If a spacecraft's guidance and control system is its nervous system, then sensors are its eyes and actuators are its muscles. One group works out which way the vehicle is facing and how it's moving; the other group does something about it. Understanding how these two families of devices work together is the key to understanding how a spacecraft controls itself in space.

‍

The Two Halves of Control

Every control system, on a spacecraft or anywhere else, needs to know two things and do one thing. It needs to measure what the vehicle is actually doing, decide what it should be doing, and then act to close the gap.

• Sensors handle the measuring. They report the spacecraft's output - things like its position, its rate of rotation and its acceleration.

• Actuators handle the acting. They apply the forces and torques that change the spacecraft's motion or orientation.

In between sits the controller, which compares where the spacecraft is with where it should be and tells the actuators what to do. Sensors in, actuators out.

‍

The Eyes: Sensors That Tell the Spacecraft Where It's Pointing

A spacecraft can't see itself from outside, so it relies on a suite of sensors to work out its orientation in space. Several different types are commonly used, each with its own strengths:

• Sun sensors, which detect the direction of the Sun.
• Star sensors and star trackers, which identify patterns of bright stars to fix the spacecraft's orientation very precisely.
• Horizon sensors, which find the edge of the Earth (or another body) to establish a reference.
• Magnetometers, which measure the local magnetic field.
• Inertial measurement units, which use internal devices such as gyroscopes and accelerometers to track changes in orientation and motion.

These sensors are often used together, because each has limitations. Inertial measurement units, for instance, are excellent at tracking change over short periods but can slowly drift over time, so a star tracker's precise fixes can be used to correct that drift. Combining sensors gives a more reliable picture than any one device alone.

‍

The Muscles: Actuators That Make the Spacecraft Move

Once the spacecraft knows it needs to turn or adjust, actuators provide the muscle. The main types each work on a different principle:

• Thrusters, which can be chemical or electrical, fire propellant to produce a force - useful for turning the vehicle and for changing its path.
• Reaction or momentum wheels, spinning wheels inside the spacecraft that let it rotate without using any propellant, by exchanging momentum with the wheel.
• Magnetorquers, which interact with a planet's magnetic field to produce a gentle turning effect.

The art of attitude control is choosing the right actuator for the job - wheels for fine, propellant-free pointing; thrusters for larger or faster manoeuvres; magnetorquers for slow, steady adjustments where a magnetic field is available.

‍

Why They Have to Work as a Team

Neither sensors nor actuators are useful on their own. A spacecraft with perfect sensors but no actuators would know exactly how it was drifting out of position but could do nothing about it. A spacecraft with powerful actuators but no sensors would be able to push and turn, but blindly, with no idea whether it was making things better or worse.

It's the loop between them - sense, decide, act, then sense again - that gives a spacecraft real control. The sensors confirm whether the actuators' actions had the intended effect, and the cycle repeats continuously, keeping the vehicle pointed correctly despite the constant small disturbances of space.

‍

A Practical Way to Think About It

When you're trying to remember which device does what, sort everything into "measures" or "moves." Anything that detects the Sun, the stars, the horizon, a magnetic field or the spacecraft's own motion is a sensor; it measures. Anything that fires, spins or pushes to change the spacecraft's orientation is an actuator; it moves. Almost every component in the guidance and control system falls cleanly into one of those two buckets.

‍

Conclusion

A spacecraft controls itself through a partnership: sensors that act as its eyes, reporting its orientation and motion, and actuators that act as its muscles, applying the forces and torques to correct it. Sun sensors, star trackers, magnetometers and inertial units feed the picture; thrusters, reaction wheels and magnetorquers respond. Linked together in a continuous loop, they let a spacecraft hold its pose and steer through space with remarkable precision. If you'd like to see how that loop is actually wired together, the natural next step is understanding open loop versus closed loop control.