Propulsion is the act of driving some-thing forwards. Rockets, cars, boats, planes and all other vehicles use some form of propulsion in order to move. Propulsion is the act of moving or pushing an object forward. The word is derived from two Latin words: pro, meaning before or forward, and pellere, meaning to drive. A propulsion system is an engine that produces thrust to push an object, such as an airplane or rocket, forward.
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. Space propulsion or in-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with launch vehicles. Several methods, both pragmatic and hypothetical, have been developed each having its own drawbacks and advantages.
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. 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. 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.
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