Field-emission electric propulsion (FEEP) is an advanced electrostatic space propulsion concept, a form of ion thruster, that uses a liquid metal as a propellant – usually either caesium, indium, or mercury.

A FEEP device consists of an emitter and an accelerator electrode. A potential difference of the order of 10 kV is applied between the two, which generates a strong electric field at the tip of the metal surface. The interplay of electric force and the liquid metal’s surface tension generates surface instabilities, which give rise to Taylor cones on the liquid surface. At sufficiently high values of the applied field, ions are extracted from the cone tip by field evaporation or similar mechanisms, which then are electrically accelerated to high velocities – typically 100 km/s or more. Although the ion exhaust velocity is high, their mass is very low, resulting in very weak acceleration forces. Their benefit comes from sustained acceleration forces over long time periods.

Due to its very low thrust (in the micronewton (μN) to millinewton (mN) range), FEEP thrusters are primarily used for microradian, micronewton attitude control on spacecraft, such as in the ESA/NASA LISA Pathfinder scientific spacecraft. The FEEP thruster was also slated for installation on Gravity Field and Steady-State Ocean Circulation Explorer spacecraft, but the Gridded ion thruster was used instead. The first FEEP thruster operated in space was the IFM Nano Thruster, successfully commissioned in Low Earth Orbit in 2018.