THE Nasa astronauts stranded aboard the International Space Station (ISS) could have "the bones of an 80-year-old when they land", an expert has told The Sun.
Butch Wilmore, 61, and Suni Williams, 58, flew to the ISS for an eight-day mission but will be unlikely to return to Earth before Christmas, Nasa announced this week.
The pair travelled to the orbital outpost on Boeing's Starliner as part of the capsule's first manned flight.
After a string of issues and delays prior to launch, the Starliner capsule has been deemed unfit for Wilmore and Williams' return.
Their short mission has now been unexpectedly extended to up to eight months, with a potential return date in 2025.
Astronauts lose between 1% and 2% of bone density for every month spent in space, according to , which can have devastating knock-on effects.
This is due to the lack of gravity taking the pressure off their legs when it comes to standing and walking.
Kyle Zagrodzky, founder of OsteoStrong, a company that treats osteoporosis, said that Wilmore and Williams' ages of 61 and 58, respectively, will influence them "poorly".
He explained: “Even if they're exercising, they're not getting sufficient force on their bones.
"They may have, in eight months in outer space, 10 to 20 years of bone loss even with exercise."
He claimed: "They could have the bones of a 75 to 80-year-old person when they land and that could be very detrimental to them if they end up having a spine or hip fracture.
“Those are very costly both to your health and longevity."
Expeditions to the ISS typically last around six months, but sometimes astronauts will be sent there for shorter missions.
Astronauts who go on the longest missions, between four and seven months, are the slowest to recover, according to Calgary University's research, co-authored by Dr Steven Boyd.
Boyd said: “The longer you spend in space, the more bone you lose."
It's why most astronauts only go to space three maybe four times (if at all) in their entire career, and why they are often carried from the capsule they landed back to Earth in.
During a briefing on Wednesday, Nasa officials said no return flight had been confirmed, but that Starliner would be their emergency plan B.
The life-altering effects of space don't stop at bone loss, Zagrodzky, 55, warned, as fractures and breakages can lead to other ailments.
He revealed: "Muscle and bone loss is really the beginning of the end for most people.
"People don't realise how important [bone density] is."
“One of the challenges with bone loss is it's linked to dementia and type 2 diabetes as well," he added.
“As your bones and muscles down-regulate to accommodate the weakening strength of your bones, your insulin receptors in your muscles are also down-regulating, so their ability to process glucose through their body diminishes as well."
Recovering from bone density loss is possible, but it's difficult.
As part of Boyd's research, published in 2022, scientists found that the shinbone density of nine astronauts had not fully recovered after a year on Earth.
Still, after 12 months, the astronauts Boyd and his team were analysing lacked about a decade’s worth of bone mass after some four to seven months in space.
Zagrodzky added: "If somebody does [get a bone fracture], 75% of people are never back to themselves again.
"The lack of mobility as we age accelerates aging.
"That's why it's not the broken hip that kills you, it's the lack of mobility and movement that does - especially when people are over 60."
The Sun has contacted Nasa for comment.
Boeing declined to comment when approached for a statement.
Space takes a significant toll on the human body, such as:
Although ISS astronauts spend an average of two hours a day exercising, muscle loss is unavoidable in space.
It takes several years to recover from a six-month spaceflight.
There can be ongoing health concerns long after astronauts have returned to Earth, including:
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