Solar Power from the Moon
Section 2.8.
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The World's Energy Future Belongs in Orbit

Dr. Gerard K. O'Neill

This is an abridged paper written by the late Dr. Gerard K. O'Neil, a renowned proponent of space development and solar power satellites.

Another option commonly considered but not discussed in this paper would be to mass-produce the solar cells right on the Lunar Surface, and to beam the energy to Earth, or elsewhere in space, from there. This avoids costly transportation, despite the facts of intermittant nights and low Sun angles.

The Link Between Energy and Industrialization

The central problem of industrial growth is energy. Building more energy efficient systems, especially transportation systems, is a way we can reduce energy needs. Much of my current efforts focus on the design of a practical transport system which can be faster than the Concorde, is virtually free of noise and environmental impact, and uses less than five times the energy as an efficient car, train, or airplane: vehicles traveling at or near surface level in a vacuum, supported and driven by magnetic forces, and guided by computers.

Despite such examples, reducing energy use overall is not an option, because producing most foods, goods, and services depends on energy. The least-developed industrial nations use only a hundredth as much energy as the most developed - but they suffer a living standard a hundred times lower in consequence. We could not reduce energy use without condemning the majority of the world's population to unending poverty.

Maintaining the present rate of energy growth in developing nations will require generating more than five times today's energy production 50 years from now. It is so difficult and challenging to do that in an environmentally benign way that we must begin by throwing out most of the ideas we have heard so far.

We are concerned enough with the rise of carbon dioxide and the greenhouse effect that our desire, as environmentalists, is to phase out the use of fossil fuels. That can be done, even for road transportation, if electricity can be made cheaply enough. Fuels like methane, propane, and butane can be synthesized using recycled carbon dioxide to put energy into portable form.

But how do we generate energy cheaply, reliably, without significant environmental impact, and without danger? Nuclear power is not very cheap, and would require operating 63,000 nuclear reactors worldwide by the middle of the next century. Reactors concentrate energy to an extreme degree, which is why there have been fatal reactor accidents in the real world of fallible, error-prone people.

Solar power received at ground level cannot be converted to energy in the amounts needed without paving much of the world with solar cells. That would further raise the Earth's temperature by increasing the heat the ground absorbs. Obtaining power from the temperature difference between surface and deep ocean waters in the tropical oceans would change the global climate profoundly by altering the heat balance at the ocean's surface. Fusion power appears still far from realization, and would not be free of radioactivity.

When conventional ideas are exhausted and no solution has been found, it is often wise to examine whether we have, unknowingly, bound our own thinking by convention. In this case we have, because all of the usual answers concerning generating energy are bound by our unspoken limitation to the surface of the Earth. I like to illustrate that tradition-bound thinking by posing the six-match puzzle. Put six matches on a table, do not break any, and use them to make four equal triangles. People who attempt the puzzle usually push the matches about the table for awhile and then give up. The solution is simple: make an equal-sided triangle of three matches, and hold a new match at each point of the triangle with the other ends of those last 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 from 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 Moon 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 as 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.

The moon's second advantage is it has no atmosphere. The combination of the moon's weak gravitational grip and its vacuum environment makes it practical to locate electric mass accelerators on its surface which would be capable of lofting a steady stream of small payloads to a precise collection point high in space.

Such machines, called "mass-drivers," were tested nearly a decade ago under the sponsorship of [a] small, quiet, nonprofit foundation, the Space Studies Institute (SSI). Mass-drivers were shown to obey their computer design programs within one percent - no new science there - just straightforward engineering. Since then SSI has sponsored laboratory research on making useful products from ores similar to lunar soils.

Can SPS Technology Deliver?

As people concerned about our environment and about the world we leave to our children we should question proposed solutions to major physical problems. As fossil fuels, nuclear energy, ground-based solar, and other conventional sources of energy all fail to make sense in the world.

First of all, there is plenty of energy in space. Even in a narrow band 25,000 miles above the equator, where a satellite can maintain a fixed orbit, plenty of solar energy streams by constantly to supply far more than enough energy for the Earth of 2050.

What of the conversion on Earth? It was demonstrated years ago. The antennas convert the radio waves with an efficiency so high that less than 100 watts of waste heat goes into the environment for every 1,000 watts that goes into power lines. For coal or nuclear the numbers are: 1,500 watts waste, 2,500 watts total; for ground-based solar they are several thousand watts waste plus another thousand to make up the total - different from an Earth without solar cells - because solar cells absorb more heat than the ground they cover.

Transmission is the question that deserves continuing research: How to send the low-density radio waves from an SPS to antennas on the Earth. I have satisfied myself that transmission does not involve significant risks. But I invite you to do your own research. One of the best sources on the subject is The Microwave Debate by N.H. Steneck (MIT Press).

The points that seem to me most important about radio transmission of energy are that people would not be in the beams; that for fundamental physical reasons the beams could not be intentionally or accidentally redirected; that their intensity would be comparable to sunlight; that unlike the massive shielding around a nuclear reactor, the only shielding necessary would be a layer of household aluminum foil; and that, unlike the present irreversible dumping of 5,000 megatons per year of fossil-fuel carbon dioxide into the atmosphere, or the generation of long-lived nuclear wastes, the SPS system would leave no chemicals or radioactives behind if our descendants decided to turn it off.

Solar Power from the Moon

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