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How do you judge Musk's million-person threshold for a self-sustaining Mars city?
Paraphrase of Musk's 27 September 2016 presentation, published in New Space in 2017: he put the threshold for a self-sustaining city or civilization on Mars at one million people.
Source / statement date: · Elon Musk, 2016 IAC presentation; 2017 New Space paper, p. 50 (PDF mirror)