Electric propulsion systems have the possibility of reducing the propellant mass needed for long-range planetary missions and drag compensation in orbiting satellites, as compared to chemical rocket systems. Electric propulsion reduces the mass needed for a given transfer of momentum to the space vehicle and thus its acceleration by exhausting the mass at a higher velocity. Electric propulsion systems are not used to launch payloads from the Earth's surface to low-Earth orbit because the force and energy required are too large. Instead, they are used in a space environment for drag compensation of space stations and for long space trips.