The astronaut candidates must hover a helicopter, take their own blood and be tested for claustrophobia in the first round of tests.
The remaining astronaut candidates face a simulated emergency splashdown where they have to escape from an underwater capsule.
The remaining astronaut candidates face more gruelling tests, including assembling a large cube underwater, learning basic Russian and attempting to dock the Soyuz Space capsule with the International Space Station.
The remaining candidates head off to a secret facility in Sweden where they are deliberately deprived of oxygen to see if they can recognise the signs of hypoxia.
The candidates' leadership skills are tested in a terrifying cliff abseil and a gruelling survival test, but weaknesses are exposed in the group. Those who succeed are sent to Florida to carry out a spacewalk dive in an underwater marine lab.
The finalists dive down to the Aquarius underwater research laboratory in the Florida keys, a training facility used by NASA. They return to London's Science Museum to find out who has won the competition and has what it takes to be an astronaut.