John McFall is the European Space Agency's first ever para-astronaut, selected to study how feasible it is for someone with a physical disability to live and work in space. BBC News joined him on board a parabolic flight, where he experienced weightlessness for the first time.

One minute John McFall is lying on the floor of a plane. The next, he starts to float upwards, still horizontal, seemingly levitating towards the ceiling.

He looks astonished - everyone on this far-from-normal flight does, as they slowly rise into the air. The sensation of being weightless, no longer pinned down by gravity, is extraordinary.

You feel totally out of control - because you are. Any tiny movement against something solid sends you catapulting around the cabin, bouncing into walls and people. It's like being in a slow-motion pinball machine.

A smile begins to form across John's face - he starts laughing. "It's brilliant, it's amazing," he says.

Then suddenly, the weightlessness is gone and he falls to the ground.

John is an astronaut candidate with a difference - he's an amputee. When he was 19 he lost his leg in a motorcycle accident, and now uses a high-tech prosthesis.

He has now been recruited by the European Space Agency (Esa) to take part in a ground-breaking study assessing how to make spaceflight accessible to people with physical disabilities.

"I saw that the European Space Agency had announced that they were looking for an astronaut with a physical disability," he says, "and I looked at the person specification and thought, 'Wow, that's me - I would love to give that a go'."

John's used to pushing himself to the limit. After losing his leg and learning to walk again with a prosthesis, he took up running - for fun at first, then competitively. He went on to win a bronze medal in the 100m at the 2008 Beijing Paralympics.

He then decided to become a doctor and is currently working as a trauma and orthopaedic registrar in Hampshire, but he's had to pause his medical career - the opportunity to work with Esa was too good to turn down.

"I'm very much following my heart and I'm following my curiosity," he explains, "and I'm following my passion for science and life."

I've joined John on board a specially adapted aircraft to take a parabolic flight. As the plane flies steeply upwards, we experience extra gravity - about twice as much as usual - where your whole body is pressed down into the floor.

Then, as the plane gets ready to nosedive back down, there's a moment when we become weightless for about 20 seconds. It's not that gravity has vanished, we're still bound by the laws of physics. Instead, we're actually in freefall - as is the plane around us - but this reproduces zero-gravity conditions. The plane repeats this manoeuvre again and again.

There's a reason parabolic flights are nicknamed "vomit comets" - it's like riding on a mid-air rollercoaster.

Thankfully, I don't feel sick, but it's safe to say I'm not a natural astronaut.

I tumble, out of control, squealing, asking for help to come down. But I end up pinned to the ceiling - until the period of zero-gravity comes to an end, and I crash down like a sack of potatoes. Luckily the floor of this plane is well padded.

Meanwhile, John is faring much better, soaring around and gaining confidence with every period of "zero-gravity".

The prosthesis John wears is technologically very complex, incorporating a microprocessor, hydraulics, a gyroscope, accelerometers and other force sensors.

"All those things together make the knee know where it is in space and how fast it's bending or straightening," John says.

He's assessing how well his prosthetic leg is operating in this unusual environment, and the challenge comes when weightlessness kicks in.

"You'll probably see I'm floating around with my leg out straight, because that gravity isn't there," he explains. "So it's harder for me to turn quickly - because my leg doesn't want to bend. I'm just getting used to that and working out how I can move myself in zero-g, but each parabola is a learning opportunity."

John thinks that his prosthesis may be too high-tech for this environment, and a simpler one might fare better. And this is the whole point of his project with Esa: working out exactly what needs to be adapted for a person with a physical disability to spend time in space. It will cover everything, from pre-mission training, to looking at whether a spacecraft would have to be modified to accommodate John's needs.

But he says everyday life in a microgravity environment is still the biggest unknown.

"Will I wear a prosthesis? And if I wear a prosthesis, will I have to have something that will accommodate variations in volume in my stump? Would I be able to run on a treadmill in space? Will we have to adapt a spacesuit for a spacewalk? If so, in what ways?" he wonders. "All these questions are things we do not have answers for."

John is acutely aware that his disability is specific to him, and he needs to take that into account while he's undertaking the study.

"I haven't got a spinal cord injury, I haven't got cerebral palsy, or spina bifida. So I'm thinking about how this applies to other disabilities as well - but we have to start somewhere."

There's a wider impact too. John hopes the project will redefine people's expectations of what someone with a physical disability can do.

As the flight nears its end, John keeps working on and improving his zero-gravity skills. He's now moving about easily and can even land standing up, while I continue to thwack down to the floor, every time.

When this project comes to an end, John isn't guaranteed a mission to space, but as the plane starts its preparations for landing, I ask if this experience has whet his appetite for space, or made him have second thoughts about his new job.

"It's given me even more hunger and excitement," he says with a huge grin. "It's just awesome - I can't wait."