In space, astronauts are expected to follow a strict exercise routine that includes running on a treadmill, cycling on a bike and lifting weights with a simulation machine in a weightless environment.
But how do you do that if you are an amputee?
The main concern is not so much about Mr McFall, 42, a former Paralympic athlete and surgeon. The questions currently under study are focused more on the hardware he would need to take with him.
The prosthesis he currently wears may not be necessary for “day-to-day stuff” like “floating around the International Space Station and doing work”, he told The National in a phone interview.
But it will be needed for exercising, an important aspect of astronaut life to minimise muscle weakness and balance-control problems.
“I’d have to wear a prosthesis to do that [exercise] and maintain the bone density on my amputated side,” Mr McFall said.
It will also be needed in emergency situations, such as running down the crew access arm should he need to rapidly exit the capsule at launch.
“I couldn’t do it without a prosthesis,” he said.
Meanwhile, the prosthesis itself has to be certified and meet all the requirements requested by Nasa and other international partners.
One aspect of the feasibility study, which is expected to conclude in late 2024, is to systematically go through each requirement.
So far, it has been a smooth ride overall for Mr McFall.
“We haven’t identified any reason why I couldn’t fly,” he said.
Throughout his career, Mr McFall, a British father of three, has embraced numerous challenges.
After his right leg was amputated following a motorbike accident in Thailand at age 18, he became an athlete and won a bronze medal at the 2008 Beijing Paralympic Games.
He then switched to medicine and practised as a surgeon until he was hired by ESA last year as a reserve astronaut.
Dreams of joining the army
In an ESA video of his first experience of weightlessness earlier this year, a clearly confident Mr McFall can be seen floating from one side of the plane to the other and quickly mastering the space around him.
“I’m really pleased I felt so comfortable out there,” he said after the flight.
By showing “that we are moving towards getting people with physical disabilities to train for and work in space”, Mr McFall said that he wanted to “inspire many, many people” and “show them that [space] is, hopefully, for everyone”.
Yet, as a young boy, Mr McFall did not dream about flying to space.
Instead, he wanted to join the army.
But there are no legal requirements for the British army to open up its recruitment to people with disabilities.
Access is barred even for surgeons like Mr McFall, who specialises in trauma and orthopaedics. After losing a leg, he knew that his childhood dream had become impossible.
For him, becoming an astronaut is the next best option.
“Being an astronaut ticked a lot of the boxes that appealed to me when I was a teenager, about the challenges of joining the army,” said Mr McFall, pointing at the sense of adventure as well as physical and emotional challenges.
“It also made me quite excited to include the academic side in it – the engineering and all the science.”
ESA made the groundbreaking decision to open up its recruitment pool to people with disabilities to become more inclusive.
“It’s really important for us to involve everybody that has an excitement about space,” ESA’s former director of human and robotic space exploration, David Parker, told the BBC last year.
The study has caused excitement in the community of space watchers.
"It’s an excellent idea to open our European astronaut corps to people with disabilities," said MEP Christophe Grudler.
"Europe is being forward thinking and showing that space can be open to everyone," said Mr Grudler, who was the the European Parliament's rapporteur this year for the EU space-based global secure communications system's regulation proposal.
"I'd be happy to see astronauts with disabilities working on scientific experiments in the International Space Station," Mr Grudler, a French politician, told The National.
"So far, we only select the most physically and intellectually fit astronauts. But that can change."
"Why not apply?"
As an athlete who knows how to work under pressure, Mr McFall met all the criteria laid out by ESA to become its first disabled astronaut.
“They wanted operational experience, like being in the military or [being] a doctor working in environments under stress, and when you’re a surgeon, you operate sometimes under stressful environments,” he said.
“Those sorts of things were advantageous, but you also had to be fit enough to be certified for a private pilot’s licence.”
Mr McFall describes himself as a curious person, which is why he went to university to study sports and exercise science after his motorbike accident.
While at university, he taught himself to run again and became a full-time athlete.
“It was an opportunity for me to really learn what I could do if I applied myself,” he said in an interview recorded by ESA in September.
Eight years after the amputation, he competed in the Beijing Paralympic Games in what he described as a “real affirmation” of his ability to overcome the trauma of losing a leg.
But he feared that he would not make a living out of being an athlete. In 2009, when he was 28, he returned to university in Cardiff to study medicine and has worked as a surgeon for the past 10 years.
When one of his colleagues sent him a message telling him that ESA was “looking for Paralympians to go to space,” his curiosity was piqued.
He read the job description and realised he met all the requirements.
Speaking to ESA, he recalled thinking “this seems really interesting” and “why not apply and see where it takes me”?
So far, a highlight of the study was his first parabolic flight.
“I learnt some very subtle things,” he said in the September interview.
“Gyroscopes in prosthetics don’t work in ZeroG [gravity] … so what is the best prosthetic solution in ZeroG?
"That is exactly the sort of stuff we’re looking at in the study.”
Mr McFall is currently shadowing ESA’s class of astronaut candidates.
He has gone through exercise training, including sea survival training, and also attends theory classes that help him better understand the physiological effects of micro-gravity on his body.
Should the study be successful, there is a real possibility that Mr McFall, or someone with an equivalent disability, will one day be sent into space.
“We’re looking after 2025 or into the later half of this decade,” he told The National.
Once it is over, he does not want the experience to have just been a “box-ticking” exercise, he said in September.
“If we are truly going to honour this and get the most out of it, then we need to explore other disabilities," he said.
"We've done a lower limb disability – what about the other disabilities out there?"