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A spacecraft is a vehicle designed to leave Earth's atmosphere and operate beyond the surface of the Earth in outer space. Spacecraft may either be unmanned or manned. Spacecraft are designed for a variety of missions which may include communications, earth observation, meteorology, navigation, planetary exploration, space tourism and space warfare. The term spacecraft is also used to describe artificial satellites.

A spacecraft is a system made up of various subsystems, dependent upon mission profile. Spacecraft subsystems may include: attitude determination and control, guidance, navigation, and control, communications, command and data handling, power, thermal control, propulsion, structures, and payload. Manned spacecraft have the additional requirement of providing life support to the crew. Though not part of the spacecraft itself, the launch vehicle used to place a spacecraft in orbit.

Spacecraft must be engineered to withstand launch loads imparted by the launch vehicle, and must have a point of attachment for all the other subsystems. Depending upon mission profile, the structural subsystem might need to withstand loads imparted by entry into the atmosphere of another planetary body, and landing on the surface of another planetary body.

Spacecraft need an attitude control subsystem in order that they may be correctly oriented in space and respond to external torques and forces properly. The attitude control subsystem consists of sensors and actuators.

Guidance refers to the calculation of the commands needed to steer the spacecraft where it is desired to be. Navigation means determining a spacecraft's orbital elements or position. Control means adjusting the path of the spacecraft to meet mission requirements.

The communications subsystem, sometimes called the Telemetry, Tracking, and Control subsystem serves as an interface between the spacecraft and the ground system, or between the spacecraft and other spacecraft. The communication subsystem receives telecommands from the ground subsystem, and transmits telemetry from the spacecraft.

Spacecraft need an electrical power generation and distribution subsystem for powering the various spacecraft subsystems. For spacecraft near the Sun, solar panels are frequently used to generate electrical power. Spacecraft designed to operate in more distant locations, for example Jupiter, might employ a Radioisotope Thermoelectric Generator to generate electrical power.

Electrical power is sent through power conditioning equipment before it passes through a power distribution unit over an electrical bus to other spacecraft components. A battery (or batteries) are typically connected to the bus via a battery charge regulator, and the battery is used to provide electrical power during periods when primary power is not available, for example when a Low Earth Orbit (LEO) spacecraft is eclipsed by the Earth.

Spacecraft must be engineered to withstand transit through the Earth's atmosphere and the space environment. They must operate in a vacuum with temperatures potentially ranging across hundreds of degrees Celsius. Depending on mission profile, spacecraft may also need to operate on the surface of another planetary body.

Spacecraft may or may not have a propulsion subsystem, depending upon whether or not the mission profile calls for propulsion. Typically though, LEO spacecraft include a propulsion subsystem for altitude adjustments and inclination adjustment maneuvers. Components of a conventional propulsion subsystem include fuel, tankage, valves, pipes, and thrusters.

The ground system, though not technically part of the spacecraft, is vital to the operation of the spacecraft. Typical components of a ground system in use during normal operations include a mission operations facility where the flight operations team conducts the operations of the spacecraft, a data processing and storage facility, ground stations to radiate signals to and receive signals from the spacecraft, and a voice and data communications network to connect all mission elements.

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