three matches meeting above the table. You have made a perfect tetrahedron - and you have escaped two-dimensional thinking to find a solution in three. LETTING THE SUN SHINE IN By close analogy, the probable best solution to our energy problem is to use the third dimension. There is, already, an utterly reliable, maintenance-free, nuclear reactor that consumes its own wastes-the sun. Everywhere except where the Earth's shadow interrupts it, sunlight is intense and reliable 24 hours a day; everywhere, except where we have tried to use it so far. There is a way to use the clean solar energy that now streams by the Earth to lose itself in the depths of space. It does not require new science, nor high temperatures, nor high energy density. It is simply to locate, in high orbit, large arrays of solar panels. Those panels convert solar energy to electricity, and then to low-density radio waves. The waves are sent to fenced-off areas on Earth where they are converted back to electricity at an efficiency rate of more than 90 percent. To make solar power satellites (SPS) practical and economical, we do not need any new science; we only need to apply what we are already doing in the more advanced industries: robotic production, computer control, and the replication by robotic machines of some of their heavier, simpler components. We do need one more thing: materials. It is neither practical, nor economical, nor environmentally acceptable to lift frqm the Earth by rockets the thousands of tons of materials needed to build an SPS that would supply Earth electricity equal to the output of ten nuclear power plants. LET THE MOO PITCH IN Fortunately, we do not have to. We were given something unique in our solar system: an enormous moon, orbiting tantalizingly nearby, and containing on its surface just the materials we need. Lunar soils contain 20 percent silicon for solar cells, and about 20 percent metals. Much of the rest, surprisingly enough, is oxygen. The moon has two other great advantages a a source of materials: its gravitational pull is only one-sixth of the Earth's, and because of its small diameter, the moon's gravitational grip is less than a twentieth of the Earth's. TRILOGY .JANUARY/FEl3RUARY 1992 INDUSTRY
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