MICROWAVE KITE

Antarctica is precious because it is unique. Its rugged shores are swept by the strongest winds on the Earth. For example, the average wind velocity at Cape Denison near Tasmania (6659'S, 14239'E) is 19 m/s (68 km/h). Wind power is proportional to the cube of wind velocity, so Antarctica seems to be a wind power bonanza. It fact, there are only a few small wind turbines providing electric power for scientists living there. The wind power is not utilized because its transmission to other continents costs too much. Superconductive cables are not much cheaper than room temperature cables. Making hydrogen from water wastes most of the energy and is not economical.

There is nothing to do with this cheap wind power other than to beam it skyward. I mean it! Electric power can be easily converted to microwaves and beamed to a tungsten screen orbiting the Earth. The screen reflects the beam back to the Earth where it is needed. This is cheaper power system than solar power satellites. Let us imagine a screen that has diameter of 1 kilometer and altitude of 10000 kilometers. The screen spins about its center of mass to maintain its flat shape. The microwave beam has the power of 10^10 watts, which is the equivalent of the entire electric power output of a small country (Finland), or 5 Hoover Dam power plants. The beam exerts a force on the screen for the same reason that sunlight exerts a force on a solar sail. If the beam is not absorbed by the screen and is perpendicular to the screen (it is not), the force equals F = 2P/c = 67 newtons. (P is the power in watts, and c is the speed of light in meters per second.) If the screen has the mass of 45 kilograms, the beam force equals the force of gravity. This means that the screen does not orbit the Earth, but levitates over Antarctica. This is great news because it means that we do not have to chase it across the sky.

The stationary screen can be used as a sort of TV comsat -- it would be cheaper and more powerful than ordinary comsats. The screen diameter and the microwave power can be relatively small because the beam reflected by the screen does not have to be very narrow.

The tungsten screen will be hot because it will absorb some microwaves. This is not a problem because there is no air that can oxidize the screen. I do not know what percentage of the microwave beam will be absorbed by the screen. If the percentage is increased from 0% to 100%, the levitation force is reduced by half. Some buckytubes conduct electricity better than any metal, so they seem to be the best material for the screen in the long term. The maximum current density of these buckytubes is about one billion amperes per square centimeter -- 3 orders of magnitude more than the maximum current density of copper.

Screens made of buckytubes weigh little and do not absorb microwaves. They are so lightweight that they can levitate at low altitude, e.g., 1000 km, and support weight of a buckytube tether anchored in Antarctica. The tether holds the screen in place like a kite - no need for rocket scientists to control its orbit. One can envision a very long tether supported by several kites and transporting small cargoes to outer space. Several kites can guide the microwave beam from one hemisphere of the Earth to the other hemisphere. To reduce atmospheric drag in strong winds, the bottom of the tether is secured to a stratospheric blimp.

The microwave screens and kites perform best when the beam has constant power. Wind velocity varies a lot, so the wind energy must be stored in Antarctica to provide power on a windless day. Although pumping stations are very common on warm continents, there is a better way to store energy in the cold climate of Antarctica. Long tunnels are cut in antarctic ice sheet with warm jets of salty water. The wind turbines have pistons instead of electric generators and they pump compressed air into the tunnels. A large turbine transforms the potential energy of the compressed air into electric energy.

Magnetron is the most efficient device which transforms electric power to a powerful microwave beam. It is also the most efficient device which transforms the beam back to the electric power.

No bibliography. The maximum practicable cargo mass is on the order of 10 kilograms.