Horizon goes behind the scenes at NASA as they countdown to the landing of a 2.5 billion-dollar rover on the surface of Mars. In six days time, the nuclear-powered vehicle - the size of a car - will be winched down onto the surface of the Red Planet from a rocket-powered crane. That's if things go according to plan: Mars has become known as the Bermuda Triangle of space because so many missions there have ended in failure. The Curiosity mission is the most audacious - and expensive - attempt to answer the question: is there life on Mars?