Triton is the largest of Neptune's seven satellites. It was discovered in 1846 weeks after the discovery of Neptune itself by William Lassell. Triton's orbit is retrograde. It is the only large moon in the solar system to orbit backwards. The most interesting feature of this satellite are its ice volcanoes probably composed of nitrogen, dust, or methane compounds from beneath its surface. Triton, Io, Venus, and the Earth are the only solar bodies that exhibit volcanic activity. Almost all that is known about Triton comes from Voyager 2 which encountered the moon on August 25, 1989.