For half a century we have been struggling and failing to do what the sun does naturally. We have been trying to take the nuclei of four atoms of hydrogen and fuse them together to make the nucleus of one atom of helium which, if done in sufficient quantity, provides as a by-product a huge amount of energy.

I do mean huge. Conversion to helium of the hydrogen in a few pints of water yields as much energy as burning a million gallons of heating oil. Successful nuclear fusion would seem to solve all the world's energy problems for all time. Unfortunately, so far we have spent many hundreds of millions of dollars and can claim one doubtful accomplishment: the hydrogen bomb.

Many people have argued that there is a better way; in fact, there is a perfect, non-polluting alternative. Since the sun is already doing the job, why not take advantage of it? Solar energy rains in on the Earth every day, and will continue to do so for billions of years. A tiny fraction of the sun's radiant energy is all we need or want for the foreseeable future.

Like most perfect solutions, this one has problems. True, the sun does shine every day, but as anyone knows who owns a house with solar panels, clouds and seasons change the amount of solar energy from day to day and from month to month. Moreover, for half of each day you can collect no energy at all because the sun is shining on the wrong hemisphere of the earth. Finally, anything that collects solar energy will be affected by weather and pollution, and will gradually decrease in efficiency and have to be replaced.

However, there is a possible solution to all these problems. Why not place your collection system out in space, and beam the energy down to the ground as microwaves?

Solar-powered satellites, as these systems are called (abbreviated to SPS), have been designed in great detail. There are three main components. First, a large array of photo- receptors, kilometers across, out in space. Each receptor captures sunlight and turns it to electricity. The second component is a device that converts electricity to a beam of microwave radiation and directs it toward earth. The third component is a large array on the surface of the earth, usually known as a rectenna, which receives the microwave radiation and turns it into electricity for distribution nationally or internationally.

Again, we have what looks like a perfect solution. If the collection array is placed in geosynchronous orbit, out where we keep many of our communications and weather satellites, there will be power interruption as the earth comes between the satellite and the sun for only an hour or so each day. The collection system contributes no pollution on Earth, nor does it generate the waste heat of ground-based power production. The rectenna on the ground must be kilometers across, but can be placed anywhere. Also, the radiation level at any particular point will not be high enough to cause damage. Some designs propose that the rectenna be placed on rangeland, with cattle grazing comfortably around the components. Finally, we will never run out of solar power, as we will one day certainly run out of fossil fuels.

Of course, the SPS can't be built without a powerful in-space manufacturing capability, something we lack today. But suppose that we had such capability. Is the SPS then a likely part of our future?

I hate to rain on the solar parade because I have friends who are big fans of the idea, but I have many reservations. If it takes half a century to develop the needed space-manufacturing skills, in that time we should have better energy production methods. Maybe we will even have controlled fusion. An orbiting fusion plant (or, for that matter, a nuclear power station like the ones we have today) would have the advantages of the SPS, without calling for the construction of a kilometers-wide collection array in space.

In addition to technological difficulties, there are economic and political ones. What might work well for rich nations will not work for poor ones. For example, SPS may seem ideal to serve the continent of Africa. This evades a key question: who will pay for the energy? Economists distinguish two kinds of demand: real demand, the need for energy of people with money to buy it; and other demand, the need for energy of people without money. Regrettably, much demand for energy is in nations with no way to pay.

Even if we solve the economic disconnect, there will still be political issues. Suppose that you build an SPS and beam down, say, 5 gigawatts to an African rectenna. That's the power delivered by a substantial fossil fuel station. However, you could generate that much energy by building a dam on the Congo River, where it drops sharply from Kinshase to the Atlantic. Now, which would you prefer if you were an African? The SPS, providing power from a source over which you have no control and whose location you might not even be able to visit? Or a dam, which in spite of all its defects sits on African soil?

Even with these problems, there are still strong SPS advocates. A recent NASA study suggested that a 400-megawatt SPS could be built and launched for $5 billion. Do I believe that number? Not in this world. We all know that paper studies often diverge widely from reality, and projects look a lot easier before you get down to doing them. Recall the euphoria for nuclear power plants in the 1940s, "electricity too cheap to meter." And that was for something we had more experience with than monster space structures.

If I sound like a technological pessimist, or even a technophobe, I'm not. I believe that we will find better answers to our energy problems. I also believe there will come a time when the idea of building power generation plants near population centers will be as unacceptable to us as the unpleasant Middle Ages habit of allowing the privy to drain into the well.

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