The Command and Data Handling (C&DH) subsystem is the central nervous system of a spacecraft. It receives commands from the ground, processes them, and generates control signals for other subsystems. It also collects, processes, stores, and transmits scientific and engineering data. The core of the C&DH is the onboard computer, which runs the spacecraft's flight software. This subsystem is critical for autonomous operations and for responding to unexpected events.

The Electrical Power System (EPS) is vital for providing the energy needed for all spacecraft functions. The primary power generation method for most spacecraft is solar arrays, which convert sunlight into electricity. This electricity is then used directly or stored in batteries for use during eclipses or when solar power is insufficient. The EPS also includes power distribution units and voltage regulators to ensure stable power delivery to all subsystems.

For crewed missions, the Environmental Control and Life Support System (ECLSS) is paramount, providing breathable air, managing temperature and humidity, and removing waste. For uncrewed spacecraft, a similar but simpler thermal control system is essential to keep electronics within their operational temperature ranges, protecting them from extreme heat and cold in space.

The Communication System is the spacecraft's link to the outside world. It comprises antennas for transmitting and receiving signals, transponders that amplify and retransmit signals, and transmitters and receivers. The choice of frequency and antenna type depends on the mission's data rate requirements and the distance to Earth or other communication nodes. High-gain antennas are often used for long-distance communication.

The Attitude Determination and Control System (ADCS) is responsible for pointing the spacecraft accurately. It uses sensors to determine the spacecraft's orientation (attitude) and actuators to adjust it. Sensors include star trackers, sun sensors, and gyroscopes. Actuators can be reaction wheels (which spin to create counter-torque), thrusters (for larger maneuvers), or magnetic torquers. Precise attitude control is crucial for pointing instruments, maintaining communication links, and controlling orbital trajectory.

The Propulsion System is what allows a spacecraft to change its velocity, which is essential for orbital insertion, station-keeping (maintaining a specific orbit), and attitude control. Common types include chemical propulsion (using propellants like hydrazine) and electric propulsion (such as ion thrusters, which are more fuel-efficient but provide lower thrust). The choice of propulsion system depends on the mission's delta-v (change in velocity) requirements and duration.

The Payload is the reason the spacecraft exists. It encompasses all the scientific instruments, cameras, sensors, communication equipment, or other devices that carry out the mission's objectives. For an Earth observation satellite, the payload might be high-resolution cameras; for a scientific satellite, it could be a telescope or particle detector. The payload is often the most complex and expensive part of the spacecraft.

The spacecraft's structure provides the physical framework that holds all the components together and withstands launch forces. The thermal control system manages the spacecraft's temperature, preventing components from overheating or freezing. This can involve radiators, insulation blankets, heaters, and heat pipes. Protection from radiation and micrometeoroids is also a critical aspect of the spacecraft's design.