An ion propulsion engine uses magnetic fields rather than chemical combustion to achieve thrust. A small amount of gas—usually of a heavy element such as xenon—is injected into an ionization chamber containing a series of magnetic coils. An electrical power supply powers the coils, and the resulting electromagnetic forces in the chamber separate the positively and negatively charged particles in the gas, creating ions and free electrons. Using powerful electric fields, those charged particles are then accelerated to very high speeds, and then pushed out of the back of the ionization chamber. Their backward motion creates forward thrust.