Laomedeia

Laomedeia was discovered by astronomers Matthew Holman, JJ Kavelaars, Tommy Grav, Wesley Fraser, and Dan Milisavljevic during a search for distant moons of Neptune.^[1] 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.^{[11]: 865 [15]} 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.^{[11]: 865–866 [15]} Through this technique, they discovered Laomedeia in images taken on 13 August 2002 by the 4-meter Víctor M. Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile.^[1] Laomedeia was among the first Neptunian moons discovered from a ground-based telescope since Nereid in 1949.^[1][15]

To determine Laomedeia'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.^[15] A team led by Brett Gladman observed Laomedeia from 3 to 5 September 2002 using the 8.2 m Very Large Telescope at Cerro Paranal Observatory,^[16] 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.^[15][17] Earlier observations of Laomedeia were found by Holman in Blanco Telescope images from 11 August 2001.^[16] The discovery of Laomedeia, alongside the Neptunian moons Halimede and Sao, was announced by the Minor Planet Center on 13 January 2003.^[16][17] The announcement raised Neptune's number of known moons to 11.^[18]

When the discovery of Laomedeia was announced, it was given the temporary provisional designation "S/2002 N 3" by the Minor Planet Center.^[16] It was later named and given the Roman numeral designation Neptune XII by the International Astronomical Union's (IAU) Working Group for Planetary System Nomenclature on 3 February 2007.^[19] In accordance with the IAU's naming convention for Neptunian irregular moons, it was named after Laomedeia (Λαομέδεια), one of the fifty Nereids or daughters of Nereus and Doris from Greek mythology.^[1][6] The moon was given a name ending with "a" to indicate its prograde orbit, in a similar fashion to the naming convention for Jupiter's irregular moons.^[20]: 414

Laomedeia is an irregular moon of Neptune, meaning it follows a distant, highly eccentric, and highly inclined orbit around the planet.^[21][d] The moon's orbit is prograde, meaning it revolves around Neptune in the same direction as the planet's orbit around the Sun.^[22]: 1 Like many other irregular moons, Laomedeia is strongly affected by the Sun's gravity, which causes substantial long-term variations in its orbit.^[22]: 4 For this reason, Laomedeia's orbit is better described by proper orbital elements, which are calculated by averaging out its perturbed orbit over an extended period of time.^[22]: 4

Over a 10,000-year time span, Laomedeia's semi-major axis from Neptune varies from 23.3 to 23.7 million km (14.5 to 14.7 million mi; 0.156 to 0.158 AU), averaging about 23.5 million km (14.6 million mi; 0.157 AU).^[b] In terms of average semi-major axis, Laomedeia is the outermost-known prograde moon of Neptune, though it is nearly tied with S/2002 N 5.^[10] On average, Laomedeia takes about 8.67 years (3,168 days) to complete one orbit around Neptune,^[10] though perturbations by the Sun can vary the orbital period from 8.59 to 8.78 years (3,137 to 3,206 days).^[b] While Laomedeia's orbit has an average eccentricity of 0.42 and an average inclination of 37° with respect to the ecliptic, both values vary substantially due to the Sun's perturbations.^[22] The moon's eccentricity ranges from 0.29 to 0.56, while its inclination ranges from 30° to 44°.^[b] Compared to other irregular moons of Neptune, Laomedeia's orbit variations are not as extreme.^[21]: 7

Laomedeia does not appear to experience any orbital resonances.^[21] Its orbit exhibits both apsidal and nodal precession with periods of 2,255 and 3,154 years, respectively.^[10] Due to apsidal precession, the moon's argument of pericenter circulates uniformly from 0° to 360°.^[21]: 7 A 2004 study led by Holman suggested that Laomedeia is unlikely to collide with Neptune's large outer moon Nereid over a period of 4.5 billion years.^[11] A 2022 study led by Jet Propulsion Laboratory astrophysicists Marina Brozović and Robert A. Jacobson found that Laomedeia and Sao can approach within 320,000 km (200,000 mi) of each other during a 30,000-year time span.^[22]: 11

Group and origin

Laomedeia shares similar orbital characteristics with two other Neptunian irregular moons, Sao and S/2002 N 5.^[8] Together, they form the Sao group, a family of prograde irregular moons of which Sao is the largest member.^[8] Laomedeia was first suspected to be related to Sao in 2004.^{[8]: 2 [11]: 867} The group is predicted to contain many more smaller members, though they are too faint to be detected with current technology.^{[8]: 9 [23]}

Because of their similar orbits, the moons of the Sao group have low mutual dispersion velocities between 100 and 200 m/s.^{[8]: 11–12} The dispersion velocity is the speed needed to eject a fragment from a parent body onto its current orbit.^[8]: 11 For fragments produced in collisions, their dispersion velocities are typically below 100 m/s, comparable to the parent body's escape velocity.^[8]: 11 The minimum dispersion velocities of the Sao group are low enough to suggest that it originated from a collision, meaning it may be a collisional family.^[8]: 12 The Sao group is believed to have originated from the destruction of a once-larger irregular moon by a collision with a passing asteroid or comet.^[1][23] This former moon was likely gravitationally captured by Neptune during the Solar System's early history, during or shortly after the formation of the planets.^[8]

If the Sao group is a collisional family, then its members are expected to are expected to share similar surface characteristics, such as color.^[22]: 11 Compared to other collisional families, the Sao group is unusual in that two of its largest members—Sao and Laomedeia—have similar brightness and estimated sizes.^[8]: 12

Little is known about Laomedeia's physical characteristics.^[1] Like most of Neptune's irregular moons, Laomedeia is extremely faint and difficult to observe due to its small size and great distance from the Sun.^[15] In red light (R band), Laomedeia has an apparent magnitude of 25.4,^[11][13] making it about 100 million times fainter than what can be seen with the naked eye.^[1][15] Because of this, Laomedeia and most of Neptune's irregular moons can only be observed through long-exposure imaging with large optical telescopes.^{[15][22]: 12}

The diameter of Laomedeia has not been directly measured, although it can be indirectly estimated from its brightness and an assumed geometric albedo.^{[12]: 173–174} Assuming a geometric albedo of 0.04, Laomedeia's diameter would be about 42 km (26 mi).^[12]: 174 For a higher albedo of 0.06, Laomedeia would have a diameter of 37 km (23 mi).^[11]: 866 Laomedeia appears to be roughly the same size as Sao, if both moons have the same albedo.^[8]: 12

• Other Neptunian irregular moons discovered in 2002:
  • Halimede
  • Sao
  • Neso
  • S/2002 N 5 (lost in 2002, confirmed much later in 2024)