Articles in the popular press have intrigued the public with wild ideas, some of which have even been legitimized under the rubric geoengineering. For instance, instead of reducing GHGs, why don’t we shield the earth from getting so much sunlight? This could be done by sending zillions of small plastic sheets up into orbit to reflect sunlight over large areas of the earth. It has also been suggested to use natural plant spores which have large area for their weight. This would not ride well with the resort business! More seriously, such a large-scale, uncontrolled experi­ment would have unpredictable consequences for our climate and for life itself. It may even trigger an ice age. Such proposals are, of course, science fiction.

The following proposal has been taken more seriously. If the sun does not shine all the time on terrestrial solar panels, why not put them in space? In a geostationary orbit, 22,000 miles (36 km) above the earth, the panels will receive the whole 1.366 kW/m2 of sunlight instead of the 1 kW/m2 that reaches the earth, and the weather is always clear. That’s only 37% more, but nights will be shorter since the satellite is so high that it will not always be in the earth’s shadow when it is nighttime on earth. Gyroscopes can keep the panels always pointed at the sun. If expensive multijunction silicon solar cells are used, the efficiency could be 40%. How much area would be required to produce the power of a coal or nuclear plant, say 1 GW (1,000 MW)? (There are thousands of such plants in a large country.) For the sake of argument, let us assume that the satellite panels get an average of 1 kW/m2. To generate 1 GW at 100% efficiency would require 1 million square meters or 1 km2 (0.39 square miles). At 40% efficiency, it would require 2.5 km2 or just about 1 square mile of panels. That is a lot to send into space! The panels would not last may years because they would be damaged by micrometeorites and solar flares. The moon’s gravity would make the satellites drift from their geosynchronous orbits, so a supply of propellant is necessary to make corrections. This supply cannot last many years either.

Then there is the problem of getting the power back to earth. It is proposed to transform the solar energy into microwaves and beam the energy back to the earth at a wavelength that is not absorbed by the atmosphere. Of course, this would be in a desert area with few storms and clouds, and that means building transmission lines to population centers. Microwaves are strongly absorbed by water vapor in the atmosphere. Low frequencies, like the 2.45 GHz (gigahertz) used in microwave ovens are well absorbed by water, which is why microwave ovens work in the first place. To get good transmission, the frequency has to be high, like 100 GHz. Such frequencies can be generated by gyrotrons, and the most advanced of these are being developed for the large fusion energy experiment ITER, which is described in Chap. 8. In the laboratory, a gyrotron has produced 1.67 MW at 110 GHz for 3 ps, and 800 kW at 140 GHz for 30 min [28]. Though continuous operation at such powers is expected to be attainable on earth, it may not be possible in space because of the lack of air and water for cooling. Gyrotrons are large devices containing heavy magnets into which energetic electron beams are injected. The magnets help convert the electron energy into microwaves, but not all the energy can be extracted because as the electrons slow down, they get out of sync. The best efficiency that can be hoped for is about 50%. The rest of the energy goes into a beam dump, which has to be cooled. One can build a heat engine that generates electricity from that heat to accelerate more electrons, but that would make the device even more complicated than it is already. There is a further loss at the receiving end in convert­ing the microwave energy into AC power. Even worse, high-power microwaves are known to break down air and make plasma that can scatter or reflect the micro­waves. Solar panels in space may gain a factor of 2 in available sunlight over those on land, but more than this is lost in transmission even if the technology can be developed. Regardless of the cost, this is a really bad idea!