The planet Saturn and three of its moons, 1981.
© National Aeronautics & Space Administration / Science & Society
This picture taken by Voyager 2 from a distance of 21 million kilometres, shows the planet and three of its moons; Tethys, Dione and Rhea, with one casting a shadow on the planet. Saturn is a gas giant similar in atmospheric composition to Jupiter, with banded cloud formations. It rotates very quickly, causing it to appear oblate (flattened at the poles). The origin and formation of the rings, which are composed of ice and ice-coated dust and rock, are not precisely understood, but it seems that tidal effects caused by some of Saturn's moons play a role in maintaining their structure. NASA's two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 2 pased Saturn in August 1981, before continuing on to Uranus and Neptune.