Just as NASA's Mars rovers rely on robust wheels to roam the Red Planet and conduct science, some orbiters rely on wheels, too—in this case, reaction wheels—to stay pointed in the right direction. Engineers and technicians at NASA's Jet Propulsion Laboratory in Southern California recently installed four reaction wheels on Europa Clipper, which will rely on them during its journey at Jupiter's icy moon Europa.

When NASA's spacecraft heads through deep space, slips into orbit around Jupiter, and collects science observations while flying dozens of times by Europa, the wheels rotate the orbiter so that its antennas can communicate with Earth and its science instruments, including cameras, can stay oriented.

Two feet wide and made of steel, aluminum, and titanium, the wheels spin rapidly to create torque that causes the orbiter to rotate in the opposite direction. Isaac Newton's third law of motion also applies in deep space and explains the underlying phenomenon: For every action, there is an equal and opposite reaction. The reaction wheels cause the spacecraft to react to the spinning action of the wheels.

Here's one way to visualize how reaction wheels work: Imagine you are sitting in a swivel chair and lift your feet off the floor so that you are free to rotate. If you jerk your torso one direction, the chair and your legs will rotate the opposite direction. The reaction wheels work the same way: As the reaction wheel's motor accelerates the metal wheel in one direction, the spacecraft experiences an acceleration in the opposite direction.