Spacecraft cannot use air to turn. They need internal momentum. A reaction wheel spins a heavy mass inside the vehicle, and when the wheel accelerates one way, the spacecraft rotates the other. Newton's third law, applied to attitude control.

Three wheels handle the three axes, though most spacecraft carry four for redundancy. The wheels are heavy and power-hungry, but they use only electricity, no propellant, which makes them ideal for long missions.

Practical limits

• Wheels saturate as they reach maximum speed
• Desaturation requires thrusters or magnetic torquers
• Bearing wear limits lifespan
• Vibration can affect sensitive instruments

When a wheel fails, the spacecraft can often limp along on the remaining three, but it loses redundancy and may not be able to point at all targets. Hubble and Kepler both suffered wheel failures that changed their missions.