The second largest planet in the solar system and the sixth in order of distance to the Sun. The outermost planet known prior to 1781, Saturn is surrounded by a beautiful system of rings, more than 30 moons, and a complex magnetosphere. Despite the planet's huge size, its mean density is so low it could float in water. Saturn is also the only planet that has a satellite (Titan) with a dense atmosphere. This distant planetary system has been visited by four spacecraft: Pioneer 11 in 1979, Voyagers 1 and 2 in 1980–1981, and Cassini-Huygens since 2004. The first three were flybys; the last is a joint United States–European mission consisting of a Saturn orbiter and a probe into the atmosphere of Titan.