A reaction wheel is a type of spacecraft attitude control device that helps manage the orientation of a satellite or spacecraft in space. It consists of a wheel that spins at high speeds, creating angular momentum. By changing the speed of the wheel, the spacecraft can rotate in the opposite direction, allowing precise adjustments to its position and orientation. These wheels are crucial for missions involving satellites, space telescopes, and space probes, where maintaining a stable orientation is essential for tasks like imaging and communication. Reaction wheels are often preferred for their ability to provide fine control without using fuel, making them efficient for long-duration missions.