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Dawn is the first NASA mission to operate in the vicinity of the two mostmassive asteroids in the main belt, Ceres and Vesta. This double-rendezvousmission is enabled by the use of low-thrust solar electric propulsion. Dawnwill arrive at Vesta in 2011 and will operate in its vicinity for approximatelyone year. Vesta's mass and non-spherical shape, coupled with its rotationalperiod, presents very interesting challenges to a spacecraft that dependsprincipally upon low-thrust propulsion for trajectory-changing maneuvers. Thedetails of Vesta's high-order gravitational terms will not be determined untilafter Dawn's arrival at Vesta, but it is clear that their effect on Dawnoperations creates the most complex operational environment for a NASA missionto date. Gravitational perturbations give rise to oscillations in Dawn'sorbital radius, and it is found that trapping of the spacecraft is possiblenear the 1:1 resonance between Dawn's orbital period and Vesta's rotationalperiod, located approximately between 520 and 580 km orbital radius.Thisresonant trapping can be escaped by thrusting at the appropriate orbital phase.Having passed through the 1:1 resonance, gravitational perturbations ultimatelylimit the minimum radius for low-altitude operations to about 400 km,in orderto safely prevent surface impact. The lowest practical orbit is desirable inorder to maximize signal-to-noise and spatial resolution of the Gamma-Ray andNeutron Detector and to provide the highest spatial resolution observations byDawn's Framing Camera and Visible InfraRed mapping spectrometer. Dawn dynamicalbehavior is modeled in the context of a wide range of Vesta gravity models.Many of these models are distinguishable during Dawn's High Altitude MappingOrbit and the remainder are resolved during Dawn's Low Altitude Mapping Orbit,providing insight into Vesta's interior structure.

The Dynamical Environment of Dawn at Vesta