American Institute of Aeronautics and Astronautics 1 In-Flight Operation of the Dawn Ion Propulsion System Through Orbit Capture at Vesta Charles E. Garner, 1 Marc D. Rayman, 2 John R. Brophy, 3 and Steven C. Mikes 4 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Ca 91109 The Dawn mission, part of NASA’s Discovery Program, has as its goal the scientific exploration of the two most massive main-belt asteroids, Vesta and Ceres. The Dawn spacecraft was launched from Cape Canaveral Air Force Station on September 27, 2007 on a Delta-II 7925H-9.5 (Delta-II Heavy) rocket that placed the 1218 kg spacecraft into an Earth-escape trajectory. On- board the spacecraft is an ion propulsion system (IPS) developed at the Jet Propulsion Laboratory which will provide most of the Δ V needed for heliocentric transfer to Vesta, orbit capture at Vesta, transfer among Vesta science orbits, departure and escape from Vesta, heliocentric transfer to Ceres, orbit capture at Ceres, and transfer among Ceres science orbits. The first 80 days after launch were dedicated to the initial checkout of the spacecraft which was followed by about ten months of full-power thrusting leading to a Mars gravity assist in February 2009 that provided 1 km/s of heliocentric energy increase and is the only part of the mission following launch in which a needed velocity change is not accomplished by the IPS. Deterministic thrusting for heliocentric transfer to Vesta resumed in June 2009 and was concluded with orbit capture at Vesta in July 2011. IPS was operated for approximately 23,400 hours, consumed approximately 250 kg of xenon, and provided a delta-V of approximately 6.7 km/s to achieve orbit capture at Vesta. IPS performance characteristics are very close to the expected performance characteristics based on analysis performed pre-launch. The only significant problem to have occurred over the almost four years of IPS operations in flight was the temporary failure of a valve driver board in DCIU-1, resulting in a loss of thrust of approximately 29 hours. Thrusting operations resumed after switching to DCIU-2, and power cycling conducted after orbit capture indicates DCIU-1 is completely operational. After about three weeks of survey operations IPS will be used to maneuver the spacecraft as needed for science operations including orbit transfers. After approximately one year of science operations IPS will then be used for escape from Vesta and begin thrusting for cruise to Ceres with a planned arrival date at Ceres in February 2015. This paper provides an overview of Dawn’s mission objectives and the results of Dawn IPS mission operations through orbit capture and the start of science operations at Vesta. I. INTRODUCTION The number of missions using electric propulsion is increasing. Deep Space 1 (DS1), launched in 1998, operated its single thruster ion propulsion system for over 16,000 hours before successfully completing its mission [1]. A PPS-1350 Hall thruster was used for primary propulsion on board the European Space Agency’s SMART-1 probe, with more flights planned [2]. European and U.S.-launched communications satellites have been launched with SPT-100 –based propulsion modules for attitude control and orbit boosting. The Hayabusa spacecraft returned to Earth after exploring asteroid Itokawa [3] and employed ion engines for primary propulsion. Several communications satellites based on the Boeing 702 bus and the Japanese ETS-VIII include ion thrusters for north- south station keeping. ESA’s GOCE mission, launched in March 2009, employs ion propulsion for precision orbital control [4], and ESA’s Artemis mission used the European RIT-10 ion propulsion system for transfer to a geostationary orbit [5]. 1 Engineer, Propulsion and Materials Engineering Section, Member AIAA 2 Dawn Project System Engineer 3 Principal Engineer, Propulsion and Materials Engineering Section, Senior Member 4 Senior Engineer, Engineer, Flight System Avionics Section