Until recently, the idea of sending humans to Mars was pure science fiction. Now the world's largest space agency is gearing up for its most ambitious project yet - a manned mission 140 million miles across space to the Red Planet. From testing their rockets and experimenting on their astronauts to designing their life support systems, Horizon follows teams across NASA as they start work. How will they overcome the impossible? Is it actually do-able? And if they do land a human on Mars, will they ever be able to bring them back again?