OK, let’s see if we could start with a much smaller habitat, but still big enough to grow its food. around 1,000 people. I have assumed a single torus with a larger radius of 500 metres, an inner radius (to the torus itself)of 200 metres, and that half the internal area of the torus is habitable.
If we use the same input costs as for the larger habitat in the previous section, total construction costs come to around $1 billion. This seems suspicious low, and almost certainly is an underestimate. When the first few habitats are constructed space mining and manufacturing will be in early stages and costs will be higher than in the longer term. Some low mass high value components (such as printed circuits)will probably need to be brought up from Earth. But even if costs are as much as ten times higher, that is not an enormous sum.
What if, however, the raw materials and parts are transported from Earth, instead of assembled in space? Apart from soil, that is, still assumed to be lunar regolith. The cost is a staggering $256 billion at $100/kg launch cost, and even at $50/kg (difficult to achieve unless a launch loop or similar is built) it costs $128 billion, almost as much as the habitat which houses 400 times as many people. Space habitats must be built in space, and thus cannot be built until there has been a lot of prior development of space mining and manufacturing.
Not only are the above costs too high, this approach does nothing to develop space manufacturing.
Note that cumulative costs (launch, assembly, maintenance) of the International Space Station is over $100 billion to date – and that only weighs 420 tons, and the living area is barely the size of a suburban house.
It is tempting to put a habitat in LEO, where under the protection of the Van Allen Belts an artificial magnetic field to protect from radiation would not be required. There are two problems, however.
The first is that it would spiral back towards Earth, although very slowly.
A more important problem is space junk. There are half a million objects circling this planet already, satellites have already been destroyed by collisions, and the problem is getting worse. Even a small habitat would be a much bigger target than a satellite.
The chart below shows the current distribution of objects in space. Source: aer.com
Most are clustered in the 7000-20,000 kms from earth range, within the protection of the outer Van Allen belt, but high enough to have long lived orbits; even so, the higher objects will spiral down over time. There is a smaller cluster at or just below 36,000 kms which is geostationary orbit, but they are exposed to more radiation (within the range of the outer Van Allen belt).
As long as radiation protection can be assured (see previous posts) then habitats are best placed beyond 36,000 kms to avoid the space junk problem, and will need to take measures to ensure that they don’t create their own space junk.
Costs per head are acceptable, at high density