The application of low-thrust ion propulsion systems to space missions requires long duration (~ 5,000-20,000 hr) component operation. Thus, components must be developed with wearout mean times to failure in excess of these required mission times. Furthermore, chance failures which occur during the useful life of components must be minimized. This paper makes an assessment of both early and wearout failure modes of ion propulsion systems by examining the results of existing developmental and long duration testing. Estimates of chance failure rates of system components are also presented along with design concepts which maximize total propulsion system reliability. I. Introduction B ECAUSE of the long time operation required for most space applications, ion propulsion system reliability is a major concern. For this reason, the long duration and space testing of critical components, subsystems, and systems associated with primary and auxiliary ion propulsion systems are of great importance. It should be recognized, however, that although a large number of long duration tests have been conducted to date on ion propulsion systems, subsystems, and components, these tests have been part of a technology development phase and are not to be considered statistical reliability testing of fully developed systems. Thus, even though space-qualified ion propulsion systems are becoming available, system reliability can only be inferred from the results of past and present developmental testing and estimated by analytical techniques.