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An ion engine (or ion thruster) is a type of spacecraft engine that runs on electricity (a type of electric thruster). Advantages of ion engines over rocket engines (i.e., powered by burning the propellant) include efficient use of propellant and ability to run continuously, while disadvantages include inability to power a launch from the ground and the need to carry along a source of power. Ion engines are a viable option while the spacecraft is in space, and depending upon the circumstances, their efficiencies can win out. To date, ion engines have been small scale, with only the power of very small rocket engines.
Ion engines run on electric power, such as from solar panels or an atomic reactor and are considered a means to make use of either of these for active space flight. A propellant is also needed (and is eventually used up), but rather than using a burnable substance (as is the fuel/propellant of ordinary rockets), it must be a substance (such as xenon) that can be ionized by the engine. Electricity is used to ionize the propellant and also is used to create electromagnetic forces to thrust the propellant backward from the engine, accelerating the spacecraft according to Newton's law, "every action has a reaction". The propellant must be thrust at considerable speed and the faster the thrust, the less propellant is needed to accomplish a particular amount of acceleration, and ion engines have the advantage of propelling it faster than do rocket engines. Though this is efficient use of the propellant, the thrust that has been accomplished is small compared to that of rocket engines, but it can be maintained for long flights, something impractical for typical rocket engines.
Some spacecraft have used ion engines merely for turning, maneuvering, and stationkeeping, but now some have used them for traveling long distances, such as between planets. Their low power (so far) has limited them to slow-but-efficient routes, e.g., using gravity assists. Missions that have incorporated it:
Likely, all current electric thruster concepts use ions, but the terms ion engine and ion thruster are used more specifically, for those that ionize a gas, accelerate it and direct it backward after allowing it to recombine with its missing electrons. Some terms for electric thrusters (such as Hall effect thruster, HET) do fit this category and can be considered subclassifications of ion thruster. Numerous commercial, and (non-astrophysics) science satellites use ion thrusters and other types of electric thrusters.
Regarding the possibility launching from the ground, current ion engines produce far too little thrust to counteract their Earth weigh (it wouldn't budge) or atmospheric drag, and the presence of air reduces the electric field (and resulting thrust) they can generate. The thrust deficit is so great that such launches won't happen in the foreseeable future. For travel within space, an ion engine's ability to run continuously makes up for the miniscule thrust: once in orbit, it isn't pinned to the ground and can accelerate and even a tiny acceleration maintained sufficiently long can accomplish much space travel. However, ion engines designed to use a surrounding (thin) atmosphere as their propellant are in development (making them something like jet engines), e.g., for stationkeeping of satellites at the lowest-altitude orbits.