A quick qualification. I do not recall stating that habitats could be
built in the near future, and I suspect it was bad phrasing on my part
that suggested it. I would not be surprised if we see early permanant
space habitats around 2100 A.D.; I would be surprised to see them before
that. Space habitats worth living in require certain technologies that
we simply do not have or have not sufficiently developed. Of these,
I think one of the most important would be self-replicating robots to
handle the enormous labor and manufacturing costs--building something
the size of a Nimitz class carrier in orbit is far beyond what we can
afford to do at this point, whether or not we could solve the many
technological problems.
On the other hand, I find it hard to imagine a future where mankind
has survived that does not eventually involve a space industrialized
civilization. It is not that people will emigrate to space in large
quantities, but if only 1% of the population leaves (comparable to
other emigrations in human history), that means you have an initial
population of several million humans (with several billion robots
working for them--self-replicators will breed as often as we want
if we give them sunlight for power and raw materials to process).
This population will gradually find living in space "normal," and
when their populations pressures start, they will simply build more
habitats. As I said before, an Earth-like planet might be able to
support 20 billion people, perhaps comfortably if we distributed
resources properly, but the asteroid belt has enough raw materials
to make habitats for half-a-trillion people, even assuming you need
one million tons of raw materials per person to make these habitats
and supply other basic needs (I assume very roomy habitats...after
all, if you're going to live in space, don't live in a metal shack,
build a small world to order, if you can afford it--advanced
technology will make it economical. Not during our lifetimes,
but I cannot imagine technology not advancing to the point where
it will be practical and cost-effective. And it is also true
that the first space habitats will be primitive and cramped, but
probably no worse than what urban dwellers endure right now--
New York City has a population density of more than 20,000 per
square mile--but as the space-based population grows larger and
richer, their habitats will become more luxurious.)
As I said before, it is not that space habitats are going to be
available very soon, or that everyone is going to suddenly rush
to leave Earth, but some people will--you always get a few with a
frontier mentality--and their children will probably stay in
space (children do leave small towns for the big cities, but
most children of colonists do not leave their new native country
for the "old country" of their parents); they will be the population
that eventually expands, growing huge because the natural resources
in space are much greater than those on any planet.
Assuming we do not destroy ourselves or turn away from space entirely,
I cannot see how this could not happen. Eventually, the technology
to support people in space will be available; a few will go, either
to get rich, or to get away from a society they dislike, or for some
other reason, and they will get things started. Sure it sounds
crazy. It was crazy to leave England in 1620 for America. It was
really crazy to set out on the Pacific in an open boat, hoping you
would find a new island to live on. People did it. (For a good
discussion of these issues, by the way, I strongly recommend a
collection of articles entitled INTERSTELLAR MIGRATION AND THE
HUMAN EXPERIENCE.)
Enjoy. And remember, by the time you finish reading this sentence,
it will be the future.