Is it true that astronauts don't age in space?

Relativity predicts two opposite effects in orbit: speed makes the spacecraft's clock run slow compared to the ground, while the weaker gravity at 400 km makes it run fast. In low Earth orbit the first effect dominates, so an astronaut comes back a tiny bit younger than if they had stayed home. The difference, after nearly a year on the station, is measured in milliseconds: irrelevant for biology, decisive for satellite navigation.

Without weight, the skeleton stops receiving the load that keeps it strong and loses between 1% and 1.5% of mineral density per month in weight-bearing areas such as the hip and spine. Postural muscles atrophy, the heart pumps a smaller volume, and fluids shift toward the head, which in some crew members deforms the eyeball and alters vision for months.

This is countered with about two hours of daily exercise on resistance machines, a harnessed treadmill and a bike. Even so, recovery on the ground takes weeks or months, and some bone mass may never fully return. On top of that comes a radiation dose far higher than on the surface, a long-term risk factor.

The comparative study of Scott and Mark Kelly, identical twin astronauts, followed one for nearly a year in orbit while the other stayed on the ground. Changes appeared in the expression of thousands of genes, in the immune system, in the gut flora and in telomere length, which lengthened in flight and shortened upon return. Most of it reversed in the following months, but overall it describes an organism under stress, not one that rejuvenated.