Elon Musk, chief executive officer for Space Exploration Technologies Corp. (SpaceX), speaks during the 67th International Astronautical Congress (IAC) in Guadalajara, Mexico, on Tuesday, Sept. 27, 2016. Musk's vision for building a self-sustaining city on Mars will require full rocket reusability, refueling the spacecraft in orbit and propellant production on the Red Planet. Photographer: Susana Gonzalez/Bloomberg via Getty Images
