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All space vehicles require some source of power for operation of the instruments and the radio communication equipment which makes possible the remote recording of the data obtained. Current satellites and space probes require relatively low power; for example, Pioneer V, whose data were recorded at distances up to 221/2 million miles from the earth, carried out its mission with a power of 30 watts.Many satellites use chemical energy stored in nickel-cadmium or silver-zinc base batteries, which have a limited life dependent on the battery weight and power requirements. Others, such as Vanguard I, use solar power, giving much longer life. Vanguard I is still transmitting after three years in orbit.Many of the hurdles that have stood in the way of the use of solar energy for generating electric power on the earth's surface are absent in the case of space vehicle applications The difficulties of earth stations have been associated primarily with the prolonged periods of cloud cover, the relatively long periods of complete darkness at night, and the bulkiness and unwieldiness of the solar energy collectors. To provide a continuous flow of power under these conditions requires a large energy storage device. Since earth satellites may enter the earth's shadow, energy storage is also required in space applications of solar power, but the requirements can be met with less difficulty because of the shorter periods of darkness. Solar Cells versus Chemical Batteries.-It is of some interest to make a brief comparison of solar energy devices with chemical sources. At the position of the earth's orbit, the solar radiation density is about 0.140 watts per square centimeter or 130 watts per square foot. The most frequently used conversion device is the photovoltaic silicon cell usually called simply a solar cell. These cells have relatively low efficiency because of losses of energy by reflection and transmission. It is reasonable to expect an output of about 10 watts per square foot from an oriented solar array. One specific design for a future spacecraft indicates a total weight of the complete solar power system of about 2 pounds per square foot. The net specific output is then approximately 5 watts per pound. By comparison, a silver-zinc chemical battery stores energy with a specific weight of about 50 watthours per pound. Since the weight of a battery is proportional to the total energy stored, whereas the weight of the solar cell source is more nearly proportional to the rated power, it is clear that a solar energy source would be far superior to a chemical battery when the power is to be used for a long time. For example, in the case cited, the solar cell array would be lighter than the silver-zinc battery if the power is required for a time longer than 10 hours.Power Requirements for Spacecraft.-An indication of the important part played by solar energy in the exploration of space may be found from a review of the estimated electric power requirements for some present and future space vehicles given in Figure 1. Some ty...

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