Newton’s third law of motion states that for every action, there is an equal and opposite reaction. Hubble utilizes this principle with its four reaction wheels, which are large, massive wheels that spin under the control of Hubble’s computer. If one of the reaction wheels turns clockwise, Hubble will turn counterclockwise. Changing the spin speed of any of the wheels produces a rotational force called torque. The combined changes in the spin speeds of the wheels allow Hubble to maneuver to point at any location in the sky. The four reaction wheels are each approximately two feet wide (smaller than a car tire), but in microgravity they can move the school-bus-sized Hubble because of their high spin rate and their mass. However, at its fastest, Hubble only moves as quickly as the minute hand of a clock (approximately 90 degrees in 15 minutes).