Nasa is planning a manned mission to Mars. But the challenges are enormous. Astronaut Scott Kelly has just returned to Earth after almost a year in space aboard the International Space Station. His mission was to understand what a long period in zero gravity does to the human body. The research is a vital part of preparing to send a manned mission to Mars. In 2010 President Obama tasked the US Space Agency Nasa with the goal of putting an astronaut in Martian orbit, and later to land on the red planet itself. The challenges, of course, are enormous. So, what would it take to put a human on Mars? This programme first aired in March 2016. (Photo: Mars Lander, Credit: Nasa)