How does The Hubble telescope orient itself in space?

Source: Robert Frost, Quora

Reaction wheels. There is a set of flywheels that can spin at high speed. Changing the speed of a wheel changes the angular momentum of the vehicle, resulting in a torque that rotates the vehicle.

The Fine Guidance Sensors (FGS) line up guide stars to determine Hubble’s attitude within 0.01 arc seconds. The reaction wheels then rotate the vehicle to the desired attitude and hold it within 0.007 arc seconds.

Reaction wheels work by creating torques that counter other torques that are influencing the attitude of the vehicle. They do this by manipulating their angular momentum.

Angular momentum (H) is equal to the product of the moment of inertia (I) and the angular velocity (omega). The moment of inertia is dictated by the physical properties of the wheel (mass, size, and shape). For most reaction wheels, it is a constant value. The angular velocity is the rotation rate and rotation direction of the wheel. That is easily adjustable by adding power to the motors that control the wheel.$$\Large \vec{H}=\vec{I \omega}$$Torque is equal to the change in angular momentum over a period of time.$$\Large \vec{\tau}=\frac{d \vec{H}}{dt}$$While the wheel is changing its angular momentum, a torque is being produced. A torque is a twisting force that causes a rotation.

So, imagine our spacecraft is a simple cube with three wheels attached to it: one on each primary axis (x,y,z).

If some other torque acts upon the vehicle (gravity gradient, atmospheric drag, or even internally caused torques like vents or leaks), the vehicle software will identify the rates on the vehicle and calculate the torques needed to null those rates. The torque for each axis is calculated and then rate commands are sent to each wheel. During their acceleration they produce the needed torques.