Sao (), also known as Neptune XI and previously as S/2002 N 2, is an irregular moon of Neptune. It was discovered on 14 August 2002 by Tommy Grav, Matthew Holman, JJ Kavelaars, Wesley Fraser, and Dan Milisavljevic at Cerro Tololo Inter-American Observatory in Chile. Named after one of the Nereids from Greek mythology, Sao follows a distant, highly eccentric, and highly inclined orbit around Neptune with an average orbital period of about 8 Earth years. Its orbit is prograde, meaning it revolves around Neptune in the same direction as the planet's orbit around the Sun. Due to Sao'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 3,000-year period. Sao shares similar orbital characteristics with two other Neptunian irregular moons, Laomedeia and S/2002 N 5, which suggests they originated from the collisional breakup of a once-larger moon of Neptune. Sao is estimated to have a diameter between 31 and 44 km (19 and 27 mi), though most of its physical properties are unknown.
Discovery Sao was discovered by astronomers Tommy Grav, Matthew Holman, JJ Kavelaars, 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 Sao alongside two other Neptunian moons—Halimede and Neso—in images taken on 14 August 2002 by the 4-meter Víctor M. Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile. Sao was among the first Neptunian moons discovered from a ground-based telescope since Nereid in 1949. To determine Sao's orbit around Neptune, astronomers conducted follow-up observations at several observatories, with the help of orbit predictions by Brian G. Marsden and Robert A. Jacobson. A team led by Brett Gladman observed Sao on 3 and 5 September 2002 with the 8.2-m Very Large Telescope at Cerro Paranal Observatory, while other teams observed it with the 2.5-m Nordic Optical Telescope at La Palma Observatory and the 5-m Hale Telescope at Palomar Observatory. Earlier observations of Sao were found by Holman in Canada–France–Hawaii Telescope images from 23 July 2001. The discovery of Sao, alongside the Neptunian moons Halimede and Laomedeia, was announced by the Minor Planet Center on 13 January 2003. The announcement raised Neptune's number of known moons to 11.
Name When the discovery of Sao was announced, it was given the temporary provisional designation "S/2002 N 2" by the Minor Planet Center. It was later named and given the Roman numeral designation Neptune XI 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 Sao (Σαώ), one of the fifty Nereids or daughters of Nereus and Doris from Greek mythology.
Orbit Sao is an irregular moon of Neptune, meaning it follows a distant, highly eccentric, and highly inclined orbit around the planet. Sao's orbit is prograde, meaning it revolves around Neptune in the same direction as 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, Sao'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, Sao's semi-major axis from Neptune varies from 22.1 to 22.4 million km (13.7 to 13.9 million mi; 0.148 to 0.150 AU), averaging about 22.2 million km (13.8 million mi; 0.148 AU). On average, Sao takes about 7.99 years (2,919 days) to complete one orbit around Neptune, though perturbations by the Sun can vary the orbital period from 7.92 to 8.05 years (2,894 to 2,942 days). While Sao's orbit has an average eccentricity of 0.30 and an average inclination of 50° with respect to the ecliptic, both values vary substantially due to the Sun's perturbations. The moon's eccentricity ranges from 0.07 to 0.63, while its inclination ranges from 39° to 55°. Among Neptune's known prograde moons, Sao has the highest average inclination. Because of its varying eccentricity, Sao can occasionally pass close to Nereid, one of Neptune's largest irregular moons. Over a period of 4.5 billion years, these close encounters could result in gravitational scattering, or even a 1.6% chance of collision. A 2022 study led by Jet Propulsion Laboratory astrophysicists Marina Brozović and Robert A. Jacobson found that Sao and Laomedeia can approach within 320,000 km (200,000 mi) of each other during a 30,000-year time span.
Kozai resonance
Sao's high inclination places it in the Kozai resonance, a dynamical phenomenon in which perturbations from the Sun cause periodic exchanges between orbital eccentricity and inclination. First recognized by Holman and colleagues in 2004, the Kozai resonance causes Sao's eccentricity and inclination to oscillate out of phase over a period of about 3,000 years, with eccentricity reaching maximum at minimum inclination and vice versa. While Sao's orbit exhibits nodal precession with a period of 4,454 years, the Kozai resonance prevents it from apsidally precessing. Instead of circulating from 0° to 360°, Sao's argument of pericenter librates around 90° with a period of about 3,000 years. Sao is the only known prograde irregular moon of Neptune that is in the Kozai resonance.
Group and origin
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