Then Dawn will do something unprecedented in real-world spaceflight: exit the orbit of one distant body, and fly to and orbit another. The second destination is asteroid Ceres. "Dawn will be the first spacecraft ever built to orbit two target bodies after leaving Earth," says Marc Rayman, Dawn chief engineer at NASA's Jet Propulsion Laboratory. "There's not even a concept for doing such a mission with conventional propulsion systems. The spacecraft would have to carry so much fuel, it would be too heavy to launch."