With the launch of the Deep Space One Mission spacecraft, the NASA Solar-Electric-Propulsion Technology Applications Readiness Program (NSTAR) began flight validation of its xenon ion propulsion system. The NSTAR system includes a 30-cm diameter ion thruster, a xenon tankage and feed system, a power processing unit, and a digital control and interface unit. Since program inception in 1995 two flight power processors have been built, tested, and integrated. The test program consisted of acceptance and qualification testing at the system level plus solar vacuum testing at the spacecraft level. In addition to the ground test program, in-flight acceptance testing was successfully completed before year-end 1998. Further a multi-year mission profile test is planned to begin early this year at the Jet Propulsion Laboratory ion engine vacuum test facility. As of January 1999, the power processing unit has demonstrated more than 850 trouble-free hours of flight operation as Deep Space One is now more than 26 times as far away from the Earth as the Moon is and continues toward an asteroid rendezvous in July, 1999.