Field-Emission Electric Propulsion (FEEP): Ion Thrust from Liquid Metal
FEEP is an electrostatic ion thruster that pulls ions directly off a liquid metal propellant using a strong electric field. It produces extremely fine, throttleable micro-thrust used for precise spacecraft attitude control and drag-free flight.
Field-Emission Electric Propulsion (FEEP) is a class of electrostatic ion thruster that generates thrust by extracting and accelerating ions directly from a liquid metal propellant. A potential difference on the order of 10 kV is applied across an emitter, producing a field strong enough (around 10^9 V/m at the tip) to deform the liquid into protruding Taylor cones and field-evaporate individual ions. The propellant is fed passively by capillary action through micrometer-scale channels, so the system needs no pumps, valves, or pressurized tanks. Common propellants are the liquid metals indium, cesium, and mercury. Cesium and indium are favored for their high atomic mass, low ionization potential (cesium ~3.87 eV), low melting points, and good wetting of the emitter. The result is exceptionally fine thrust — from sub-micronewton to millinewton — with exhaust velocities of 100 km/s or more and throttle resolution better than one part in 10,000. That precision, not raw power, is FEEP's purpose: it provides drag-free station-keeping and micro-attitude control where tiny, finely tunable forces matter. It flew on ESA/NASA's LISA Pathfinder gravitational-wave technology mission, and the IFM Nano Thruster became the first operational FEEP system in low Earth orbit in 2018. Heavy metals like gold could in principle serve as FEEP propellant — gold's high mass and low ionization energy are favorable — but indium and cesium are used because they are cheap and well characterized. See Can Gold Be Spacecraft Fuel? Ion Drives, Mass Drivers, and Orion.