“We need to figure out how to launch multiple times a day,” Musk said. “The thing that’s important in the long run is establishing a self-sustaining base on Mars. In order for that to work—in order to have a self-sustaining city on Mars—there would need to be millions of tons of equipment and probably millions of people. So how many launches is that? Well, if you send up 100 people at a time, which is a lot to go on such a long journey, you’d need to do 10,000 flights to get to a million people. So 10,000 flights over what period of time? Given that you can only really depart for Mars once every two years, that means you would need like forty or fifty years.”