Many people have written pages, papers, even entire books on the subject of Space Based Solar Power. Certainly many of the authors are better qualified than I am. What my posts add to the mix is the cost reduction and simplicity gained by using an asteroid captured into Earth orbit for the bulk of the materials.
Also, some of my sources are more recent than many older papers – which makes a huge difference. For example, my earlier posts used a figure of 125,000 tons for a 4 gigawatt SPS, based upon references quoting 20,000 tons per gigawatt, plus structure and the transmitter. But more recent sources propose a very lightweight design requiring only 1,000 tons per gigawatt – a 20-fold reduction. For these posts, I’ll be using an intermediate figure of 5,000 tons per gigawatt, or a total of 25,000 tons for a 5 gigawatt SPS (measuring 4 km on a side).
Even with current high launch costs, at least one company believes they can build a cost-effective SBSP system using all Earth-sourced materials. I don’t see how, since fifty to a hundred or more 100-ton payload launches would be required. But the people with their money on the line have put more thought into justifying the ROI, and I wish them well.
More commonly, pundits believe that Lunar materials (typically using a launch-rail system to lower the cost of launching materials from the Moon) might reduce the total cost to an acceptable level for as few as 50 to 100 SPSs (to amortize the costs of landing a great deal of equipment and people on the Moon).
Asteroidal resources are much more readily available, and several sources have proposed robotic mining missions to various asteroids (chosen for a low net delta-V, especially for returning material to Earth orbit). Some proposals return raw material to Earth orbit for processing; others return materials processed to some degree (such as iron). Robotic missions are often assumed to eliminate life-support and radiation shielding costs.
Note that a 100-meter radius Kalpana One style habitat (housing and feeding up to 8,000 residents) can be built from a single 120-meter asteroid, along with 12 five-gigawatt Solar Power Satellites, using the slag from iron smelting as radiation shielding.
My proposal (see Capturing an Asteroid) is to use gravitational slingshot maneuvers (around the Earth, the Moon, or even Mars or Venus when appropriate) to capture one or more asteroids into Highly Eccentric Earth Orbit (HEEO), then to launch the tools needed to mine the asteroid, smelt it into valuable materials, and build the SPS network in orbit. Of course, this is simply a logical extension of the proposals to return asteroid raw materials to Earth orbit. I propose to send the whole asteroid (which we can do given the extraordinary special circumstance of an existing close approach of an asteroid).
This approach requires large numbers of people in space, because people are good at problem solving. Using robotic approaches requires that the engineers anticipate all possible contingencies, and, speaking as an experienced computer programmer, I absolutely guarantee that we will never succeed at anticipating every possibility.
But people are resourceful, and their ingenuity will solve all of the little problems, and likely the big ones that crop up, as well. The number of people needed is huge, because each SPS is itself huge (kilometers wide), and there is a lot of work required. We’ll need to smelt ore into steel at impressive rates (but typical of a foundry on Earth). We’ll need to form that steel into plates, girders, pipes, tanks, cables, etc.. We’ll also need tables, and chairs, and sinks, and toilets. It’s expensive to bring those things up from Earth.
We’ll need to use part of that steel (and much of the slag) to build habitats for space workers (see Designing a Space Habitat). We’ll need to produce magnesium for mirrors (assuming turbine-driven electric generators) or silicon for solar voltaic cells (or both – we won’t know what is best until we try). We’ll need to weld or bolt all the pieces together into immense structures, and then maintain them (because parts fail, regardless of how well-built they are). We are likely speaking of thousands of people to build that first SPS, and thousands more to build and maintain a network of them.
Carbonaceous chondrite asteroids and extinct comets certainly contain everything needed. These comprise at least 75% of all asteroids, although they are relatively rare among Earth-crossing ones. However, any undifferentiated asteroid (such as ordinary chondrites) should suffice. All of these contain vast quantities of iron, oxygen, and magnesium. Most asteroids will contain small but significant quantities of carbon and hydrogen, easily extracted by simple heating. The asteroids to avoid are those from parent bodies that melted and differentiated, since that would isolate most metals into an iron core, and deplete the volatiles, leaving ore as poor and dry as moon rocks.
HOWEVER, there are significant ground-based costs to consider as well. In addition to building an SPS with a microwave transmitter to deliver power to an Earth station, we must build that ground station, which (in principle) is easy and low-tech, but is still large (and thus expensive, partly because of land-use costs), plus we must build the power distribution network to get the electricity to the consumer. I do not have a good handle on this cost, which I am certain will vary tremendously from site to site, as it should be much cheaper where land is cheap (and power is not needed), and will be much more expensive close to major cities where the power is needed but land is expensive plus there is always resistance to putting the receiving rectenna in our back yards.
Note that the receiver may typically require an oval roughly three miles wide and six long, but it is sparse and may be placed over a farm with little impact on crops or livestock below. It may also be place in a forest just above the treetops, where it would not even be all that visible. As a very rough estimate, I will assume that the ground-side costs will equal the in-orbit costs per SPS, although I will also plan that all of the up-front capital costs are for the SPS itself (a simplistic approach that ignores the cost of lawyers and politicians).
My next post will discuss specific design considerations for a Solar Power Satellite.