Why do astronauts stay younger in space?

Astronauts Butch Wilmore and Suni Williams are stuck on the International Space Station (ISS) until February 2025 due to a Boeing spacecraft failure, but it’s not all bad news. In addition to getting to see the majestic Earth from space, they’re aging more slowly than people on the ground.

The difference is subtle. After six months on the ISS, astronauts age about 0.005 seconds slower than people on Earth. So after about nine months in space, Williams and Wilmore will be about 0.0075 seconds younger than if they had returned to Earth in June 2024 as originally planned. There are a number of scientific reasons behind this phenomenon, starting with Albert Einstein’s theory of relativity.

Time dilation due to gravity

According to Einstein’s theory of relativity, space and time are combined into a single fabric that permeates the universe: spacetime. Spacetime is not flat but curved, and can be distorted by matter and energy. This effect leads to gravitational time dilation: Time appears to flow more slowly near a massive object because the object’s gravity warps spacetime.

This means that a watch worn on the ankle will gradually slow down compared to a watch worn on the wrist because the foot is closer to the Earth’s center of gravity. But this effect is so small that humans will not notice it.

Astronauts on the ISS are about 418 km farther from Earth’s center of gravity than the average person. However, astronauts age more slowly due to another phenomenon: time dilation due to relative velocity.

Time dilation due to relative velocity

Relative time dilation is when time slows down as one person moves faster. Imagine a pair of twins, one of whom is in a spaceship traveling at nearly the speed of light, while the other remains on Earth. When he returns, the space traveler has aged only a few years, but his twin on Earth has aged dozens of years.

No one has actually tested this, but there is evidence to support it. Scientists once launched an atomic clock into orbit and brought it back while keeping an identical clock on Earth. The orbiting clock ran slightly slower than its “twin.” Astronauts on the ISS experienced the same effect.

The ISS is orbiting Earth at about 8 kilometers per second (29,000 km/h), according to NASA. This means that time passes more slowly for astronauts than it does for people on the ground. For astronauts, the time dilation caused by velocity has a greater effect than the time dilation caused by gravity. So they age a little slower in space.

Astronauts’ telomeres lengthen in space

Another strange phenomenon astronauts experience may also help fight aging. Some recent preliminary studies have shown that astronauts’ telomeres grow longer while in space. Telomeres are the caps at the ends of chromosomes that help protect against damage. As people age, telomeres shorten.

But after astronaut Scott Kelly spent a year in space, he returned to Earth with longer telomeres than when he left. The same happened to the four commercial astronauts on SpaceX’s Inspiration4 flight in 2021. Interestingly, they only spent three days in space.

However, experts are still unclear about whether longer telomeres are associated with a longer life. They are still researching this issue.

Plus, aging slowed by a few fractions of a second due to space-time factors wouldn’t make much of a difference. That’s not the main reason humans go into space. According to NASA, space exploration unites the world to inspire the next generation, make groundbreaking discoveries, and create new opportunities.