How do reaction wheels control spacecraft attitude?

Reaction wheels are spinning flywheels that use conservation of angular momentum for attitude control. When wheel speed changes, the spacecraft body rotates in the opposite direction (Newton's third law). Advantages: Precise pointing, No propellant consumption, and Continuous operation. Configuration: Typically 3 wheels (one per axis) plus redundant wheel, or 4 wheels in tetrahedral arrangement. Limitations: Saturation - wheels reach maximum speed over time due to external torques; requires momentum dumping using thrusters or magnetic torquers. Power consumption while operating. Mechanical failures (bearings) limit lifetime. Applications: Earth observation, communications satellites, space telescopes. Wheel size ranges from CubeSat momentum wheels (~10 mNms) to large wheels for massive spacecraft (~100+ Nms).