My first year seminar in college was all about Colonizing Mars, and it was built around the Mars Trilogy (which I've been writing about rather a lot these past few weeks, but I must admit that my first encounter here was not as fruitful.) Even though I spent most (all?) of my college years away from the science fiction world (long story, sorry BSFFA), the whole idea of space travel and colonization has been a fascinating problem.
And not just the recent hubbub about one way journeys to Mars, which I think Karl Schroeder does I fine job of dispelling, though I think there's a much more thorny problem around population pressures and the cost of emigration that deserves some attention than the common discussion about space settlement has really been able to spawn so far.
Space is really big. Right? Like huge. The colonization of space if it is to be successful--in its own right, building settlements takes the efforts of a lot of people--is really about moving millions upon millions of people to Mars or the Moon not to mention other possibilities like constructed outposts and longer range colony ships. And that's an incredibly huge proposition. Of course, the there would be some sort of pioneering group, but you'd need to be able to commit to being able to send a great deal of people out there in pretty short order. Think a million people a year for ten or twenty years? That's 2,700 people a day and some change. If you wanted to ship a billion people off world over ten years, you'd need to lift almost 274,000 people a day.
Which isn't anywhere near feasible, with transportation costs, equipment costs, let alone the logistics costs of getting a quarter of a million people to do anything in a concerted fashion every day for ten years. But the truth is to make a settlement viable out there you need a lot of people, perhaps I think more than can be easily transported from Earth.
Space is huge, though. Even our little corner of space is huge. Mars, because it doesn't have oceans, has as much landmass as Earth. That's a lot of room, and while I'm certainly not saying "we have to work to fill up the rest of the solar system as fast as possible," I think there are likely critical mass and critical densities of people that would be required to make the settlement of space viable.
On top of Schroeder's simple point about the role of launch costs (using current/Apollo-era technology), there is a whole other issue of "what to do when we get there," (and how we're going to sport that, whatever it is) are equally important considerations.
I guess my point, insofar as I have one, is that while we might start to feel a bit crowded on earth in the next hundred years--which may or may not be real--using off planet settlements as a "population shunt" is difficult. (Anyone else have a good way to get 20k people a day off world. Or more? Way more?) It would be hard to move people off world fast enough to make a dent in the population here, and also hard to move people off world fast enough and in great enough quantities to sufficiently populate those settlements.
One of the things I adore about Samuel Delany's Stars in my Pocket Like Grains of Sand is the huge scope of the universe, and that it's huge not just in the traditional conception of "space is big," where we put a lot of thought into "Stars are really far apart," but also in the "Planets hold a lot of people." There's a segment, which is otherwise not particularly notable, where the narrator talks about the population of the settled universe, and he is--to our eyes--incredibly out of touch with the actual number of people in his "world" (universe,) and I think the figure the narrator gives is unspecific to the tens of billions of people. Which is sort of boggling.
And I think I'll leave you on that note. Food for thought! Cheers!