File:Cesium Ion thruster (54223631982).jpg

English: This is one the earliest spacecraft ion thrusters (from EOS circa 1962), Ben Longmier, lead designer of the SpaceX thruster, helped identify the old one I had in my collection. It appears to be “a development unit or early flight unit for a Cesium thruster from EOS (Electro-Optical Systems). The black component underneath the thruster looks to be the propellant tank and you can see several heaters wrapped around and brazed in place. A porous plug would have been used as a ‘valve,’ which takes advantage of a metal wetting and vapor pressure trick to throttle the propellant flow vs. temperature of the porous sintered metal plug.”

The final EOS thruster is in the Smithsonian. It was successfully tested twice in space on flights of Air Force Blue Scout missiles in 1964.

Back in 1912, Goddard postulated that high-velocity streams of electrons and positive ions could be “energized” by solar-electric power supplies to provide thrust for an interplanetary spacecraft. He went further to suggest that the source of the ions could come from exposing alkaline atoms, such as mercury or cesium, to hot tungsten surfaces. He was spot on!