So I was at the school library today doing some photocopying, and I decided to wander through the small section of space books in the basement. Aside from feeling vaguely like a murderer was going to jump out from one of the aisles, the library basement is actually fun, because it's where most of the cooler (by which I mean farthest from cool) books are kept.
After picking up a book which referenced Gerard K. Oneill's pioneering work on the idea of space settlement, I realized that the library probably had O'neill's original article in Physics Today from 1974, in which O'neill proposed not colonizing Mars or the Moon, but building new, massive habitats in space capable of housing tens of thousands of people. For people who've seen Babylon 5, B5 (the station) is a classic O'neill design - a rotating cylinder enclosing an earthlike environment and gravity. O'neill had a modestly popular book called The High Frontier in which he advocated construction of these habitats, whose primary industry (in the beginning) would be making solar power satellites, which I mentioned earlier. For a good summary of O'Neill's ideas, start here.
One of the most interesting things about reading the Physics Today article is how much more evangelical it is than even High Frontier. O'Neill advocates building colonies bigger than many small countries, capable of supporting hundreds of millions of people - in the most extreme case, a cylinder forty miles long consisting of nothing bur farmland and apartment complexes could support 700 million people indefinitely, according to his calculations. The costs were estimated at roughly equal to the Apollo program, or $100 billion in today's money. That comes out to one year's worth of US oil consumption, 90 days of US military spending, or an hour and a half of global currency speculation. Of course, that cost isn't for one colony, it's for a whole system to construct that colony and more like it. Better yet, 90% of the costs are startup, so the next colony costs $10 billion, not $100 billion. If we could move people off the planet fast enough (and that's the hardest part) we could move the entire human population in to space in less than 20 years. Now, obviously not everyone is going to want to go, but given some of the impending climate crises we're in for, they may change their minds.
When I write about the Iron Denominator, there's something we should keep in mind: even with the most optimistic assumptions about renewables and sustainability, it may not be possible to support the projected human population of this planet without a) serious social hardship (famines, epidemics, etc.) or more importantly b) human activity totally monopolizing the biosphere's capacity to support life. So we should be thinking about this, if only to keep the world's few remaining wild spots wild. In the long-term, I think a world with a population of less than one billion, with most humans living in space and coming to the mother world as tourists would be wonderful. But then again, I'm a big nerd, so that figures.