A Backwards Moon

Voyager 2 mosaic of Neptune's largest moon Triton

At 1,680 miles across, the frigid and wrinkled Triton is distant Neptune’s largest moon. It orbits the planet backwards – that is, in the opposite direction that Neptune rotates – and is the only moon one of only two moons in our solar system to do so. This leads many astronomers to believe that Triton is a captured Kuiper Belt Object that fell into orbit around Neptune at some point in our solar system’s nearly 4.7-billion-year history.

Triton is the largest by far of Neptune’s 13 known moons. This beautiful portrait of Triton is a false-color mosaic taken in 1989 by NASA’s Voyager 2 spacecraft. It is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn’s moon, Titan). It’s frozen surface is made mainly of nitrogen ice, with some methane ice at the south pole. Voyager 2 observed some active geyser-like plumes erupting from the surface, perhaps like those active now on Jupiter’s Saturn’s moon Enceladus. Like Phoebe’s orbit around Saturn, Triton orbits Neptune in the opposite direction than the orbits of most objects in the Solar System.

The image above is one of many selected to be part of the 2011 From The Earth To The Solar System (FETTSS) collection. This is a series of over 90 images of Earth as well as some of the worlds that make up our solar system, chosen for their beauty and scientific value. These images are available as high-resolution downloads to registered parties wishing to print and display them in public places for others to experience. Many such events – assembled with private funding – have already then place in cities and towns all across the world, typically with a great turnout!

Read more about FETTSS here.

Image Credit: NASA/JPL/USGS



  1. Jeff Barani says:

    There’s a “little error” in the comment.
    Enceladus isn’t a Jupiter’s moon but surely a Saturn’s moon no ?? 😉
    Jeff Barani from Vence (France)


    1. J. Major says:

      That’s true! I’ll have to let them know. Thanks!


  2. thereviewer says:

    Nitrogen ice! Fascinating, beautiful and bizarre how Triton anti-orbits…


  3. The first paragraph says Triton is the only retrograde moon in the solar system…but then the quoted paragraph says Phoebe orbits Saturn backwards. So is Triton not the only one?


    1. J. Major says:

      Good catch…. yes, Phoebe also orbits in a retrograde fashion. Triton is the only large “major” moon to do so, since Phoebe is rather small – 132 miles wide. https://lightsinthedark.wordpress.com/2011/05/12/from-the-litd-archives-a-primordial-moon/


Comments are closed.