Initial Results of the Dawn Gravity Investigation at Vesta

The Dawn gravity investigation utilizes the precision X-band Doppler tracking and landmark tracking from optical images to measure the gravity fields of Vesta to a half-wavelength surface resolution better than 90-km. For given error budget assumptions, the spherical harmonic gravity field will be determined to somewhere between degrees 15 and 25. The gravity fields together with shape models determined from Dawn's framing camera constrain models of the interior from the core to the crust. The gravity field is determined jointly with the spin pole location, rotation, Vesta ephemeris and spacecraft modeling parameters; precision orbits incorporating surface landmarks from images will be used to improve positioning to aid in geodetic control. The second-degree harmonics together with assumptions on obliquity or hydrostatic equilibrium may determine the moments of inertia; they constrain the core size and density. To date, data from the survey and high altitude orbital phases (HAMO) have lead to a determination of GM to better than 0.1 percent and a gravity field of degree 6 with 140-km resolution. J2 is consistent with the core size of one half the mean radius. Models of crustal and mantle structure will be developed constrained by bulk composition estimates derived from HED meteorites and observations of surface composition from Dawn's VIR.