A few decades from now, people could be living on Mars. The reason: The red planet is Earth’s closest neighbor in outer space, 35 million miles away, and the one that seems most able to support human life. As close as Mars is, though, humans will have to overcome much to get there and live there. “But nothing great has ever been accomplished without risk,” Robert Zubrin, president of the Mars Society, an organization devoted to manned Mars missions, said Thursday from Lakewood, Colorado.