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.