Wearing helmets weighing 100 kilos, spinning in a centrifuge and exercising while weightless: Russian cosmonauts and astronauts from abroad have to undergo a gruelling training process before blasting off into space. Helped by an instructor at the famed Star City outside Moscow, cosmonaut Sergei Ryazansky slowly puts on his helmet as he hangs from the ceiling suspended by a thick metal cord and practises opening a lock while wearing a thick spacesuit. Space walks are especially tough because they are often carried out weeks after astronauts arrive at the orbiting space laboratory when their muscles are already growing weaker due to lack of gravity.