- Time can appear to move faster or slower to us relative to others in a different part of space-time.
- That means astronauts on the International Space Station age slower than people on Earth.
- The effect is minuscule, though, amounting to only fractions of a second.
Astronauts Butch Wilmore and Suni Williams are stuck in space on the International Space Station until February, but it's not the worst that could happen.
There are a lot of perks to spending time in space from the epic views to the company. The one that you rarely hear about, though, is the aging effect.
Astronauts age more slowly than people on Earth. The difference isn't noticeable though — after spending six months on the ISS, astronauts age about 0.005 seconds less than the rest of us.
So after spending about 9 extra months in space, Williams and Wilmore will be about 0.0075 seconds younger than if they'd returned to Earth in June as scheduled.
There are a couple of scientific reasons for why this happens, and it all starts with Albert Einstein's theory of relativity.
At first, it seems like astronauts should age faster, not slower
We've all heard the phrase that "time is relative," but it can be difficult to wrap our minds around what that actually means.
The phrase came from Einstein's theory of relativity that joined space and time and created the idea of a fabric that permeates the whole universe: "space-time."
We all measure our experience in space-time differently. That's because space-time isn't flat — it's curved, and it can be warped by matter and energy.
This effect leads to something called gravitational time dilation. Time appears to move slower near massive objects because the object's gravitational force bends space-time.
Technically, that means a watch strapped to your ankle will eventually fall behind one strapped to your wrist because your feet are nearer to Earth's center of gravity than your hands. But the effect is so minuscule that you'd never notice.
Astronauts aboard the ISS, on the other hand, are much farther from Earth's center of gravity than the rest of us — about 260 miles farther. So, in that case, astronauts actually age slower.
But time is weird, and there's another phenomenon called relative velocity time dilation that usurps gravity's effect.
Why astronauts age slower
Relative velocity time dilation is where time moves slower as you move faster.
The classic example of this is the twin scenario. One twin blasts off in a spaceship traveling close to the speed of light, and one twin stays behind on Earth. When the space-traveling twin returns to Earth, she's only aged a couple of years, but she's shocked to find that her Earth-bound sister has aged over a decade.
Of course, no one has performed that experiment in real life, but there's evidence that it's real. When scientists launched an atomic clock into orbit and back — while keeping an identical clock here on Earth — it returned running ever so slightly behind the Earth-bound clock. It's the same effect for astronauts on the ISS.
The space station is whizzing around Earth at about five miles per second (18,000 mph), according to NASA. That means time moves slower for the astronauts relative to people on the surface.
Now, it's not as slow as it could be because gravitational time-dilation is actually making them age a smidge faster compared to the rest of us on Earth. But their velocity time dilation has a bigger effect than their gravitational time dilation, so in the end, astronauts age slightly slower while in space.
Astronaut's telomeres also grow longer while in space
Unrelated to the physics of space-time is another peculiar phenomenon astronauts experience that suggests an anti-aging effect.
A couple of preliminary studies in recent years have found that astronauts' telomeres grow longer while in space. Telomeres are caps at the ends of your chromosomes that protect against damage. As you age, your telomeres shorten.
But when astronaut Scott Kelly spent a year in space, he came back with telomeres that were longer than when he left. The same was found true for four commercial astronauts who were part of the 2021 SpaceX Inspiration4 spaceflight. Interestingly, they only spent three days in space.
It's unclear if longer telomeres are linked to a longer life, though. Research is ongoing in that field of work.
Moreover, aging fractions of a second slower due to the quarks of space-time isn't going to make a difference either. That's not why we go to space.
We go to space because, "space exploration unites the world to inspire the next generation, make ground-breaking discoveries, and create new opportunities," according to NASA.
So the next time you find yourself wishing the weekend would last longer, stay low to the ground and move really fast. It won't feel like your weekend got any longer, but technically you may gain a teeny, tiny fraction of a fraction of a second. Remember, time is relative.
An earlier version of this story was published on January 1, 2023, and it was most recently updated on September 18, 2024.