[sobellian]

For what it's worth Musk believes that Starship will bring a >10x improvement for $/kg to LEO. Not throwing away your second stage will do magical things. At any rate a Martian colony would be mostly self-sustaining with trade being a relatively small fraction of gross domestic product (like with international trade right now). On planetary scales, moving trade from 0.01% Gross Martian Product to 0.1% would more than justify the development costs for such an engine.

I think we can safely say that we are very far from 1 Martian colonist, let alone 1e6. I think we would outperform expectations to set up a very small research station by the end of the century at great expense on the Martian surface. So there's clearly a lot of magical thinking going on here. If we allow for some amount of aspiration, there are a few ways to provide a return on long time scales (much more than 1 century, with capex far exceeding $175B just looking at the cost of staging the necessary propellant in LEO):

- Such a base will have a propellant depot in a much shallower gravity well than Earth and a far thinner atmosphere. This is a huge comparative advantage for launches into deep space.

- Necessity breeds ingenuity. A Mars colony would likely generate many advances useful to Earth that do not make sense to pursue terrestrially. Think hydroponics, insulation, radiation shielding, so on and so forth.

- There are plenty of completely desolate locations on the Martian surface - ideal locations for radio telescopes, neutrino observatories, and other hyper-sensitive experiments.

- ??? (We're trying to predict centuries ahead, after all!)