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Chris Hadfield, Dr Kevin Fong and Dr Iya Whiteley put 12 candidates through a series of physical and psychological tests to find out who has what it takes to be an astronaut.
The remaining astronaut candidates are put through more demanding tests. They have to escape from an underwater capsule and operate a groundbreaking robot - the Mars Rover.
Old fears surface when the remaining candidates face some space agency swim tests. Their teamwork and mental agility are scrutinised in an underwater test, assembling a large cube.
The competition becomes more intense as the remaining candidates are deprived of oxygen to see if they can recognise the signs of hypoxia, critical for an astronaut on a spacewalk.
The astronaut candidates fight for their place in the final. Their leadership skills are assessed in a survival test and they head off to the Kennedy Space Centre in Florida.
The finalists head to the Shuttle Landing Facility on Florida's Space Coast for a tough test in weightlessness, and back in London the winner is decided.
