Ion engine mechanism, illustration. The blue exhaust stream at right is ionized atoms of a propellant gas (such as xenon) emerging from the engine to provide thrust. The non-ionised gas (blue) is injected at left, with a negatively charged cathode (yellow rod) firing out electrons (orange dashed lines). Voltages increase the acceleration of the particles through a negative (yellow) and positive (red) grid at right. Designed for use in spacecraft, the thrust is very small but it provides more thrust for weight of fuel than chemical rockets. Ion propulsion was first proposed in the 1950s, but was not used until the 1990s.