When in space, the purpose of a propulsion system is to change the velocity, or v, of a spacecraft. The reaction control and orbital maneuvering systems provide the propulsive force for orbit maintenance, position control, station keeping, and spacecraft attitude control. The main engines used in space provide the primary propulsive force for orbit transfer, planetary trajectories and extra planetary landing and ascent. In-space propulsion begins where the upper stage of the launch vehicle leaves off performing the functions of primary propulsion, reaction control, station keeping, precision pointing, and orbital maneuvering. A portfolio of propulsion technologies should be developed to provide optimum solutions for a diverse set of missions and destinations. With a wide range of possible missions and candidate propulsion technologies, the question of which technologies are "best" for future missions is a difficult one. These propulsion technologies are intended to provide effective exploration of our Solar System and will permit mission designers to plan missions to "fly anytime, anywhere, and complete a host of science objectives at the destinations" and with greater reliability and safety. Hypothetical in-space propulsion technologies describe the propulsion technologies that could meet future space science and exploration needs. Interplanetary vehicles mostly use chemical rockets as well, although a few have used ion thrusters and Hall-effect thrusters (two different types of electric propulsion) to great success.
Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north–south station-keeping and orbit raising.
Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control. Several methods of pragmatic spacecraft propulsion have been developed each having its own drawbacks and advantages. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry. Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. Bipropellant rocket engines of the Apollo Lunar Module reaction control system (RCS)
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