A retrorocket is a type of rocket engine designed to be fired when a ship or object needs to reduce its speed or stop. The term is short for retrograde rocket. The positioning of a retrorocket is usually facing opposite of the main form of propulsion. When fired, the retrorocket will apply thrust in the opposite direction of movement, slowing the vehicle down until it is at an acceptable speed or has stopped. There is extensive use of retrorockets in space and aerospace vehicles, including satellites and space shuttles.
One reason a retrorocket is necessary when maneuvering a vehicle in space is the lack of friction or gravity in space. Once thrust is applied to an object, it will move in a straight line through space until some other force acts on it. Theoretically, a satellite that is launched in one direction will travel indefinitely in that direction until another force, such as gravity or a physical object, stops it.
Most vehicles used in space are designed to accelerate so they can reach their targets faster, and this acceleration can slowly continue over a long time depending on the type of engine the vehicle is using. To slow a vehicle down so it can perform tasks such as exploration or enter into orbit around an astronomical body, a force needs to be applied in the opposite direction of travel. Firing a retrorocket is the solution to this problem. The rockets can be designed to fire in a long, slow burn that gradually slows down the craft, or they can be fired in short, intense bursts for a fast stop.
Using specifically installed retrorockets avoids having to perform complex maneuvers with a vehicle. Without a retrorocket system, vehicles that need to slow down or stop in space would have to fire another set of engines to spin the craft around and face the opposite direction. Once facing backward, it would then fire the main propulsion unit to apply thrust slowly in the opposite direction. A retrorocket system simply needs to be fired in sufficient amounts to slow the craft and no complex movements are necessary.
Satellites and other vehicles that exist in orbit around the Earth use retrorockets to end their orbital period. By firing the retrorocket, the speed of the object is slowed. Once slowed, it will begin to be drawn toward the Earth and lose altitude. The gravity and atmosphere of the Earth will further slow down the object until it actually enters the atmosphere and falls to the surface. This is how space shuttles re-entered the Earth’s atmosphere before their retirement in 2011, although their descent was always very tightly controlled.