It takes something special to be an astronaut – an extraordinary combination of bravery, fitness, intelligence, lightning-fast decision-making and calmness under the most extreme pressure. It's known as "the right stuff".

When Nasa was selecting its first spacemen in the late 1950s, it looked no further than the best military and test pilots the United States had to offer. The Soviet Union did the same, only it also specified that its cosmonauts had to be no taller than 170cm (5ft 6in) – to fit in the tiny Vostok capsule – and parachutists, so they could eject from the spacecraft as it re-entered the atmosphere. Unlike the Americans, it also recruited a woman.

Since those early days, there have been scientists, engineers and medical doctors selected for spaceflight. But for the most part, 60 years on, those original ‘right stuff’ criteria for the ideal astronaut still hold true. Take the European Space Agency’s (Esa) most recent astronaut intake in 2009, for example. Of the six astronauts selected, three are military pilots and a fourth is a commercial pilot. The other two astronauts’ lists of hobbies include skydiving and mountaineering.

But despite selecting the best of the best, humans are still poorly suited to life in space. We are products of 3.8 billion years of evolution in a comfy 1g oxygen-rich biosphere, protected by a magnetic bubble (the magnetosphere) from the harshness of the Universe. Away from the Earth, astronauts are bombarded by cosmic radiation and suffer nausea, muscle and bone loss, deteriorating eyesight and even weakened immune systems as a result of zero gravity.

Esa astronaut, Luca Parmitano, says he was astonished how rapidly his body changed during his five-and-a-half months in orbit on the International Space Station. “There’s an adaption that feels like a transformation,” he tells me. “You see your legs get skinnier and your face becomes round – slowly your body gets into a new state of normality.”