Ground-track resonances around Vesta: potential threats for Dawn?

The aim of Dawn mission is the acquisition of data from orbits around (4)-Vesta and (1)-Ceres, the two most massive asteroids. Due to the low thrust propulsion, Dawn will slowly transit through ground-track resonances (abbreviated by GTR), where the perturbations on Dawn orbit may be significant. In this context, to safety go the Dawn mission from the approach to the lowest orbit, it is essential to know the properties of the crossed resonances. We analytically investigate one of the properties of the 2:3 GTR. This resonance makes the eccentricity of Dawn increase until 0.2. Then the radius of the Dawn orbit changes about 260 km dangerously leading Dawn nearer to Vesta. We determine that this increases is due to the C22 coefficients (characterizing the equatorial ellipticity of the asteroid).