SpaceX plans to build a “self-sustaining city” on Mars, according to its founder Elon Musk. But, while we now know a lot more about how SpaceX plans to get to Mars, details about how people will actually survive up there remain sketchy.

Musk dropped the news on Tuesday during an address at the International Astronautical Congress meeting in Guadalajara, Mexico, where he had promised to reveal how the company planned to send people to live on Mars.

“I don’t have an immediate doomsday prophecy,” said Musk, but he noted that he saw only two possible paths forward. “One path is to stay on Earth forever, and there will be some extinction event. The alternative is to become a multi-planetary species, which I hope you will agree is the right way to go.”

Somewhere deep at NASA’s Marshall Space Center, in an unmarked beige hangar, there is a spaceship. A spaceship built with spare parts, scrap hardware from the International Space Stations, a left-over aluminum-lithium cylinder and even museum mockups. One day, it may become the vessel that takes humans to Mars.
NASA engineers lead by Paul Bookout are talking about it at the the Fifth Wernher von Braun Memorial Symposium, happening now in Huntsville, Alabama. Bookout’s team is working with a team from the Johnson Space Center in Houston led by astronaut Benjamin Alvin Drew, a USAF Colonel who’s been to space twice, including on the last mission of the Space Shuttle Discovery.
According to Bookout, the team is using its spaceship habitat to look at volume studies: “Are the crew quarters going to be the right size, the waste and hygiene compartment, the wardroom, the exercise area—we’re looking at all those for this extended stay.”