Our solar system has three classes of planets: the rocky “terrestrial” planets (Mercury, Venus, Earth and Mars); beyond them the giant planets (Jupiter, Saturn, Uranus and Neptune); and the third zone, of primitive icy bodies that are found principally in the Kuiper Belt beyond Neptune. These objects are believed to be representative of the material which condensed to form the other planets. Their growth into full sized planets was arrested early in the history of the solar system. Hence they hold clues about the distant past of the solar system and the chemical endowment of all the planets including our Earth. There may be as many as a billion of these objects of greater than 10 km in diameter. The National Academy of Sciences placed the exploration of the third zone in general - and Pluto-Charon in particular - among its highest priority planetary mission rankings for this decade. As the first mission to investigate this class of planetary bodies, New Horizons will fill an important gap in our knowledge of the solar system. Following its visit to the Pluto/Charon/Nix/Hydra system in July 2015, the spacecraft will proceed deeper into the Kuiper Belt to study one or more of the icy mini-worlds in that vast region, up to a billion miles beyond Neptune's orbit.
