Imagine this scenario. The year is 2030 or thereabouts. After voyaging six months from Earth, you and several other astronauts are the first humans on Mars. You’re standing on an alien world, dusty red dirt beneath your feet, looking around at a bunch of mining equipment deposited by previous robotic landers. Echoing in your ears are the final words from mission control: “Your mission, should you care to accept it, is to return to Earth–if possible using fuel and oxygen you mine from the sands of Mars. Good luck!”