The Application of a Vestoid-Derived Photometric Model to the Surface of (4) Vesta

In anticipation of the Dawn Mission to Vesta, we conducted a ground-based campaign of Bessel BVRI filter photometry of five V-type near-Earth asteroids over a wide range of solar phase angle. Our disk-integrated photometry gave us access to solar phase geometries not available to ground-based observers of (4) Vesta but will be experienced by the Dawn spacecraft throughout its approach and mapping phases. The majority of our observations were obtained at the JPL 0.6-m Table Mountain Observatory but multiple nights were also contributed by the Calar Alto 1.2-m and 2.2-m telescopes, as well as the Purple Mountain 1-m Schmidt. Restricting our data to the vestoids with low light-curve amplitude, our disk-integrated photometry allowed us to derive f(α), assuming a spherical body, expressed as a fourth-order polynomial plus an exponential term. By scaling with a relative reflectance spectrum, our model can be expanded to cover a wavelength range from the optical to the near-IR. Our model was further constrained by Dawn photometry of (4) Vesta generated from approach and orbit insertion optical navigation images, when the disk of (4) Vesta was less than the field-of-view of the camera. In this work, we will focus on the application of our vestoid-derived photometric model on calibrated disk-resolved Dawn Framing Camera (FC) filter images Dawn Visual and Infrared (VIR) spectral cubes of (4) Vesta obtained during the early stages of Dawn's mapping mission. We shall explore and validate our model across a wide range of surface albedo and terrain types. Deviations from our model will likely give clues towards variations in texture and surface scattering properties. This work was funded by NASA.