Five-Millisecond Time Gap: Relativity in Real Life on the ISS

The ISS travels at about 17,500 mph, causing time to pass slightly slower up there; after 340 days in orbit, Scott Kelly was about 5 milliseconds younger than his Earthbound twin Mark. The effect is real and purely relativistic, with gravity counteracting some aging, but the velocity effect dominates on the station. NASA studied the twin pair to track biological changes—telomeres lengthened, some gene expression stayed altered months after return, and various physiological shifts occurred—illustrating how long-duration spaceflight acts as a tiny, measurable form of time travel for humans.