Abstract: A field-emission ion source in which, under the influence of an electric field, ions are released from a metal or metal alloy present in an enclosed space in the liquid state. The ions are emitted from this space through a very narrow slit. This slit may be straight or curved. The field-emission ion source can be used in an ion thruster apparatus comprising an emitter module, an electrode system, and a power supply unit. A plurality of emitter modules can be combined to form an ion thruster apparatus having a greater ion current output. Instead of a liquid metal as the propellant, a metal in the solid phase can be supplied to the emitter module, which metal is melted in the emitter module.