An spacecraft propulsion that is currently being researched by NASA and the ESA. Call ion drive. An ion drive is considered to be more efficient than traditional solid or liquid propellant rockets and in most cases provide more thrust.
The answer lies somewhere in between. Ion engines date back to at least 1959. Two ion engines were even tested in 1964 on the American SERT 1 satellite - one was successful, the other was not.
The principle function
The principle is simply conventional physics - you take a gas and you ionise it, which means that you give it an electrical charge. This creates positively charged ions of gas, along with electrons. The ionised gas passes through an electric field or screen at the back of the engine and the ions leave the engine, producing a thrust in the opposite direction.
The ion thruster is powered by large solar panels. The power ionizes the fuel (Xenon) and then accelerates it with an electric field between two grids. Electrons are injected into the beam after acceleration to maintain a neutral plasma.
Operating in the near vacuum of space, ion engines shoot out the propellant gas much faster than the jet of a chemical rocket. They therefore deliver about ten times as much thrust per kilogram of propellant used, making them very 'fuel-efficient'.
Although they are efficient, ion engines are very low-thrust devices. The amount of push you get for the amount of propellant used is very good, but they do not push very strongly. For example, astronauts could never use them for taking off the surface of a planet. However, once in space, they could use them for maneuvering around, if they are not in a hurry to accelerate quickly.
Ion drives can get up to high speeds in space, but they need a very long distance to build up to such speeds over time.
NASA's Deep Space One probe, which uses a conventional ion engine.