While they are designed to fulfill separate and specific mission objectives, the craft share much in common. Each spacecraft consists of various scientific instruments selected for a particular mission, supported by basic subsystems for electrical power, trajectory and orientation control, as well as for processing data and communicating with Earth.
Electrical power is required to operate the spacecraft instruments and systems.
NASA uses both solar energy from arrays of photovoltaic cells and small nuclear generators to power its solar system missions. Rechargeable batteries are employed for backup and supplemental power. To help prevent such a mishap, a subsystem of small thrusters is used to control spacecraft. The thrusters are linked with devices that maintain a constant gaze at selected stars. Just as Earth's early seafarers used the stars to navigate the oceans, spacecraft use stars to maintain their bearings in space.