What Is the Minimum Size for a Planetary Settlement?

Charlie Stross (seen via TJIC) asks:

There's a deceptively simple question that's been bugging me this week, and it is this:

What is the minimum number of people you need in order to maintain (not necessarily to extend) our current level of technological civilization?

and suggests:

I'd put an upper bound of about one billion on the range, because that encompasses basically the entire population of NAFTA and the EU, with Japan, Taiwan, and the industrial enterprise zones of China thrown in for good measure. (While China is significant, more than half of its population is still agrarian, hence not providing inputs to this system).

I'd put a lower bound of 100 million on the range, too. The specialities required for a civil aviation sector alone may well run to half a million people; let's not underestimate the needs of raw material extraction and processing (from crude oil to yttrium and lanthanum), of a higher education/research sector to keep training the people we need in order to replenish small pools of working expertise, and so on.

and concludes:

More realistically, we won't have autonomous off-world colonies unless and until they can cover all the numerous specialities of the complex civilization that spawned the non-autonomous, dependent-on-resupply space program. Or, to put it another way: colonizing Mars might well be practical, but only if we can start out by plonking a hundred million people down there.

Well… The important figure isn't how many people are needed for today's society; it's how many people would be needed for a society of the “Mad Men” era. I suspect that's much less. You might be able to run the Mars colony (at least in the initial stages) on ferrite-core computers (ObSF: A Little Knowledge by Poul Anderson). I also suspect that those parts that cannot be made by a small colony might be light enough (microchips) or rare enough (humans with specialized training) that resupply is feasible.

One way to get a handle on this is to investigate what the South-Pole Station has to import. Another way to find out is to try to get Hollywood to produce a new “Survivor” program: “Survivor: Post-Holocaust.” Contestants get a junkyard and a square mile of park and have to survive for a year. (Actually, this came from a discussion of whether civilization could be restarted after a drastic population loss.)