After having been in development for many years at the Glenn Research Center (formerly the Lewis Research Center), the national aeronautics and space administration-designed, 30 cm, ring-cusp, xenon ion engine was launched on the Deep Space 1 (DS1) spacecraft on 24 Oct. 1998 from the Kennedy Space Center in Florida. It has since accumulated 2200 h of in-space thrusting at input power levels ranging from 0.52 to 1.96 kW, has successfully enabled the spacecraft to fly by the asteroid Braille in July 1999, and is now thrusting DS1 along a trajectory towards its comet destinations in 2001. The design, assembly, test, integration, and operation of this thruster comprise a unique path of technical determination, artful design choices, persistent engineering and analysis, and mastery of vacuum chamber operations. The testing program over the development years, the assembly and integration periods, and the flight operational period thus far have shown that the project test philosophy of segregating effects against unique causes proved itself most useful. The 8000 h life test, the culmination of the pre-launch ground test plan, not only met its goals but surpassed them with margin. This article will explain the thruster test program from beginning to end, illustrating the technical and programmatic decision making along the way. It will justify the use of engineering models as an inexpensive method of determining answers to key design questions and will explain why testing of the thruster alone only solved a portion of the system operations task. The highlight of the test program proved to be the vacuum firing of the ion engine during the spacecraft's solar thermal vacuum test. A comparison of the preflight data with postflight data shows that high confidence was warranted for executing a successful flight to the asteroid and beyond.