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The Dawn spacecraft is under construction at Orbital Sciences Corporation in Dulles, Virginia, in preparation for launch in June 2006. After a flyby of Mars, Dawn will rendezvous consecutively with Vesta and Ceres. These are the largest asteroids and are among the few remaining protoplanets surviving from the formation of the solar system. Vesta formed 'dry', subsequently melted, and experienced core formation. Volcanic basalts cover its surface. Vesta is thought to be the source of HED meteorites. Ceres formed 'wet', differentiated into a rocky core and icy mantle, and may possess a subsurface ocean. Dawn will be the first NASA science mission to use ion propulsion. PSI Senior Scientist and Director Mark Sykes is a member of the Dawn Science Team.