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. Therefore it is critical that spacecraft have extremely reliable retrorockets. A Redstone rocket, part of the Mercury program A rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust gas from within a rocket engine. ... An engine is something that produces some effect from a given input. ... Thrust is a reaction force described quantitatively by Newtons Second and Third Law. ...

Due to the high reliability demanded by retrorockets, Mercury spacecraft used a trio of solid fuel, 1000 lbf (4.5 kN) thrust retrorockets strapped to the heat shield on the bottom of the spacecraft that fired for 10 seconds each. One was sufficient to return the spacecraft to earth if the other two failed. Gemini used a similar system as Mercury, just scaled up to four rockets of 2500 lbf each (11 kN) that burned for 5.5 seconds each. These rockets were part of the Reentry Control System mounted forward of the pressurized cabin. Project Mercury was the United States first successful manned spaceflight program. ... Project Gemini insignia Project Gemini was the second human spaceflight program in which the United States of America sent humans into space, between Projects Mercury and Apollo, during the years 1963-1966. ...

Project Apollo did not require retrorockets for lunar flights, as the flight from the moon was directed to fly the spacecraft directly back to earth, and not enter orbit. However, the flights in earth orbit for tests required retrorockets, so the large, versatile Service Propulsion Module on the Service Module was used to decelerate the spacecraft. The Space Shuttle would use a similar multipurpose engine for reentry. Apollo Program insignia Apollo CSM in lunar orbit. ... The Space Shuttle Columbia seconds after engine ignition, 1981 (NASA). ...

The Space Shuttle Orbital maneuvering system provides the vehicle with a pair of powerful liquid fueled rockets for both reentry and orbital maneuvering. One is sufficient for a successful reentry, and if both systems should fail, the Reaction control system can slow the vehicle enough for reentry. The Orbital Maneuvering System, or OMS, is a system of rocket engines used on the Space Shuttle for orbital injection and modifying its orbit. ... A reaction control system (abbreviated RCS) is a subsystem of a spacecraft. ...

Retrorockets are also used in landing spacecraft on other astronomical bodies, such as the Moon and Mars, as well as enabling a spacecraft to enter an orbit encircling such a body, when otherwise it would scoot past and off into space again. As pointed out above (in connection with Project Apollo) the main rocket on a spacecraft can be re-oriented to serve as a retrorocket. Apollo Program insignia Apollo CSM in lunar orbit. ...