An ion thruster is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions with electricity. The term refers strictly to gridded electrostatic ion thrusters, but may more loosely be applied to all electric propulsion systems that accelerate plasma, since plasma consists of ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic thrusters use the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic thrusters use the Lorentz force. In either case, when an ion passes through an electrostatic grid engine, the potential difference of the electric field converts to the ion's kinetic energy. Ion thrusters have an input power spanning 1–7 kW, exhaust velocity 20–50 km/s, thrust 25–250 millinewtons and efficiency 65–80%. The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by 4300 m/s while consuming less than 74 kilograms of xenon. The Dawn spacecraft broke the record, reaching 10,000 m/s. Applications include control of the orientation and position of orbiting satellites (some satellites have dozens of low-power ion thrusters) and use as a main propulsion engine for low-mass robotic space vehicles (for example Deep Space 1 and Dawn). The ion thruster is not the most promising type of electrically powered spacecraft propulsion (although the most successful in practice). An ion drive would require two days to accelerate a car to highway speed. The technical characteristics, especially thrust, are considerably inferior to the prototypes described in literature, technical capabilities are limited by the space charge created by ions. This limits the thrust density (force per cross-sectional area of the engine). Ion thrusters create small thrust levels (the thrust of Deep Space 1 is approximately equal to the weight of one sheet of paper) compared to conventional chemical rockets, but achieve high specific impulse, or propellant mass efficiency, by accelerating the exhaust to high speed. The power imparted to the exhaust increases with the square of exhaust velocity while thrust increase is linear. Conversely, chemical rockets provide high thrust, but are limited in total impulse by the small amount of energy that can be stored chemically in the propellants. Given the practical weight of suitable power sources, the acceleration from an ion thruster is frequently less than one thousandth of standard gravity. However, since they operate as electric (or electrostatic) motors, they convert a greater fraction of input power into kinetic exhaust power. Chemical rockets operate as heat engines, and Carnot's theorem limits the exhaust velocity. Ion thrust engines are practical only in the vacuum of space and cannot take vehicles through the atmosphere because ion engines do not work in the presence of ions outside the engine. Spacecraft rely on conventional chemical rockets to initially reach orbit.

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