Dawn is the first NASA mission to operate in the vicinity of the two most massive asteroids in the main belt , Ceres and Vesta . This double-rendezvous mission is enabled by the use of low-thrust solar electric propulsion . Dawn will arrive at Vesta in 2011 and will operate in its vicinity for approximately one year . Vesta ’ s mass and non-spherical shape , coupled with its rotational period , presents very interesting challenges to a spacecraft that depends principally upon low-thrust propulsion for trajectory-changing maneuvers . The details of Vesta ’ s high-order gravitational terms will not be determined until after Dawn ’ s arrival at Vesta , but it is clear that their effect on Dawn operations creates the most complex operational environment for a NASA mission to date . Gravitational perturbations give rise to oscillations in Dawn ’ s orbital radius , and it is found that trapping of the spacecraft is possible near the 1:1 resonance between Dawn ’ s orbital period and Vesta ’ s rotational period , located approximately between 520 and 580 km orbital radius . This resonant trapping can be escaped by thrusting at the appropriate orbital phase . Having passed through the 1:1 resonance , gravitational perturbations ultimately limit the minimum radius for low-altitude operations to about 400 km , in order to safely prevent surface impact . The lowest practical orbit is desirable in order to maximize signal-to-noise and spatial resolution of the Gamma-Ray and Neutron Detector and to provide the highest spatial resolution observations by Dawn ’ s Framing Camera and Visible InfraRed mapping spectrometer . Dawn dynamical behavior is modeled in the context of a wide range of Vesta gravity models . Many of these models are distinguishable during Dawn ’ s High Altitude Mapping Orbit and the remainder are resolved during Dawn ’ s Low Altitude Mapping Orbit , providing insight into Vesta ’ s interior structure . Ultimately , the dynamics of Dawn at Vesta identifies issues to be explored in the planning of future EP missions operating in close proximity to larger asteroids .