Pluto, Mars Water and More! The Biggest Space Stories of 2015

NASA's New Horizons probe reached Pluto after a decade of travel across interplanetary space, exploring the most well-known dwarf planet. Ceres, Mars and the moons of Saturn also revealed their secrets. NASA's New Horizons made history in July as it zoomed by Pluto, passing within 7,800 miles (12,500 kilometers) of the dwarf planet's icy surface.