In week one, the 12 candidates are thrown in at the deep end. They are first challenged to hover a helicopter, with no previous experience, to test how quickly they can learn a new skill. Astronaut Chris Hadfield sets them this test as it's one astronauts need to master and it shows the candidates just how high the bar is being set. Dr Kevin Fong then puts the candidates through a Beep Test that pushes them to their physical limits. Next, Chris runs a test for mental acuity used by space agencies to weed out candidates during their own selection processes. Psychologist Dr Iya Whitely then interviews the candidates to look for facial micro-expressions that could reveal who would be best-suited to long-term missions in space. Next, with no hospital in space every astronaut is trained in basic medicine and the candidates must face their fears by taking their own blood and, finally, they're confined to small personal spheres as a test for claustrophobia. Not everyone meets the high standard Chris has set, but which candidates will have what it takes to get through to the next stage of the selection process?

In week two the candidates must put themselves through a test that is a matter of life and death: since water covers two thirds of the world's surface, every astronaut must learn to escape from a space capsule which has crash-landed at sea. The candidates are tested on their ability to remain calm whilst escaping from an underwater capsule, but not all of them are able to face their fears successfully. The candidates' ability to adapt to life in the weightlessness of space is also put to the test as they don goggles which invert their vision, so that everything they see is upside down. This is followed by a visit to the largest space-tech company in Britain, which is responsible for building the next European Space Agency Mars Rover. Here they must operate a prototype of the Rover in a simulated Martian landscape to test their operational decision-making. Finally, a social event allows astronaut Chris Hadfield and the panel - former NASA medical researcher Dr Kevin Fong and psychologist Dr Iya Whiteley - to discover more about the candidates, and whether they've realised that during astronaut selection if you are in the presence of an assessor you are always being assessed...

Astronaut selection is getting tougher as the candidates move from basic to advanced stages of selection. This week the candidates are heading to some of the most specialised astronaut training facilities in Europe. But first they must overcome a key astronaut selection test - in the swimming pool. An extreme airless environment, water offers the closest analogue on Earth to working in space, and as astronaut Chris Hadfield says: "A comfort and an ability on and in the water is just an essential skill to be an astronaut." As well as swimming lengths in their flight suits and shoes, they must tread water, and work as a team to assemble a weighted ‘Soma cube' on the bottom of the pool.

In week four the candidates leave the German Space Agency behind and head to a secret location in Sweden to face astronaut Chris Hadfield's toughest tests so far. Here, in a military installation, the panel will discover who can cope with a crisis in space. The candidates will be put through a potentially life threatening test of hypoxia – a deadly condition in which the body is deprived of oxygen.

The last remaining candidates are put through a series of challenges to determine who will go through to the final stage. A survival test and a 30-metre abseil put their leadership abilities under scrutiny, before they face a series of gruelling interviews by two guest experts. Those who are chosen to continue head to Florida's Kennedy Space Centre, for a spacewalk training session in an underwater lab.

The finalists go to the shuttle landing facility on Florida's Space Coast, where they experience weightlessness for the first time, have their motor skills put to the test at 30,000 feet and are confronted with the dangers of space travel. They also undergo further training at a NASA underwater facility before Chris Hadfield reveals who the panel have chosen as the winner. Last in the series.