Orbit Time: Astronauts Age Slightly Less Than Earthbound Counterparts
Astronauts in low-Earth orbit experience a tiny amount of time travel into the future due to special and general relativity: their clocks run slower because they move fast, while being higher up slightly speeds up time; the net effect is about 20–25 microseconds per day, amounting to a few milliseconds over six months and about nine milliseconds after a year; the phenomenon is real and measurable (as shown by Hafele–Keating experiments and GPS timing), but it is far too small to impact health—spaceflight’s health effects dominate.
