(written as Triton)satellite of Neptune discovered by Lassell in 1846. Triton has a circular, retrograde orbit 355,000km from Neptune and takes 5.9 days to complete. An icy planet (although the ices at its surface may be dominated by solid nitrogen rather than water ice) resembling Pluto and Charon, and some 4000km across, Triton is in a decaying orbit which will apparently lead to its destruction by tidal forces in under 100 million years. Triton seems to have a thin methane atmosphere. There has been speculation that it may be one of a clutch of thousands of such bodies in the outer reaches of the early solar system, the rest being expelled into the Kuiper Belt or the Oort Cloud by gravitational perturbation. Its surface features indicate a past when it may have been warmer than now, with liquids present, as well as faulting that suggests that Triton may have been geologically active at some point in its past.