For many years those of us for whom the emptiness of the night sky is a challenge have assumed that the colonization of space would spring from the natural urge to express such drives as curiosity, the thirst for adventure, and an eagerness, apparently innate in the species, to explore the unknown.
Many still feel this way. However, it has been obvious since Pathfinder that before long there will be no place, on Earth or off, that humans are curious about that machines will not have been monitoring exhaustively for decades. There will be no place not be as familiar (to those interested in it) as, say, the San Francisco Bridge is to most Americans. It is hard not to suspect this familiarity will undermine the exploration motive, probably fatally. There might still be interest in visiting a place 'physically' or 'in reality', but it is hard to imagine a person devoting many years of their life travelling to see a sight that has been accessible with a click on a link for years previously, especially given the degree of multi-sensory fidelity we can expect from the technology in a few decades.
There may well be an appeal to the weaker sense of adventure embodied in tourism -- people still visit the San Francisco bridge -- but tourists are not colonists, and are unlikely to be interested in getting away from the grandchildren for more than a month or so at most. Given the limits imposed by light speed, tourism alone seems unlikely to scatter the species very far.
A second rationale is that space will serve as a safety valve and outlet for those feeling that their religion or ethnic group is oppressed by society. This argument has the virtue of referring to a driver responsible for a fair fraction of historical emigration and being related to colonization in ways that exploration and tourism are not. Further, it is easy to see world society falling into a repressive local minima and never climbing out. If peace is the upside of unified societies, the downside is an unchallenged collapse to sterile mannerism. Something much like this happened in and to China in the early 15th century; had it not, the inhabitants of North America would probably be speaking Mandarin now. So world society may be very well be worth escaping from in a few hundred years.
However, it is not clear that escape will be possible -- the world state might be able to afford faster spaceships than malcontents for quite some time -- and in any case the fraction of the population that minds being oppressed is clearly quite small. (I have read that 40% of the US population thinks the government ought to approve stories before the media can carry them.) Once again we seem to be left with only a weak motive to take the big step.
A third rationale grounds the colonization of outer space not on the drive to explore the unknown or the thirst for freedom but something far deeper and more general in the species: the urge to accumulate stuff. Life in space has several advantages to offer a determined consumer: the attic is infinitely extensible, at least if you stay away from planets; energy is abundant, especially at lower solar orbits; and there are lots of atoms floating around in various forms, including dust. So you can make a lot of stuff, you can keep it around, and you can power it all. What else is there to life?
The objection might be made that there is a lot of unused space here on earth, and that it is true enough. If you assume the mantle is habitable to an average of ten miles beneath the surface, the Earth's 200 million square miles of surface area yields a stock of about one cubic mile of space to distribute to every three or four people. Of course not every last cubic inch of that can be devoted to empty space, but it sounds like quite a lot. However, new homes always fill up, and even that volume will probably end up seeming like less than we thought. An interesting home white-water rafting setup could eat up a billion cubic feet right there, and anyone reading this could come up with a dozen similar applications -- a "room" for each of the Earth's major ecological types -- in a minute.
Plus there is another issue to weigh: the trajectory of green values. If it turns out that the intensity of the environmentalism of 2030 : today's :: ours : 1970, then any sort of construction on the surface not directly related to the restoration of habitat might be extremely difficult. Following the line out a bit further, the environmentalism of 2060 or 2090 might strongly disapprove even of underground construction. After all, who are we to eradicate the habitat of the lithic bacteria? What makes us elevate our needs above theirs? And why should be we feel empowered to destroy the natural expression of earth's geological processes? In a hundred years these questions might not look as ridiculous as they seem today.
So sooner or later (depending on how green the society becomes) developers might announce orbiting habitats that promise every toy you might want, now or in the future, free of environmental restrictions and limitations, or at least guilt. Perhaps they will be cross- marketed as the environmentally responsible lifestyle, since humans in orbit leave no footprint at all on Mother Earth, consume none of her resources, and interfere with not one of her creatures. A hundred years from now actually living on Earth might be the equivalent of wearing real fur today; nominally legal, but socially declasse.
The green yuppie model of space colonization differs from either the heroic explorer or freedom-seeking victim of oppression model in absolutely requiring nanotechnology, since only nanotech offers the degree of control over fabrication required to build habitats with interiors containing several cubic miles of volume. In fact, one can reverse the logic: given nanotechnology, the gentrification of the stellar reaches is probably inevitable.
Nanotechnology is above all a philosophy of manufacturing and
from that point of view no more or less interesting than (say)
continuous flow processing or lithography. However, it contains
within it two great dreams that are not readily achievable
without it. The first is very long life spans; the second, if
the train of logic in the text here is right, is the
colonization (as opposed to the exploration) of space. It might
make sense to define nanotechnology not in terms of its basic
manipulations (atomically precise manufacturing) but of its most
dramatic applications, one of which being the vision of vast
mansions, floating with the stars, filled with experiences that
Kings could not imagine.