On the day Apollo 11 left for the moon, Wernher von Braun said “You give me 10 billion dollars and a 10 years and I’ll have a man on Mars.” Perhaps he could have solved the rocketry problem in time and under budget, but the biggest obstacles to visiting Mars are not rocket science. The biggest obstacle may be psychology. How do you keep a crew from sane all the way to Mars and back? Or politics: How do you stir up sufficient public interest in the project without a cold war? Or biology: How would astronauts handle years of exposure to cosmic radiation?