A retrorocket is a rocket engine used for providing thrust to oppose the motion of a space vehicle, thereby causing deceleration of the vehicle. When the vehicle is slowed, its orbit will decrease in altitude until the point at which aerodynamic forces begin to rapidly slow the motion of the vehicle, and it (hopefully) returns safely to the ground. Without such rockets, spacecraft would remain in orbit for years until their orbits naturally slow, and reenter the atmosphere at a much later date.