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For operational purposes, solar-cell power systems are being used in the 30 or 40 different spacecraft now planned for flight. The range of power is from 15 to 500 watts. Both nonorientation and orientation of paddles to the sun are being used. An approximate break-even point between excessive weight of solar cells for nonoriented systems and weight of the orientation servomechanism system for solar-oriented systems occurs for a power level of 100 watts. Nickel-cadmium batteries have been used for the storage of electrical energy, whether for providing power during shadow periods encountered by low-altitude spacecraft or providing peak power during telemetry for interplanetary flights. The specific weight of such power systems varies from 750 or 1000 lb/kw to 500 lb/kw for orbits from 300 miles (statute) to 22,000 miles (synchronous orbit). Reduction in weight of batteries as well as solar cells needed for battery charging as altitude increases is responsible for this decrease in specific weight because of the decreasing time of shadow encountered by an orbiting spacecraft.