Electrostatic propulsion is a form of electric propulsion in which the thrust is produced by accelerating charged particles in an electrostatic field. It includes three types of device: electron bombardment thrusters, contact ion thrusters, and field emission/colloid thrusters. Of these, the first two involve the production and acceleration of separate ions and are therefore forms of ion propulsion. The third type involves the production and acceleration of charged liquid droplets. Only electron bombardment thrusters have been used operationally aboard spacecraft.