Saturn and its moon Tethys, Cassini spacecraft image. Tethys is the small round object just below and left of centre, above Saturn's rings. Tethys is some 1070 kilometres in diameter and it orbits just less than 300,000 kilometres from Saturn. Saturn itself is a gas giant planet, some 120,500 kilometres in diameter. It is composed almost entirely of hydrogen, with a rocky core and traces of other gases. It has a prominent ring system, made up of orbiting chunks of ice and rock. The shadows of the rings are seen at upper left. This image was taken on 10 June 2005, while Cassini was around 1.4 million kilometres from Saturn.