Reaction wheels are devices used in spacecraft to control their orientation in space. They consist of spinning wheels that create angular momentum. When a wheel spins in one direction, the spacecraft rotates in the opposite direction, allowing for precise adjustments to its position. These wheels are essential for maintaining stability and pointing accuracy for instruments like cameras and antennas. By varying the speed of the wheels, spacecraft can perform maneuvers without using fuel, making them efficient for long missions. Reaction wheels are commonly found in satellites and space probes, contributing to successful operations in the space industry.