[Question] Is a near-term, self-sustaining Mars colony impossible?

Some argue that Elon Musk’s plan to build a self-sustaining colony on Mars is effectively impossible for the foreseeable future without discontinuous tech progress. (See e.g. Jess Riedel’s claim here, which Robin Hanson agrees with.)

Is there a simple explanation for why this might be so? I’d be especially interested in Fermi estimates of key bottleneck resources.

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