Experienced astronauts Sunita “Suni” Williams and Commander Barry “Butch” Wilmore arrived at the International Space Station (ISS) in June 2024 thinking it was a routine eight-day stay.

Little did they know they would end up stranded in space for nine months, as the aircraft that brought them there – Boeing’s new Starliner – suffered several technical issues that made it too risky to make the return flight.

They are finally set to head home in the coming days, hitching a ride on a SpaceX aircraft that will land them in Florida.

But why have they been away for so long and what are the effects of such a lengthy stay in space?

How did they get stuck?

Ms Williams and Mr Wilmore blasted off on 5 June in Starliner’s first crewed mission.

The whole point of the mission was to test the spacecraft, which was tipped to be a critical alternative to Space X’s Crew Dragon spacecraft, which is currently the only shuttle for US, European, Canadian and Japanese astronauts to and from the ISS.

They arrived safely, but the space capsule had five helium leaks, five dead manoeuvring thrusters and a propellant valve that failed to close completely.

In the following weeks, Boeing and NASA conducted a series of tests both in space and on the ground, using a Starliner replica, to try and fix the problems the aircraft faced.

Even as late as August, Boeing insisted it was confident the Starliner, which was developed on a reported $5bn NASA contract, could safely bring back the astronauts.

But both NASA and Boeing eventually deemed it too risky, so the Starliner returned to Earth unmanned last September, leaving Ms Williams and Mr Wilmore behind at the ISS.

They are not alone, as NASA astronaut Nick Hague and Russian cosmonaut Alexander Gorbunov joined them at the ISS in September.

How are they getting back – and why has it taken so long?

Mr Hague and Mr Gorbunov arrived in a SpaceX Dragon capsule with four seats, two for them and two extras to accommodate Ms Williams and Mr Wilmore on the journey home.

They are set to depart the ISS on an SpaceX craft and return to Earth, splashing down off Florida’s coast, where a recovery vessel will pick up the crew before they are helicoptered back to shore.

But they did not come just to grab the stranded pair and bring them home.

Their trip was planned before Ms Williams and Mr Wilmore got stuck, as part of NASA’s SpaceX Crew 9 mission, aimed at carrying out science experiments and tech demonstrations aboard the ISS.

Crew 9, originally made up of four astronauts, was cut in half to make room for Ms Williams and Mr Wilmore on the return flight. But instead of bringing them home straight away, the pair agreed to join Crew 9’s six-month mission, taking them to February.

Then, in December, their scheduled February return was pushed back once more because of problems with the SpaceX rocket that was going to bring them back.

Now the four aboard the ISS are waiting for a new crew of four – NASA’s SpaceX Crew-10 – to come and take their places before leaving in a different SpaceX craft.

Crew 10 are now on their way and, if all goes to plan, there will be a brief handover period between the crews before Ms Williams, Mr Wilmore, Mr Hague and Mr Gorbunov head home.

Trump and Musk claim they have been ‘left’ there

Ms Williams and Mr Wilmore have missed an awful lot on Earth during their time away – including Donald Trump’s historic election win over Kamala Harris.

It means the pair left Joe Biden’s America and return amid Mr Trump’s second term.

That has prompted the president to claim his predecessor “left” the astronauts stranded in space, while his chief enforcer Elon Musk – who owns SpaceX – similarly said they were “left up there for political reasons”.

As stated previously, a rescue mission has never been sent for Ms Williams and Mr Wilmore; instead, they have joined another mission.

But Mr Musk has been challenged on this assertion, particularly by famed astronaut Andreas Mogensen, who accused him of lying.

Responding to the claims on social media, Mr Mogensen said to Mr Musk: “You know as well as I do, that Butch and Suni are returning with Crew-9, as has been the plan since last September.

“Even now, you are not sending up a rescue ship to bring them home. They are returning on the Dragon capsule that has been on ISS since last September.”

Mr Musk doubled down, writing: “SpaceX could have brought them back several months ago.

“I OFFERED THIS DIRECTLY to the Biden administration and they refused. Return WAS pushed back for political reasons. Idiot.”

In a news conference from space, Mr Wilmore, 62, responded to the comments by saying that politics did not play a part in his and Ms Williams’ return date.

What are the physical effects of space travel?

Human beings have been through millions of years of evolution to become perfectly adapted to life on Earth, so spending time in space – with no gravity, no sunlight and different levels of radiation – poses a real challenge physically.

These are the main hazards astronauts face in space:

Space radiation – Astronauts are exposed to high-energy radiation in space, unlike on Earth, where the atmosphere and planetary magnetic field provide a shield against it.

This can lead to DNA damage, increased cancer risk, neurodegenerative effects, cardiovascular issues and immune system dysregulation, experts say.

The good news for those aboard the ISS is Earth’s magnetosphere – the region of space dominated by the planetary magnetic field – provides some protection, as they are fairly nearby, being 250 miles away.

It’s when astronauts travel beyond that – such as on missions to the moon, which is roughly 238,855 miles away – where astronauts experience much higher radiation doses.

Effects of microgravity – Floating around the space station may look like a lot of fun, but gravity plays a critical role in regulating bodily functions, and its absence has negative impacts on human health, according to experts.

With their bodies not being engaged by the same forces they are used to on Earth, this can have two particularly concerning effects on astronauts.

Firstly, they are known to lose bone density, meaning bones get weaker and more brittle.

For every month in space, astronauts’ weight-bearing bones become roughly 1% less dense if precautions aren’t taken, according to NASA.

They also experience muscle atrophy, the wasting or thinning of muscle tissue, during their time in space.

Both bone density and muscle atrophy tend to occur on Earth from normal aging, an inactive lifestyle and illnesses, and they can cause serious injuries due to falls, osteoporosis, or lead to other medical problems.

The lack of gravity also makes bodily fluids shift upward, leading to facial swelling and increased intracranial pressure, which can affect vision.

Headaches are also widely reported for people in space. A study published last year found 22 out of 24 surveyed astronauts who travelled in space for up to 26 weeks experienced them frequently while away from Earth.

What are the psychological effects?

Living in confined and isolated spaces, with limited social interaction, is known to impact peoples’ mental health, and these are the conditions astronauts accept when they leave Earth.

It can lead astronauts to experience stress, sleep disturbance, cognitive performance declines and mood disorders, according to Afshin Beheshti, director of the Centre for Space Biomedicine at the University of Pittsburgh.

Being away from friends and family also inevitably impacts astronauts, especially given how far away they are from home.

Mr Wilmore and Ms Williams have regularly talked about missing their loved ones during news conferences on the ISS, saying they can’t wait to get back to them.

Are there long-term effects of space travel?

Data is hard to come by, because only around 700 people have ever been to space.

But studies so far suggest most astronauts who spend a while in space recover from most of its biological effects in proportion to how long they spend away.

If they were in space for five months, for example, it would take roughly the same amount of time to fully recover from most of space’s effects.

Some issues can persist, however.

Research published in 2022 documented bone loss in 17 ISS astronauts in missions averaging about five-and-a-half months.

It showed that a year after returning, the astronauts on average exhibited 2.1% reduced bone density of the tibia – one of the bones of the lower leg – and 1.3% reduced bone strength.

Some astronauts can have lasting impairment to their eyesight due to microgravity-induced fluid shifts and changes in intracranial pressure affecting the eyes, research suggests.

It’s often due to a condition known as Spaceflight-Associated Neuro-Ocular Syndrome (SANS).

For those who only partake in short-term missions lasting a few days in low-Earth orbit, about 95% of all biological damage sustained appears to be reversed upon return.

NASA astronaut Frank Rubio, who came back from a 371-day space trip in 2023, shed some light on how astronauts reintegrate themselves after a long time away.

Similarly to Ms Williams and Mr Wilmore, Mr Rubio’s six-month mission ended up taking over a year after the capsule that was supposed to bring him home was damaged.

Nearly four months after getting back, the American told Time Magazine: “The first two or three months after your return is really focused on [recovery], just kind of reincorporating yourself into Earth, your family, and then also rehabilitating your body.

“You adapt incredibly quickly to being in space, but then unfortunately, the readaptation process back to earth can sometimes be a little bit longer and more difficult.

“And that’s just, I think, because the forces of gravity and the forces at play here on Earth tend to have a stronger effect on your body.”

What is being done about the negative effects?

Researchers say more data is needed to fully understand the risks of space travel, and hope to be able to eventually map out personalised risk profiles and mitigation strategies for astronauts, considering their health backgrounds and the type of missions they are undertaking.

They say there are gaps in our knowledge of how spaceflight affects people long-term, and its impact – particularly when it comes to serious implications like lung function, cancer risk and cellular damage, is an area of active study.

Scientists also say they lack a comprehensive understanding of how all the aforementioned challenges space poses impact cognitive function, mental health and neuroplasticity – the brain’s ability to change and adapt – over long durations.

But NASA is conducting experiments and research centred around its astronauts all the time, much of it focusing on how to improve the space experience and prevent as many side effects as possible.

For example, it’s developing new radiation detectors to monitor how exposed crews are, and advanced materials to improve radiation shielding for future missions.

There is also unique exercise equipment on the ISS to help counter the effects of microgravity, and a lot of research focuses on determining the right combination of diet, exercise, and medication to keep astronauts healthy during missions and when they return to Earth.

NASA also studies people in isolated and confined environments to investigate how they can best protect astronauts from mental health challenges, and it regularly assesses staff.

All experiments and research regarding astronauts are aimed at countering potential health issues for current ISS staff, but they are also largely looking ahead to more taxing missions to the moon and eventually Mars, where all of space’s risks will be amplified.

How have the stranded astronauts fared?

Mr Wilmore and Ms Williams may have only expected a brief ISS visit, but in news conferences they have been adamant that they were fully prepared for a longer stay if the mission called for it.

The pair are space veterans, having both been up for two other flights, and have not reported any health concerns.

In November, NASA shared a photo of Ms Williams which sparked some concern over her health, with some members of the public suggesting she looked like she had lost a significant amount of weight.

The 59-year-old responded to the comments during a video interview with the New England Sports Network Clubhouse Kids Show, saying her supposed weight loss was just “rumours”.

She added any changes in her appearance would be down to “fluid shifts” caused by the lack of gravity.

She said she was healthy and that she had weighed herself using NASA’s spring-operated mass measuring device, and weighed the same as when she left Earth.

She added she had gained muscle due to all the exercises she was doing to keep in shape.

“My thighs are a little bit bigger, my butt is a little bit bigger,” she said. “We do a lot of squats.”

The pair have said throughout their stay they are feeling good mentally, insisting they “do not feel like castaways” and do not feel abandoned.

“Eventually we wanna go home,” Ms Williams said in January. “We left our families a little while ago.

“But we have a lot to do up here and we have to get that stuff done before we go.”

She then called the space station her “happy place”.

What have they been doing up there?

First and foremost, they have been living the typical space dweller life, aboard the 356ft-long ISS.

The space station has its own oxygen-generating systems, and about 50% of oxygen exhaled from carbon dioxide is recovered.

As for water, the station has a urine-into-drinking-water recycling system, and a part of that system also captures moisture released into the cabin air from the crew’s breath and sweat.

There’s always plenty of food reserves on board, but supplies have been sent up several times since they docked last year.

You can read more about their day-to-day lives on the ISS – including details like how they sleep and go to the toilet with no gravity – by reading this piece from last year.

But the pair have not spent all their time gazing out at Earth – they have been hard at work, like all astronauts on the space station.

They, along with the rest of Crew 9, have completed more than 900 hours of research between more than 150 “unique scientific experiments and technology demonstrations” during their stay, according to NASA.

Some of the crew’s most notable work has been done off the ISS, with Ms Williams and Mr Wilmore stepping out for a spacewalk together in January to help fix the NICER telescope, which studies neutron stars and other cosmic phenomena.

It was the first spacewalk by NASA astronauts since an aborted one last summer when water leaked into an airlock from the cooling loop in an astronaut’s suit.

Here is a summary of some of the other work the crew has undertaken:

• Helped grow arthrospiramicro-algae on the ISS – organisms that could be used to convert carbon dioxide exhaled by crew members into oxygen
• Used a new exercise device to track how it can help astronauts counter bone and muscle loss and improve cardiovascular health while in space
• Studying the spread of flames in microgravity could help improve safety on future missions
• Part of their mission actually has involved Earth-gazing, as they have added to the collection of millions of photos taken by astronauts of our planet, to help keep a record of how it changes over time.

What we know about the astronauts

Mr Wilmore is a veteran of two previous spaceflights and a former captain in the US Navy.

His first mission to space was in 2009, where he piloted a maintenance flight to the ISS, spending a total of 11 days in space.

Then, from September to November 2014, he served as flight engineer aboard the ISS for Expedition 41 and then as commander of Expedition 42 from November 2014 to March 2015, totalling 167 days in space.

When Mr Wilmore is on this planet, he resides in Tennessee with his wife Deanna and their two daughters, Daryn and Logan.

Ms Williams was made an astronaut by NASA in 1998, having served in the US Navy as a pilot.

Before heading to space, she lived underwater for nine days as part of NASA’s NEEMO mission, which sends groups of astronauts, engineers and scientists to live in Aquarius, the world’s only undersea research station.

Like her Boeing test partner Mr Wilmore, she has had two other ISS missions; one being from December 2006 to June 2007 as a flight engineer, and another from July to November 2012, spending 127 days in space.

NASA says she spends a lot of her time on Earth hiking and camping, and she’s got similar habits in space, too, having completed an impressive eight spacewalks during all her time at the ISS.

She and her husband Michael, who live in Massachusetts, also enjoy hanging out with their dogs, working out, and working on houses, cars and airplanes, according to NASA.

What is the longest anyone has spent in space?

Russian cosmonaut Valeri Polyakov set that record in the mid-1990s, spending 437 days off Earth.

He orbited the Earth more than 7,000 times before returning.

Polyakov had trained as a physician and wanted to demonstrate that the human body could endure extended periods in space.

When landing, Polyakov declined to be carried out of the Soyuz capsule, as is common practice to allow readjustment to the pull of gravity.

Instead, he was helped out of the capsule, and he walked to a nearby transport vehicle himself.

He died in 2022 aged 80. His cause of death was not disclosed.