FactSentinel

Astronauts aboard the International Space Station do age slightly slower than people on Earth, a direct consequence of Einstein's theory of relativity. This effect combines two forms of time dilation: velocity-based time dilation from special relativity and gravitational time dilation from general relativity. The ISS travels at roughly 17,500 miles per hour, and moving at high speed causes time to pass more slowly relative to a stationary observer. This effect dominates over the gravitational component in low Earth orbit, since astronauts are still close enough to Earth that reduced gravity plays a smaller role.

The net result is that astronauts age a tiny fraction of a second less than people on Earth for every six months spent in orbit. Scott Kelly, who spent nearly a year aboard the ISS, aged about 8.6 milliseconds less than his twin brother Mark, who remained on Earth. This is not speculation but a measurable, well-documented phenomenon confirmed through precise atomic clock experiments and GPS satellite calibrations, which must account for relativistic effects to function accurately.

A common misconception is that this effect is dramatic or noticeable in daily life. In reality, the time difference is minuscule, measured in milliseconds even after extended missions, and has no perceptible impact on an astronaut's biological aging or health. Another misunderstanding conflates this relativistic effect with the physiological effects of spaceflight, such as bone density loss or muscle atrophy, which are unrelated biological consequences of microgravity rather than time dilation. The scientific consensus on relativistic time dilation itself is robust and experimentally verified beyond reasonable doubt.