The Cassini-Huygens mission is one of the most ambitious space research projects ever undertaken. Launched on 15 October 1997, the American-European planetary probe journeyed through space for nearly seven years enroute to Saturn, the second-largest planet of the Solar System. The probe reached the Ringed Planet in the middle of 2004 – a journey of 3500 million kilometres. Before Cassini-Huygens' arrival, three American space probes had passed Saturn: Pioneer 11, Voyager 1 and Voyager 2. Cassini-Huygens will investigate the Saturnian system for four years.