So depending on our position and speed, time can appear to move faster or slower to us relative to others in a different part of space-time. And for astronauts on the International Space Station, that means they get to age just a tiny bit slower than people on Earth. That's because of time-dilation effects.

The phenomenon of astronauts returning "younger" than their counterparts on Earth is a real effect of physics known as Time Dilation, a core concept of Albert Einstein's Theory of Relativity. There are two types at play: Velocity Time Dilation and Gravitational Time Dilation. In orbit, the International Space Station (ISS) travels at roughly 17,500 mph. According to Special Relativity, as an object moves faster, time for that object slows down relative to a stationary observer. Simultaneously, General Relativity states that time moves faster when you are further away from a massive gravitational body like Earth. In the case of astronauts on the ISS, the "speed" effect is stronger than the "gravity" effect. After spending six months in space, an astronaut has actually aged about 0.007 seconds less than people on Earth. While this difference is imperceptible to the human body and doesn't provide a "fountain of youth" in the biological sense, it is a significant factor for the precision atomic clocks used in GPS satellites, which must be constantly adjusted to account for these relativistic time shifts to remain accurate.