New Horizons: Reconnaissance of the Pluto-Charon System and the Kuiper Belt C. T. Russell Originally published in the journal Space Science Reviews, Volume 140, Nos 1-4, 1-2. DOI: 10. 1007/s11214-008-9450-0 (c) Springer Science+Business Media B. V. 2008 Exploration is mankind's imperative. Since the beginnings of civilization, men and women have not been content to build a wall around their settlements and stay within its con nes. They explored the land around them, climbed the mountains, and scanned the horizons. The boldest among them pushed exploration to the most distant frontiers of the planet. As a result, much of the Earth was inhabited well before the days of the renowned European - th th plorers of the 15 and 16 centuries. Exploration did not cease, after the circumnavigation of the globe; it continued to the present. Today explorers are going in new directions, not just east and west, north and south. They explore backward in time and upward in space. Arc- ology explores the shorter time scales, and geochemistry the longer time scales of geophy- cal events: asteroidal and cometary collisions, magnetic reversals, continental formation and more. However, on Earth we cannot go back inde nitely, for much of the evidence of the very earliest days has been lost.