Ceres is a dwarf planet and the largest object in the asteroid belt. Ceres may have a rocky inner core, an icy mantle, and a thin, dusty outer crust inferred from its density and rotation rate of 9 hours. Ceres is approximately 590 miles (950 kilometers) across and was first discovered in 1801. In January 2014, astronomers have discovered direct evidence of water on the dwarf planet Ceres in the form of vapor plumes erupting into space. After visiting Vesta, NASA's Dawn spacecraft, launched on 27 September 2007, is began to explore Ceres in spring 2015.