Two cesium-contact ion engines were launched aboard the ATS-4 spacecraft on August 10, 1968. Flight test objectives were to measure thrust and to examine electromagnetic compatibility with other spacecraft subsystems. The 5-cm-diameter thrusters were designed to operate at 0.02 kW and provide about 89 mN thrust at about 6700 s specific impulse. Thrusters had the capability to operate at 5 setpoints from 18 mN to 89 mN. Thrusters were configured so they could be used for East-West stationkeeping. Prior to launch, a 5-cm cesium thruster was life tested for 2245 hours at the 67-mN thrust level.

During the launch process the Centaur stage did not achieve a second burn, and the spacecraft remained attached to the Centaur in a 218 km by 760 km orbit. It was estimated that the pressure at these altitudes was between 1.3 x 10-4 Pa and 1.3 x 10-7 Pa.35 Each of the two engines was tested on at least two occasions over the throttling range. Combined test time of the two engines was about 10 hours over a 55-day period. The spacecraft re-entered the atmosphere on October 17, 1968.

The ATS-4 flight was the first successful orbital test of an ion engine. There was no evidence of IPS electromagnetic interference related to spacecraft subsystems. Measured values of neutralizer emission current were much less than the ion beam current implying inadequate neutralization. The spacecraft potential was about -132 V which was much different than the anticipated value of about -40 V.

The preceding was an excerpt from:
Sovey, J. S., Rawlin, V. K., and Patterson, M. J.: "Ion Propulsion Development Projects in U. S.: Space Electric Rocket Test 1 to Deep Space 1." Journal of Propulsion and Power, Vol. 17, No. 3, May-June 2001, pp. 517-526.