The second year of Dawn mission operations: Mars gravity assist and onward to Vest

Dawn launched in September 2007 on a mission to orbit main belt asteroids (4) Vesta in 2011–2012 and (1) Ceres in 2015. The mission is enabled by an ion propulsion system, which will be operated for the majority of the interplanetary cruise. Following 10.5 months of thrusting that concluded in October 2008, the spacecraft began a period of optimal coast that ended in June 2009. A Mars gravity assist in February 2009 provided an effective Δv of 2.6 km/s. The mission flexibility afforded by the use of ion propulsion provided relatively simple targeting at Mars. Additional engineering activities were conducted during the coast period after Mars, including loading new software into the spacecraft's central computer. This paper describe the progress of the mission, including the approach to Mars, the encounter itself, special activities conducted prior to the resumption of ion thrusting, and the continuation toward Vesta.