Psamathe (), also known as Neptune X and previously as S/2003 N 1, is an irregular moon of Neptune. It was discovered by Scott Sheppard, David Jewitt, and Jan Kleyna at Mauna Kea Observatory on 29 August 2003. Named after one of the Nereids from Greek mythology, Psamathe follows a distant, highly eccentric, and highly inclined orbit with an average orbital period of about 25 Earth years. Like the majority of irregular moons, Psamathe's orbit is retrograde, meaning it revolves around Neptune in the opposite direction to the planet's orbit around the Sun. Psamathe is estimated to have a diameter between 36 and 40 km (22 and 25 mi), though most of its physical properties are unknown. Psamathe shares similar orbital characteristics with Neptune's two outermost moons, Neso and S/2021 N 1, which suggests they originated from the collisional breakup of a once-larger moon of Neptune billions of years ago.
Discovery Psamathe was discovered by astronomers Scott Sheppard, David Jewitt, and Jan Kleyna on 29 August 2003. At the time, the team was searching for distant moons of Neptune, using the 8.2-meter (27 ft) Subaru Telescope at Mauna Kea Observatory, Hawaii. Their search strategy involved using the telescope's camera to photograph patches of the sky around Neptune at roughly 33-minute intervals, which revealed the planet's moons as faint, slowly-moving points of light. The team detected Psamathe in images taken on 29 and 30 August 2003, and reported their discovery to the Minor Planet Center (MPC) on 1 September. Sometime later on 1 September 2003, Matthew Holman reported the identification of a candidate moon of Neptune in observations from 19 August 2003 and 11 August 2001. These observations came from an earlier search for Neptunian moons, which was led by Holman at Cerro Tololo Inter-American Observatory, Chile. Brian G. Marsden of the MPC recognized that Holman's moon candidate might turn out to be Psamathe, which became confirmed after Holman uncovered more observations from August 2002 and July 2003. With over two years of recorded observations, the MPC announced the discovery of Psamathe on 3 September 2003.
Name When the discovery of Psamathe was announced, it was given the temporary provisional designation "S/2003 N 1" by the Minor Planet Center. Sheppard's team named the moon "Psamathe" in a 2006 paper describing its discovery, although the name did not receive official approval by the International Astronomical Union (IAU) until 3 February 2007. The moon was named after Psamathe (Ψαμάθη), one of the fifty Nereids or daughters of Nereus and Doris from Greek mythology. The moon was given a name ending with "e" to indicate its retrograde orbit, in a similar fashion to the naming convention for Jupiter's irregular moons. When the name was approved, the IAU gave Psamathe its Roman numeral designation "Neptune X".
Orbit Psamathe 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, Psamathe'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, Psamathe'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, Psamathe's semi-major axis from Neptune varies from 45.7 to 50.2 million km (28.4 to 31.2 million mi; 0.305 to 0.336 AU), averaging about 47.6 million km (29.6 million mi; 0.318 AU). In terms of average semi-major axis, Psamathe is the third-outermost known moon of Neptune, behind Neso and S/2021 N 1. At its average distance, Psamathe occupies 41% 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, Psamathe takes about 25.05 years (9,149 days) to complete one orbit around Neptune, though perturbations by the Sun can vary the orbital period from 23.50 to 27.06 years (8,583 to 9,884 days). While Psamathe's orbit has an average eccentricity of 0.41 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 0.07 to 0.88, while its inclination ranges from 117° to 148°.
Since Psamathe 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 May 1988, Psamathe passed periapsis at a distance of 30.8 million km (19.1 million mi; 0.206 AU), whereas in October 2014, Psamathe passed periapsis at a farther distance of 36.1 million km (22.4 million mi; 0.241 AU). Likewise, Psamathe passed apoapsis in September 1999 at a distance of 63.0 million km (39.1 million mi; 0.421 AU), whereas Psamathe passed apoapsis at a closer distance of 57.3 million km (35.6 million mi; 0.383 AU). Psamathe is not in an orbital resonance, unlike its neighbors Neso and S/2021 N 1. Over a period of 4.5 billion years, Psamathe has 1.3% chance of colliding with Nereid, one of Neptune's largest irregular moons. Its orbit exhibits both apsidal and nodal precession with periods of 1,484 and 1,333 years, respectively. Psamathe has a longer apsidal precession period than its nodal precession period, which causes its longitude of pericenter (ϖ) to precess in the opposite direction from its orbital motion. This behavior makes Psamathe a "reverse circulator".
… excerpt ends here. Continue reading the full article.
