The structural body and primary system of a space vehicle is commonly referred to as a spacecraft bus. The spacecraft bus is used as a transport mechanism for a spacecraft payload much like an ordinary city bus is a transport vehicle for its passengers. Although each spacecraft payload may be quite different from another, all spacecraft buses are similar in their makeup. The spacecraft bus consists of several different subsystems, each with a unique purpose. The structural subsystem consists of the primary structure of the spacecraft, and supports all the spacecraft hardware, including the payload instruments. The structure, which can take various forms depending on the requirements of the particular mission, must be designed to minimize mass and still survive the severe forces exerted on it during launch and on its short trip to space.
The electrical power subsystem provides power for the payload, as well as the rest of the bus. This is usually achieved through the use of solar panels that convert solar radiation into electrical current. The solar panels sometimes must be quite large, so they are hinged and folded during launch then deployed once in orbit. The subsystem also may consist of batteries for storing energy to be used when the spacecraft is in Earth's shadow. Another major subsystem is command and data handling, which consists of the computer "brain" that runs the spacecraft, and all the electronics that control how data is transported from component to component. All other subsystems "talk" to this subsystem by sending data back and forth through hundreds of feet of wiring carefully routed throughout the spacecraft bus.
The communications subsystem contains components such as receivers and transmitters to communicate with controllers back on Earth. Many operations the spacecraft must perform are controlled through software commands sent from Earth by radio signals. Another important subsystem is the attitude control subsystem. This consists of specialized sensors able to look at the Earth, Sun, and stars to determine the exact position of the spacecraft and the direction in which it should point. Many operations spacecraft perform require very precise pointing, such as positioning imaging satellites that must point at specific spots on Earth.
In order to adjust the orbit to maintain the spacecraft in orbit for many years, a propulsion subsystem is sometimes required. There are many types of propulsion systems, but most consist of various types of rocket thrusters, which are small engines that burn special fuel to produce thrust. One additional crucial subsystem worth discussion is the thermal control subsystem, which maintains the proper temperatures for the entire spacecraft bus and all its components. This is achieved through the use of small heater strips, special paints and coatings that either reflect or absorb heat from Earth and the Sun, and multi-layered insulation blankets to protect from the extreme cold of space.
see also Exploration Programs (volume 2); Government Space Programs (volume 2); Robotic Exploration of Space (volume 2); Sensors (volume 2); Solid Rocket Boosters (volume 3).
Fortescue, Peter W., and Stark, John P. W. Spacecraft Systems Engineering. Chichester, West Sussex, UK: John Wiley & Sons, 1991.