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C00002 00002	NOTES ON SPACE COLONIES
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NOTES ON SPACE COLONIES


	Dyson () has advanced several reasons for colonizing space: to disperse
mankind so that nuclear war can't destroy it, to provide a frontier so that
dissident groups can do things their own way.  Another reason is that living
in space might be fun.  It doesn't prevent Malthusian difficulties and since
it would be very expensive, it won't even help immediate problems, since a
person can be provided a capital base for good living on earth much more
cheaply than in space.

	I want to discuss the problem of space colonization from a different
standpoint than that of O'Neill (1972).  O'Neill proposes to simulate earth
environment rather well and to provide lots of living space.  In Appendix A
of this paper, I will show that O'Neill's proposals are impossibly expensive
in their present form.  Rather than start where he does and discuss ways of
reducing the cost, I will start at the other end of the scale.  Namely, I
want to explore the minimum cost alternatives on the assumption that there 
will be colonists willing to change their way of life drastically when they
emigrate.  Perhaps our colonists will need stronger motivations than his.

	As with O'Neill and Dyson before him, I shall assume that materials
obtained from the asteroid belt in the steady state are used to construct
vessels that use solar power and have closed ecologies.

	Unlike O'Neill, I shall assume that people will adapt to zero g (perhaps
they will prefer it even if they won't be able to visit earth without
re-adaptation) and I will assume an atmosphere of 5 lbs per square inch
of pure oxygen.

	Under these conditions, it turns out that the amount of material in
required for the pressure vessel is independent of the size of the vessel
and depends only moderately on the shape.  We shall start with a spherical
shape which is the most economical.

	According to the principle of virtual work,
when a spherical shell of radius r, thickness t, tensile strength T, and
internal pressure p expands by dr and is just at its breaking point,
we get the equation

	4π(r↑2)p dr = d(4πr↑2)tT,

and putting p = 5 lbs/in↑2 and T= 150,000 lbs/in↑2 (O'Neill's more optimistic
assumption), we get t/r = 1/60000.  Supposing the steel to have a specific
gravity of 10, we get .5 kg of steel per cubic meter of living space.  If
we allocate to a 75 kg person, 75 kg of steel to the shell that contains
his air, we get 150 cubic meters per person of living space.  Comparing this
with housing with 2 meter ceilings, we get 75 square meters or 750 square feet
per person.  This is moderately generous for housing space, but looks a bit
cramped if we remember that this is the entire space allotment per person
including working space.
At this allotment, however, the cost of getting the person's container into
space would be the same as getting the person himself up there.  At estimated
Shuttle costs of $100 per lb to earth orbit, we may estimate costs of $100,000
per emigrant assuming a quite austere system.  There are a large number of
families that could assume such a debt, and another factor of ten in cost
would make the system quite affordable.