SpaceX CEO Elon Musk went into a bit more detail about the timelines and vehicle requirements to not only reach Mars, but to set up a sustainable base on the Red Planet that can serve as an actual city, supporting a local population. That’s the long-term vision for Musk and his space technology company, after all — making humans an interplanetary species. The timeline that Musk discussed today, replying to fans on Twitter, might be incredibly impressive or incredibly ambitious, depending on your perspective.

Addressing a question about comments he made earlier this week at the U.S. Air Force startup pitch day event in California, Musk said that his stated launch cost of only around $2 million per Starship flight is essentially required, should the final goal be to set up a “self-sustaining city on Mars.” In order to make that city a reality, he added, SpaceX will need to build and fly around 1,000 Starships according to his estimates, which will need to transport cargo, infrastructure and crew to Mars over the course of around 20 years, since planetary alignment only really allows for a realistically achievable Mars flight once every two years.

Read more: yahoo