The thruster for the ATS-4 and ATS-5 tests consisted of two contact ionization engines using cesium propellant. The thruster anode diameter was 5 cm, and an Yttrium-doped tungsten neutralizer was used. Beam power supply voltage was 3000 V, and the power per thruster was 0.02 kW, producing 0.089 mN thrust at a specific impulse of 6700 seconds. About 0.05 kg of propellant was carried. The longest ground test prior to launch was 2245 hours, and on ATS-4 the thruster operated for about 10 hours (however the test was terminated early because the satellite was put in a useless short-life parking orbit). This was the first successful orbital test of an ion engine, and no electromagnetic interference with satellite subsystems was detected. The second attempted test, on ATS-5, could not be conducted because of a 4G spacecraft spin which could not be damped.