How an ion engine works. Electrons attracted into the discharge chamber collide with xenon atoms from the propellant gas supply, making charged atoms (ions). Current-carrying coils, inside and outside the doughnut-shaped discharge chamber, sustain a magnetic field oriented like the spokes of a wheel. By the Hall effect, ions and electrons swerving in opposite directions in the magnetic field create an electric field. This expels the xenon ions in a propulsive jet. Other emitted electrons then neutralise the xenon, producing the blue jet.
SMART-1 is the first of ESA’s Small Missions for Advanced Research in Technology. It will head for the Moon using solar-electric propulsion and carrying a battery of miniaturised instruments.