Comparing Dawn, Hubble Space Telescope, and ground-based interpretations of (4) Vesta

Abstract

Observations of Asteroid (4) Vesta by NASA’s Dawn spacecraft are interesting because its surface has the largest range of albedo, color and composition of any other asteroid visited by spacecraft to date. These hemispherical and rotational variations in surface brightness and composition have been attributed to impact processes since Vesta’s formation. Prior to Dawn’s arrival at Vesta, its surface properties were the focus of intense telescopic investigations for nearly a hundred years. Ground-based photometric and spectroscopic observations first revealed these variations followed later by those using Hubble Space Telescope (HST). Here we compare interpretations of Vesta’s rotation period, pole, albedo, topographic, color, and compositional properties from ground-based telescopes and HST with those from Dawn. Our goal is to provide ground truth for prior interpretations and to help identify the limits of ground-based studies of asteroids in general. The improved rotational period measurement from Dawn is 0.222588652day (Russell, C.T. et al. [2012]. Science 336, 684–686), and is consistent with the best ground-based rotation period of 0.22258874day (Drummond, J.D., Fugate, R.Q., Christou, J.C. [1998]. Icarus 132, 80–99). The pole position for Vesta determined by Dawn is 309.03°±0.01°, 42.23°±0.01° and is within the uncertainties of pole orientation determined by Earth-based measurements (Li, J.-Y. et al. [2011]. Icarus 211, 528–534: 305.8°±3.1°, 41.4°±1.5°). Similarly, the obliquity of Vesta is 27.46° based on the pole measurement from Dawn and all previous pole measurements put the obliquity within 3° of this value. The topography range from the Dawn shape model is between −22.45 and +19.48km relative to a 285km×285km×229km ellipsoid. The HST range is slightly smaller (−12km to +12km relative to a 289km×280km×229km ellipsoid) than Dawn, likely due to lower spatial resolution of the former. We also present HST and Dawn albedo and color maps of Vesta in the Claudia (used by the Dawn team) and IAU coordinate systems. These maps serve to orient observers and identify compositional and albedo features from prior studies. We have linked several albedo features identified on HST maps to morphological features on Vesta using Dawn Framing Camera data. Rotational spectral variations observed from ground-based studies are also consistent with those observed by Dawn. While the interpretation of some of these features was tenuous from past data, the interpretations were reasonable given the limitations set by spatial resolution and our knowledge of Vesta and HED meteorites at that time. Our analysis shows that ground-based and HST observations are critical for our understanding of small bodies and provide valuable support for ongoing and future spacecraft missions.