Miami: Nasa has outlined the many challenges that remain before humans can set foot on Mars, calling the problems "solvable" but setting no firm date for an astronaut mission to the Red Planet. Updated details of the US space agency's Mars strategy were contained in a 36-page document released to the public late Thursday, ahead of upcoming talks with Congress about budgets for space exploration and a major international meeting of the space industry to be held in Jerusalem next week. Astronauts who journey to Mars could spend three years in deep space, where radiation is high and so are the risks of cancer, bone loss and immune problems, said the document, called "Nasa's Journey to Mars: Pioneering Next Steps in Space Exploration." "Living and working in space require accepting risk, and the journey is worth the risk," it said, calling Mars "an achievable goal" and "the next tangible frontier for expanding human presence."