Discovered in 1846, Triton is the largest moon of Neptune - but it’s unlike any other. Scientists believe it was once a free-floating planet before Neptune’s gravity captured it.

Ganymede, Jupiter's largest moon and the solar system's largest, is bigger than Mercury and the dwarf planet Pluto, with a diameter of 5,268 kilometers.

Potentially launching around 2039 is a nuclear-powered mission to Neptune to study its ring, atmosphere, and the moon Triton. The orbiter mission would aim to explore the habitability of the ...