(aerospace engineering) An on-board assemblage of equipment to generate and distribute electrical energy on satellites and spacecraft.

On-board assemblages of equipment to generate, store, and distribute electrical energy on satellites and spacecraft. A reliable source of electrical power is required for supplying energy to the spacecraft and its payloads during launch and through several years of operational lifetime in a space environment. Present-generation spacecraft fly power systems from tens of watts to several kilowatts. Each of the three fuel cells on the space shuttle delivers 12 kW continuous power and 16 kW peak power. The International Space Station's solar arrays will generate 110 kW total power, with approximately 46 kW available for research activities. See also Satellite (spacecraft); Space flight; Space probe; Space shuttle; Space station.

With few exceptions, the power systems for United States satellites have used photovoltaic generation for power, batteries for energy storage, and a host of electrical equipment for appropriate regulation, conversion, and distribution. Fuel cells have been limited primarily to use in the crewed space program, supplying power for Gemini, Apollo, and the space shuttle. Radioisotope thermoelectric generators (RTGs) have powered, or augmented solar power on, many planetary missions and probes, powered lunar instruments left on the Moon by the Apollo missions, and augmented the solar-array battery power system on at least one Earth-orbiting spacecraft. See also Energy storage; Fuel cell; Solar cell.

Many factors influence the final configuration of a power system. Basic to the initial consideration are the nature of the mission (Earth-orbiting or planetary) and mission lifetime. Other relevant factors include (1) spacecraft and payload requirements with consideration to average and peak power loads; (2) effect of environment such as orbit period, length of time the spacecraft is in sunlight and shadow, radiation particle damage, and space charging; and (3) constraints imposed by the spacecraft such as weight, volume, spacecraft shape and appendages, satellite attitude control, electromagnetic radiation limitations, characteristics of payloads, and thermal dissipation. For spacecraft that are shuttle-launched, additional considerations include compatibility with shuttle payload safety requirements and, in some cases, the retrievability by the shuttle for return to Earth. The weight of the power system ranges from 15 to 25% of the spacecraft weight.