TABLE OF CONTENTS Constraints on available spacecraft power have imposed major limitations on space vehicle design since the beginning of the space age. The earliest orbiting vehicles flown by both the United States and Russia depended on batteries. The limited energy storage capabilities of the batteries then available prevented operations of more than a few days. This was not satisfactory for missions of the duration required for detailed scientific observations or military reconnaissance, and solar power arrays quickly appeared on the scene. Although not highly efficient in turning sunlight into electricity, solar arrays (or solar panels) were in many ways admirably suited to powering spacecraft. Because no consumables were used in generating electrical power, the life expectancy of the power system was limited only by degradation of the components of which it was composed. Spacecraft operating lifetimes of several years became feasible with the development of these photoelectric arrays, with batteries used to handle peak load requirements and to provide energy storage for those periods when the spacecraft was in eclipse.
Solar panels and batteries in combination have powered the majority of unmanned spacecraft so far launched. Exceptions include a few short-lived battery-powered systems, some outer-planet missions using radioisotope thermoelectric generators (RTGs), and some spacecraft (mostly Russian radar imaging satellites) powered by nuclear reactors. Early manned spacecraft, including Mercury, some Gemini spacecraft, and the Russian Vostok/Voshkod vehicles (which were essentially the same design) used batteries. The later Gemini spacecraft and the Apollo command and service module (CSM) and Lunar Module (LM)...
