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Mr. Cook raises a common suggestion for clean energy.

Fusion power, in theory, seems like an ideal solution. In fact, in 1977, I worked with a group of excellent physicists and engineers who hoped to get a contract to design a test facility. The contract went elsewhere, but I have been watching developments before and since that time.

The science is fairly well understood, but the technological and engineering problems involved in confining and sustaining a plasma long enough to draw more energy out than you put in have been daunting. To use a bumper-sticker phrase, it's like trying to make a sun in a bottle.

In the 1970s, engineers were predicting that a practical fusion power plant was 30-40 years in the future. These days, although there has been continuous R&D work in several countries, including international collaborations, the most optimistic researchers are still saying it will probably take a few decades for fusion to be a practical source of power.

Now no one can predict a breakthrough in such a circumstance, and I certainly advocate continuing research at its current level (at least). But we certainly can't rely on fusion.

It remains "the power source of the future." The only problem is we don't know when that future will be.

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)