Cassini-Huygens at Saturn (NASA/ESA, 2004–2017)

After a 7-year, 3.5-billion-km journey, Cassini entered Saturn orbit in July 2004, beginning a 13-year investigation that produced more than 450,000 images and 635 GB of science data. The mission's most dramatic discovery came in 2005: Cassini observed geysers of water ice, salt, and organic molecules erupting from the south pole of Enceladus, a moon only 500 km across. The plumes originate from a global subsurface liquid water ocean in contact with a rocky seafloor — the conditions considered most likely to support life anywhere in the solar system. The ESA-built Huygens probe descended through Titan's atmosphere in January 2005, becoming the first spacecraft to land in the outer solar system.