NASA astronauts Barry 'Butch' Wilmore and Sunita 'Suni' Williams launched into space from Florida in June. The mission was supposed to last eight days.
The Americans were testing Boeing's new Starliner on its maiden crewed voyage, but technical problems with the spacecraft caused it to return to Earth without them.
They've been living inside the International Space Station (ISS) ever since. Now, after nine months in orbit, the astronauts are returning home.
Experts say they face a long and intense recovery due to both the physiological and psychological impacts of space travel.
'Even our very DNA changes'
Nine months may sound like a long time, but some astronauts stay in orbit for up to a year. ISS missions typically last six months.
Still, experts say prolonged time in the low gravity environment of space, known as microgravity, takes a toll on the body.
Dr Rebecca Allen, the co-director of the Space Technology and Industry Institute at Swinburne University of Technology, said: "Our most important muscle, our heart, pumps blood differently in space because fluids don't flow, of course, without gravity there."
"Our immune system function, our gut microbiome, even our very DNA changes," she explained.
Allen said without the constant tug of gravity, muscle mass and bone density start to deplete.
"Our muscles get strong because they're used to holding us up against gravity. When you take that away, they no longer have that resistance to really build up strength, and so they very quickly deteriorate," she said.
"But also the bones that they're attached to, they don't have that continuous pull of the muscles on them, so our bones also start to break down."
Dr Brad Tucker, an astrophysicist and cosmologist at The Australian National University, said studies have shown that six months in space can result in the equivalent of 20 years worth of bone density loss.
"So [Wilmore and Williams] being in their late fifties, early sixties, they're going to have the bones of an 80-year-old now, coming down," he said.
Prolonged exposure to microgravity also affects the vestibular system, located in the inner ear and is crucial for maintaining balance and spatial orientation. That can cause balance and coordination issues, Tucker said.
He said eyesight can also be impacted.
"The ocular nerve, through pressure changes in space, creates a vision distortion. So when [the astronauts] come back down, their vision will be distorted."
Return to Earth
Allen said astronauts who've spent an extended time in space are typically taken away for an immediate medical assessment.
"They're pretty much immediately sat down. And that's because their muscles are not able to cope with gravity and hold them up," she said.
"So immediately they're assessed to see how extreme this has been for them on their body. So things like their respiratory and cardiovascular systems and how their brain activity is functioning are immediately assessed.
"And then in terms of actually recovering and recuperating, they will go into for hours, multiple times a day, rehabilitation to build up that muscle strength and try to get the right nutrients back."
Allen said in space, the body is deprived of the key nutrients it depends on.
Psychological impacts
Tucker said Wilmore and Williams will also need to partake in psychological rehabilitation.
Long-duration space missions require astronauts to live in confined and isolated environments with limited social interaction.
"It is a transition that is hard for some astronauts," he said.
"The whole world has kind of been watching [them] seemingly left in space and stuck there for a prolonged mission. Now they have to wake up, make breakfast and get stuck in traffic going to work.
"And also catching up with the personal life that they have missed for nine months. They were only planning maybe two weeks away, but they've missed birthdays and other scenarios, and the life admin that they now have to catch up on."
Allen also notes that in space, crews experience multiple sunrises and sunsets each day.
"They orbit the Earth every 90 minutes. So that means a sunrise and a sunset every hour and a half," she said.
"So things like sleeping, even though they've solved the fact that there's no gravity and you can tether yourself and be in a sleeping bag, the fact that the sun is coming up and setting so rapidly really affects our circadian rhythm, that fundamental pattern in human beings that helps us sleep."
Mars ambitions
NASA has plans to send humans to Mars as early as 2035. A round-trip to the red planet takes three years, so is such a mission actually possible?
Tucker said extended trips to space, like Wilmore and Williams', are key to understanding that question.
"I think this is the untold part of space travel … how do we make humans survive the trip? How do we solve all of the health issues, the psychological issues, the food supplies? We can't send a resupplied mission to Mars," he said.
"So right now, it is logistically almost impossible to get a human safely to Mars.
"But by studying and doing these experiments on the space station and eventually the moon, that will hopefully give us ways of solving these issues so you can have a successful mission to Mars."
Allen believes work towards such a mission is well underway.
"I would say there are astronauts that have already been preparing for Mars for years and years … that have already been undertaking probably over a decade of training for that mission."
