On Tuesday, a space capsuleA small spacecraft or the part of a larger one that contains the instruments or crew. splashed down off the coast of FloridaA coastal state in the south-east of the US, between the Atlantic Ocean and the Gulf of Mexico. Though nicknamed "the sunshine state", it also experiences phenomenal rainstorms.. Inside were four astronauts. As the doors opened, a pod of dolphins appeared and swam around the capsule for several minutes. It was the perfect return to Earth.

For two of the astronauts this day was a long time coming. Last June, Suni Williams and Butch Wilmore set off on a week-long mission to the International Space StationA joint project between the US, Russia, Japan, Canada and the European Space Agency. (ISS). 

It was to prove the longest week of their lives. Their pioneering capsule faced technical issues. They were stuck on the ISS for nine months until a rescue mission could be arranged.

Williams and Wilmore smiled and waved as they re-entered Earth's atmosphere. One of their rescuers Nicholas Hague said: "I see a capsule full of grins, ear to ear". Once they left the vessel, medical teams wheeled them away on stretchers.

Both astronauts will need to undergo intense training to undo the effects of space on their bodies.

These are wide-ranging and serious. Astronauts often experience skin thinning, fuzzy vision, weight loss and an increased risk of cancer from space radiationEnergy given off when radioactive material decays. It can be extremely harmful and sometimes fatal, causing vomiting, skin damage, hair loss and cancer..¹

Without having to resist the tug of gravity, our muscles begin to shrink. Richard Gray writes for BBC Future: "After just two weeks muscle mass can fall by as much as 20% and on longer missions of three-to-six months it can fall by 30%."²

Lack of gravity has a similar effect on human bones: astronauts can lose 1-2% of their bone mass every month. It can take up to four years for this to return to normal.³

Time in space also affects our minds. The Daily Mail reports that changing pressures in the brain, alongside the stress and lack of sleep, "has been linked to cognitive declineA gradual loss of thinking abilities, such as learning, remembering, paying attention, and reasoning, that can occur as a person ages or because of various medical conditions. in some astronauts". 

Despite all these risks, there are many who believe that humans can - and should - live in space. Elon Musk wants to build a city on Mars. He claims that he can get millions of people living there in 20 years.

Some think that we can move to space to escape Earth's problems. The late scientist Stephen Hawking said: "We are running out of space, and the only places to go to are other worlds."⁴ Amazon boss Jeff Bezos wants to turn Mars into a factory to protect Earth's environment.

Yet others think this is a pipe dreamSomething that will never happen - that a smoker might dream about while puffing on a pipe.. According to science writer Sarah Scoles: "Humans evolved for and adapted to conditions on Earth. Move us off our planet, and we start to fail - physically and psychologically."

Space is a hostile environment. Mars is colder than AntarcticaThe most southerly continent, surrounding the South Pole. Its area is estimated at 5.5 million square miles, most of it covered in ice.. As Kelly and Zach Weinersmith write: "the least-bad places still have high radiation, dangerous soil, and low gravity with unknown medical effects. Also there's no air, no running water, and despite cost-drops and new tech, the best options are far away, insanely expensive, or both."

Money is a big issue. The mission to bring back Williams and Wilmore alone cost £108m.⁵ The cost of sending up enough people, food, water, equipment and buildings to establish a colonyAn area or country under the control of another nation. would be prohibitivelyIn a way that prevents something from happening. expensive.

Will humans ever live in space?

Yes: Humans can do amazing things. To bet against people in space is to bet against human genius.

No: A space colony would be so extraordinarily expensive and difficult that it makes no sense. And we are not even sure yet if we can survive in space for a long period of time. We were made for Earth.

Or... The likes of Elon Musk salute the idea of living in space. But they have not grappled with the reality that, even when it is possible, most people might not want to leave Earth for a barren, hostile wasteland.

Space Capsule - A small spacecraft or the part of a larger one that contains the instruments or crew.

Florida - A coastal state in the south-east of the US, between the Atlantic Ocean and the Gulf of Mexico. Though nicknamed "the sunshine state", it also experiences phenomenal rainstorms.

International Space Station - A joint project between the US, Russia, Japan, Canada and the European Space Agency.

Radiation - Energy given off when radioactive material decays. It can be extremely harmful and sometimes fatal, causing vomiting, skin damage, hair loss and cancer.

Cognitive Decline - A gradual loss of thinking abilities, such as learning, remembering, paying attention, and reasoning, that can occur as a person ages or because of various medical conditions.

Pipe dream - Something that will never happen - that a smoker might dream about while puffing on a pipe.

Antarctica - The most southerly continent, surrounding the South Pole. Its area is estimated at 5.5 million square miles, most of it covered in ice.

Colony - An area or country under the control of another nation.

Prohibitively - In a way that prevents something from happening.