Dawn Departs Asteroid Vesta Sept 4 for Dwarf Planet Ceres

Dawn spacecraft departs Sept 4th from asteroid Vesta for dwarf planet Ceres. Dawn will skip some final low res. pix due to reaction wheel problem spacecraft experienced previously. Dawn mission scheduled to reach Ceres in February 2015 and end of primary mission slated for July 2015? Why an entire year orbiting a rock while less than half that time around an orb composed of up to 25% water? Where does Dawn venture after July 2015? Does spacecraft stay at Ceres and go offline?