Exploring the smallest terrestrial planet: Dawn at Vesta

The Dawn mission is designed to map Vesta and Ceres from polar orbit for close to one year each. The ion-propelled Dawn spacecraft is illustrated in Figure 1. Dawn carries a framing camera with clear and color filters, a visible and infrared mapping spectrometer, a gamma ray and neutron spectrometer, and obtains radiometric data on the gravity field. The camera obtains stereo imagery from which a global shape and topography model are derived. The mapping spectrometer determines the mineral composition of the surface and the gamma and neutron spectrometer determines the elemental composition. As Dawn approaches Vesta, as illustrated in Figure 2, it measures the rotational characteristics of the body to determine the orientation of the rotation axis. This in turn determines when solar illumination reaches the north pole and when mapping can be completed. As shown in Figure 3, there are three science orbits: Survey at a radial distance of 3000 km and a period of 69 hr; high-altitude mapping at a radial distance of 950 km and a period of 12.3 hr; and low-altitude mapping at a radius of 465 km and a period of 4 hours. Vesta is the ultimate source of the HED meteorites from which much has been learned about their parent body. By the time of this presentation we will have surveyed the region around Vesta for moons, determined a much more accurate mass and rotation axis for Vesta, and have preliminary information on surface features and composition from the survey orbit.