Fig. I. Mass and volume scales for power systems. sun. One could have fusion energy on a large scale without having to build the thousands to tens of thousands of fusion reactors necessary to provide 20,000 GW or more of power to Earth early in the 21st century. Nuclear fusion will likely require much larger and more complex installations on the ground to produce power that will be required to receive microwave energy from SPS or LPS. Parkins (1978) (28) estimated that the mass of very high technology components (superconducting magnets, power recycling circuits, power extraction devices, vacuum containers) around a 10 GWe complex of fusion reactors would exceed 300,000 tons. These components will generally be as inaccessible as the core of a nuclear fission reactor. The 5,000 tons of simple and open electrical equipment for receiving power from space will certainly be much cheaper than the highly sophisticated components of a 10 GWe fusion power plant. Both short and long life radioactive materials will be produced in fusion plants. Turning off a fusion reactor will not turn off the induced radioactivity in the reactor. In addition, it will not be clear for several decades whether or not controlled fusion is possible within reasonable engineering limits. SPS presents major problems. The first is getting the components from Earth to space for assembly. This is a very major challenge which can be appreciated by referring to the stream of water shown in Fig. 1. Suppose hydrogen and oxygen are used to fuel the rockets taking SPS components to orbit. The quantity of water which would have to be electrolyzed and cooled into cryogenic liquids to place one SPS into low orbit corresponds to a stream 660 m long by 300 m across and 10 m deep. This is liquid handling on the scale of four supertankers. Rockets of 10,000 tons gross
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