Figure 1: An artist’s rendition of the planet Neptune.
Neso is a small, irregular natural satellite in a far-flung orbit around the planet Neptune. Not much is known about Neso. It was discovered in August 2002 and given the provisional designation S/2002 N4. In February 2007, it was named “Neso” by the International Astronomical Union (IAU). Neptune’s large distance from the Sun means that its gravitational field can dominate an extremely wide region of space around it, giving it a large gravitational sphere of influence. As a consequence, Neptune can have moons that orbit around it at large distances and still remain gravitationally bound to Neptune.
Neso orbits Neptune at a mean distance of 50.26 million km. However, the orbit of Neso is fairly elongated. Neso comes as close as 28.93 million km to Neptune and recedes as far as 71.58 million km, its maximum distance from Neptune, making it the most distant known moon of any planet. Neso is in such a wide orbit around Neptune that it takes a whopping 26.67 years to go around Neptune just once. The orbital period of Neso is more than twice the orbital period of Jupiter and nearly as long as the orbital period of Saturn.
In fact, when Neso is furthest from Neptune, the distance between it and Neptune actually exceeds the maximum distance of Mercury from the Sun. Neso orbits Neptune in a retrograde orbit, which means it is in the opposite direction to the rotation of Neptune. Neso’s orbit appears rather similar to the orbit of Psamathe, another natural satellite of Neptune. Both objects might have originated from the break-up of a larger moon. From its observed brightness and assuming an albedo of 0.04, the diameter of Neso is estimated to be ~60 km.
Figure 2: A plan view showing the orbits of some of Neptune’s moons. The orbit of Neso is the one in red, elongated towards the lower left. The dashed circle shows the theoretical outer limit of stability for any moon around Neptune. The orbit of Triton, Neptune’s only large moon, is barely visible on this scale and is represented by the black dot at the center. Sheppard et al. (2006).
Sheppard et al. (2006), “A Survey for “Normal” Irregular Satellites around Neptune: Limits to Completeness”, The Astronomical Journal 132: 171-176.