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 in 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. The team used the shift-and-add technique to align and combine the images according to Neptune's motion across the sky, which enhanced the faint moons as points of light. Through this technique, they discovered Neso alongside two other Neptunian moons—Halimede and Sao—in images taken on 14 August 2002 by the 4-meter Víctor M. Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile. To determine Neso's orbit around Neptune, astronomers conducted follow-up observations at several observatories. A team led by Brett Gladman observed Neso on 3 September 2002 with the 8.2-m Very Large Telescope at Cerro Paranal Observatory, while Holman's team observed it later that day with the 2.5-m Nordic Optical Telescope at La Palma Observatory. Neso was next recovered by Holman on 19 August 2003 with the 4-m Blanco Telescope, followed by additional observations with Las Campanas Observatory's 6.5-m Magellan Clay Telescope from 28 to 30 September. After more than a year of observations, the Minor Planet Center announced Neso's discovery on 30 September 2003. It was the fifth and last moon of Neptune announced in 2003, bringing the planet's count of known moons to 13.
Name When the discovery of Neso was announced, it was given the temporary provisional designation "S/2002 N 4" by the Minor Planet Center. It was later named and given the Roman numeral designation Neptune XIII by the International Astronomical Union's (IAU's) Working Group for Planetary System Nomenclature on 3 February 2007. In accordance with the IAU's naming convention for Neptunian irregular moons, it was named after Neso (Νησώ), one of the fifty Nereids or daughters of Nereus and Doris from Greek mythology.
Orbit Neso is an irregular moon of Neptune, meaning it follows a distant, highly eccentric, and highly inclined orbit around the planet. Like the majority of irregular moons, Neso's orbit is retrograde, meaning it revolves around Neptune in the opposite direction to the planet's orbit around the Sun. Due to its great distance from Neptune, the moon is strongly affected by the Sun's gravity, which causes substantial long-term variations in its orbit. For this reason, Neso's orbit is better described by proper orbital elements, which are calculated by averaging out its perturbed orbit over an extended period of time. Over a 10,000-year time span, Neso's semi-major axis from Neptune varies from 47.7 to 52.8 million km (29.6 to 32.8 million mi; 0.319 to 0.353 AU), averaging about 49.9 million km (31.0 million mi; 0.334 AU). At such distances, Neptune would appear as a tiny, barely-visible disc from Neso with an average angular diameter of roughly 3 arcminutes. The scale of Neso's orbit is comparable to that of Mercury, which orbits the Sun at a semi-major axis of 57.9 million km (36.0 million mi; 0.387 AU). In terms of average semi-major axis, Neso is the second-outermost known moon of Neptune, behind S/2021 N 1. At its average distance, Neso occupies 43% of Neptune's Hill radius, placing it just over halfway to the outer limit at which retrograde moons can stably orbit the planet.
On average, Neso takes about 26.84 years (9,805 days) to complete one orbit around Neptune, though perturbations by the Sun can vary the orbital period from 25.05 to 29.23 years (9,150 to 10,675 days). Due to its long orbital period, it is unlikely to collide with other irregular moons. While Neso's orbit has an average eccentricity of 0.46 and an average inclination of 128° with respect to the ecliptic, both values vary substantially due to the Sun's perturbations. The moon's eccentricity ranges from 0.14 to 0.88, while its inclination ranges from 117° to 145°. Since Neso follows an eccentric orbit, its distance from Neptune varies by several tens of millions of km as it moves from periapsis to apoapsis in its orbit. However, because of its variable semi-major axis and eccentricity, its periapsis and apoapsis distances can change. For example, in October 2010, Neso passed periapsis at a distance of 17.8 million km (11.1 million mi; 0.119 AU), whereas in January 2038, Neso will pass periapsis at a closer distance of 10.6 million km (6.6 million mi; 0.071 AU). Likewise, in July 2025, Neso passed apoapsis at a distance 85.6 million km (53.2 million mi; 0.572 AU), whereas in July 2050, Neso will pass apoapsis at a further distance of 89.0 million km (55.3 million mi; 0.595 AU).
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