An anonymous reader writes: Elon Musk's ambitions for SpaceX keep getting bigger. First he wanted to make the trip to Mars affordable, then he wanted to establish a city-sized colony, and now he's got his eye on the future of humanity. Musk says we need a million people on Mars to form a "sustainable, genetically diverse civilization" that can survive as humanity's insurance policy. He continued, "Even at a million, you're really assuming an incredible amount of productivity per person, because you would need to recreate the entire industrial base on Mars. You would need to mine and refine all of these different materials, in a much more difficult environment than Earth. There would be no trees growing. There would be no oxygen or nitrogen that are just there. No oil." How fast could we do it? Within a century, once the spacecraft reusability problem is solved. "Excluding organic growth, if you could take 100 people at a time, you would need 10,000 trips to get to a million people. But you would also need a lot of cargo to support those people. In fact, your cargo to person ratio is going to be quite high. It would probably be 10 cargo trips for every human trip, so more like 100,000 trips. And we're talking 100,000 trips of a giant spaceship."