Neso (moon)

Neso (), also known as Neptune XIII and previously as S/2002 N 4, is the second-outermost-known moon of Neptune, behind S/2021 N 1. It was discovered on 14 August 2002 by Matthew Holman, JJ Kavelaars, Tommy Grav, Wesley Fraser, and Dan Milisavljevic at Cerro Tololo Inter-American Observatory, Chile. Named after one of the Nereids from Greek mythology, Neso orbits Neptune at an average distance of 49.9 million km (31.0 million mi)—almost as far as Mercury's orbital distance from the Sun—and takes about 26.8 Earth years to complete one orbit. It is classified as an irregular moon because it follows a distant, highly eccentric, and highly inclined orbit.

Due to Neso's great distance from Neptune, its orbit is significantly influenced by the Sun's gravity. As a result, the moon experiences the Kozai resonance, which causes large, synchronized variations in its orbital eccentricity and inclination over a 600-year period. Neso shares similar orbital characteristics with two other Neptunian irregular moons, Psamathe and S/2021 N 1, which suggests they originated from the collisional breakup of a once-larger moon of Neptune billions of years ago. Most of Neso's physical properties are unknown, though telescope observations suggest it has a reddish color and a diameter between 43 and 60 km (27 and 37 mi).

Discovery

Neso was discovered by astronomers Matthew Holman, JJ Kavelaars, Tommy Grav, Wesley Fraser, and Dan Milisavljevic during a search for distant moons of Neptune. Led by Holman, the search began in 2001 and employed large optical telescopes on Earth to obtain numerous long-exposure images of the sky around Neptune.