Attitude control is crucial for satellites to maintain their orientation in space, ensuring instruments are pointed correctly and communication links are stable. Actuators are the components that physically execute the commands from the attitude determination and control system (ADCS) to change the satellite's orientation. This module explores the primary types of actuators used in satellite attitude control.

Actuators convert electrical energy into mechanical motion or force. In the context of attitude control, they are responsible for generating torques that alter the satellite's angular momentum, thereby changing its attitude. The choice of actuator depends on factors such as required torque, precision, power consumption, mass, and cost.

Several types of actuators are commonly employed for satellite attitude control, each with its advantages and disadvantages.

Reaction wheels are perhaps the most common actuators for fine attitude control. They consist of a spinning rotor driven by an electric motor. By changing the speed of the rotor, a reaction torque is generated on the satellite body, according to the conservation of angular momentum. They offer high precision and are momentum-biased, meaning they are always spinning to provide immediate control authority.

Magnetorquers are coils of wire that generate a magnetic dipole moment when current flows through them. This dipole interacts with the Earth's magnetic field to produce a torque. They are simple, reliable, and consume little power but are limited by the strength and gradient of the Earth's magnetic field, making them less effective in higher orbits or for rapid maneuvers.

Thrusters expel propellant to generate thrust, which can be directed to produce torques for attitude control. They are capable of generating large torques and are essential for momentum dumping (offloading excess momentum from reaction wheels) and for large attitude maneuvers. Common types include cold gas thrusters, monopropellant thrusters, and bipropellant thrusters. Their main drawbacks are the consumption of propellant (limiting mission life) and the potential for contamination.

CMGs are more complex than reaction wheels but can generate significantly larger torques. They consist of a spinning rotor whose angular momentum vector can be changed by tilting the rotor's gimbals. This change in angular momentum produces a torque on the satellite. CMGs are typically used for large, rapid maneuvers, often on larger spacecraft like the International Space Station.

The selection of actuators is a critical design decision. A typical satellite might use a combination of actuators to leverage their respective strengths. For instance, reaction wheels might be used for precise pointing, while thrusters are used for momentum dumping and larger maneuvers. Understanding the mission requirements, such as pointing accuracy, maneuver speed, mission duration, and available power, is key to making an informed choice.