Why astronauts age slower in space: Here’s how much younger stranded explorers will be when they return to Earth

It’s an entirely different kind of space age.

Astronauts Butch Wilmore and Suni Williams aredue to their faulty Boeing craft — but there’s one upside to their extended adventure in orbit.

The pair will return a smidge younger than they would have been if they had stayed on Earth.

That’s because the European Space Agency says that six months on the International Space Station (ISS) saves 0.005 seconds of aging.

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But while the pair may only shave a fraction of a second off their Earth age, the concept is still incredibly intriguing.

As Albert Einstein’s theory of relativity explains, the closer a person is moving towards the speed of light (almost 300 million meters per second), the quicker time passes and the slower age does.

The ISS orbits around Earth at a speed of 27,500 kilometers per hour, according to the BBC.

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There is also gravitational time dilation, which ties into Einsten’s theory. It causes slower aging the further away a person is from the Earth’s core due to weaker gravity and the planet’s curvature.

The subtle changes in time due to gravity’s denser passage on the Earth’s surface are a major reason why experts call for the moon to be given its own time zone — as even the planet’s best atomic clocks will be off fractions of a second in time.

Researchers at Texas A&M found this minor lag even affects people at the top of buildings and skyscrapers.

Regarding human age change, NASA previously studied the impacts of space by researching biological changes in twin astronauts Mark and Scott Kelly.

While they both spent time on the ISS, Scott had spent about ten times longer in space as opposed to his brother Mark, who was born six minutes ahead of Scott.

The study found Mark to be an additional five milliseconds older than his brother due to the extended time Scott had spent in space at higher rates of orbital speed among other factors.

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What does it mean for the human body back home?

While in an astrophysical sense, astronauts may shave off a few milliseconds of age, space can take years, if not a decade, away from their bone health upon return to Earth.

“Long periods in space damage bone structure irreparably in some cases and can make parts of the human skeleton age prematurely by up to 10 years,” according to a 2022 paper from Gemrany’s University of Erlangen-Nuremberg.

In a study of 17 astronauts, nine of them had not completely recovered a year after their mission and had shown “a reduction in bone strength” and up to a 2% decrease in bone mineral density.

“This may not sound like much, but it corresponds to age-related bone loss of at least a decade,” said researcher Anna-Maria Liphardt.

“For those affected, this means they will have to expect a much earlier onset of osteoporosis and susceptibility for fractures.”

Additionally, extended stays in space put astronauts at extended risks of absorbing doses of radiation, putting them at risk for developing cancer, warns the Japanese Aerospace Exploration Agency.