Spending time in space and having an unrivalled view of planet Earth is an experience many of us dream of.

However, the human body evolved to function in the gravity of Earth. So time in the weightlessness of space can take years to fully recover from.

Astronauts Suni Williams and Butch Wilmore are back on Earth after their eight-day mission aboard the International Space Station (ISS) unexpectedly became a nine-month enforced stay. Now, their recovery begins.

“Space is by far the most extreme environment that humans have ever encountered and we’ve just not evolved to handle the extreme conditions,” Prof Damian Bailey, who studies human physiology, at the University of South Wales, says.

Entering space changes the human body – and initially that feels awesome.

“It feels like a holiday,” astronaut Tim Peake, who went to the ISS in 2015, says.

“Your heart is having an easy time.

“Your muscles and bones are having an easy time.

“You’re floating around the space station in this wonderful zero-gravity environment.”

Imagine spending weeks lounging around in bed and never having to get up – this is actually one technique scientists use to investigate the impact of zero gravity – and you start to get the picture.

But when it comes to muscle, it is a case of use it or lose it.

Even the simple act of standing still uses muscles throughout the body to hold you upright.

And that is not happening in the microgravity on board the ISS.

Muscle strength takes on a different meaning when everything is practically weightless.

The heart and your blood vessels also have an easier time as they no longer have to pump blood against gravity – and they start to weaken.

And the bones become weaker and more brittle.

There should be a balance between the cells breaking down old bone and those making new.

But that balance is disrupted without the feedback and resistance of working against gravity.

“Every month, about 1% of their bone and muscles are going to wither away – it’s accelerated ageing,” Prof Bailey says.

And this becomes apparent on the return to Earth.

The video below shows the astronauts needing support to get their bodies out of the capsule and on to a stretcher.

All of this is why astronauts go up to space in tip-top physical condition.

Then, their daily routine involves two hours of exercise – a combination of treadmill, cycling machine and weights – to maintain as much muscle and bone health as possible.

And now, Suni and Butch will start an intense exercise training programme to regain their lost function.

“It will probably take them a few months to build up their muscle mass,” Dr Helen Sharman, who was the first Briton in space, says.

Bone mass could take a “couple of years” until it recovers – but even then, there are “subtle changes in the type of bone that we do rebuild after returning to Earth that may never return to completely normal”.

But that is just muscle and bone – space changes the whole body.

Even the types of good bacteria living in us – the microbiome – are altered.

The fluids in the body also shift in microgravity.

Instead of being pulled down towards the legs as on Earth, fluid drifts up towards the chest and face.

A puffy face is one of the first noticeable changes in the body.

But this can also lead to swelling in the brain and changes in the eye, including to the optic nerve, retina and even the shape of the eye.

And this “spaceflight-associated neuro-ocular syndrome” can lead to blurred vision and potentially irreversible damage.

Microgravity also distorts the vestibular system, which is how you balance and sense which way is up.

In space, there is no up, down or sideways.

It can be disorientating when you go up – and again when you return to Earth.

Tim Peake says: “That initial phase of stopping feeling dizzy, of regaining your balance and having strength to walk around normally, that’s just two or three days.

“Those first two or three days back on Earth can be really punishing.”