Saturns Moon Tethys . ... It was visited by both Voyager spacecraft - Voyager 1 in November 1980 and by Voyager 2 in August 1981. Tethys is now known to be composed almost completely of water ice. Tethys shows a large impact crater that nearly circles the planet. ... Two smaller moons, Telesto and Calypso , orbit Saturn just ahead of and behind Tethys. Giovanni Cassini discovered Tethys in 1684. In 1997, NASA launched a spacecraft named Cassini to Saturn that will arrive in 2004. ...