During each orbit of Venus, Magellan used to rotate four times: away from the planet to aim the HGA earthward for data transmission, towards space to scan stars for precisely determining any spacecraft-orientation errors, again toward Earth to resume data transmission, and back towards the surface of Venus for mapping. Throughout each elliptical orbit's mapping pass, the spacecraft continuously maneuvered in small increments to adjust the pointing of the HGA as the distance to the surface of the planet changed.
Throughout the mapping phase of the mission, there were 1,852 orbits requiring 7,408 major attitude changes in 243 days. If these attitude changes were performed solely with rocket thrusters, there would be more than 14,800 thruster burns for each of the spacecraft's three control axes (one to start a maneuver, another to stop it, and periodic thruster burns to control the rate). Indeed, Magellan would need immense fuel tanks.
However, Magellan had very less fuel in its single, small, propellant tank. The repetitive attitude changes were instead accomplished with reaction wheels that used the principle known as Newton 's Third Law: for every action there is an equal and opposite reaction.
When it was desired to turn Magellan in a particular direction, an electric motor inside the spacecraft was commanded to spin a reaction wheel (a rotatable mass approximately 36 centimeters in diameter) in the opposite direction. By Newton 's Third Law, the spacecraft turns in the intended direction while the reaction wheel spins in the opposite direction.
The job of keeping track of the spacecraft's current orientation was accomplished with gyroscopes and a star scanner. The gyroscopes sensed any rotational rate, which was then integrated by the attitude-control computers to determine the spacecraft's current orientation. When the spacecraft orientation was more than a preset amount (usually 0.0l degree) from what it should be, appropriate reaction wheels were commanded to speed up or slow down until the orientation was again within bounds.