Why do astronauts age faster?

Special relativity predicts that a moving clock ticks more slowly: launched at orbital velocity, the ISS therefore accumulates a time lag. General relativity predicts the opposite for altitude: the farther one moves from Earth's gravity well, the faster time passes, and at the station's altitude this gain genuinely exists. The two effects do not cancel each other out. In low orbit, speed wins out by a wide margin, and the net result is a lag. This is the reverse of GPS navigation satellites, placed in much higher orbits: there, the gravitational effect dominates and their clocks run fast, which requires constantly correcting their frequency so that positioning stays accurate.

The magnitude remains tiny. Cosmonaut Sergei Krikalev, who has logged 803 days in space, is often cited as an example of this "journey into the future": only a minuscule offset. No perceptible effect, but a genuine experimental verification of a century-old theory.

The confusion comes from elsewhere: spaceflight biologically mimics aging. In the absence of mechanical loading, bones demineralize at a rate of 1 to 1.5% of density per month in weight-bearing segments, postural muscles waste away, the cardiovascular system deconditions, and cosmic radiation damages DNA. NASA's study of twins Scott and Mark Kelly revealed alterations in gene expression, telomere length and the immune system during the year spent in orbit, most of which returned to normal after landing.

These changes are largely reversible and countered by two hours of daily exercise. This is precisely what interests medicine: the ISS offers an accelerated and reversible model of senescence, useful for understanding osteoporosis or sarcopenia on the ground.