration mission health planning.