Voyager 2, launched in 1977, is considered the most productive unmanned space probe because it is the only spacecraft to have visited all four outer planets (Jupiter, Saturn, Uranus, and Neptune) and their moons. It discovered numerous moons, active volcanoes on Io, and provided unprecedented data about planetary atmospheres. Voyager 1 only visited Jupiter and Saturn before leaving the solar system, Cassini-Huygens orbited Saturn, and Spirit was a Mars rover.