Cassini-Huygens (1997)

The joint NASA/ESA Cassini-Huygens mission spent 20 years studying Saturn, its rings, and its moons from launch in 1997 to its intentional destruction in Saturn's atmosphere in 2017. The Huygens probe became the first to land on a moon in the outer solar system when it touched down on Titan in January 2005. Cassini discovered active geysers on Enceladus (indicating a subsurface ocean), found hydrocarbon lakes on Titan, and sent back over 453,000 images.