Laomedeia
A distant, prograde irregular moon of Neptune.
Laomedeia, designated Neptune XII and originally S/2002 N 3, is a distant, irregular moon of Neptune. It was discovered on August 13, 2002, by a team including Matthew Holman, JJ Kavelaars, Tommy Grav, Wesley Fraser, and Dan Milisavljevic, using the 4-meter Víctor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. Its name comes from a Nereid in Greek mythology. Laomedeia follows a prograde orbit around Neptune—meaning it moves in the same direction as the planet’s orbit around the Sun—but this orbit is highly eccentric and inclined, taking roughly 8.7 Earth years to complete. The moon is estimated to be between 37 and 42 kilometers (23 to 26 miles) in diameter, though most of its physical properties remain unknown. Its orbital characteristics are similar to those of two other Neptunian irregular moons, Sao and S/2002 N 5, suggesting they all originated from the breakup of a larger moon.
**Discovery**
Laomedeia was found during a search for distant Neptunian moons led by Matthew Holman, which began in 2001. The team used large ground-based optical telescopes, taking many long-exposure images of the sky around Neptune. They applied a shift-and-add technique, aligning and combining images to match Neptune’s motion, which made faint moons appear as points of light. This method revealed Laomedeia in images captured on August 13, 2002, at Cerro Tololo. It was one of the first Neptunian moons discovered from a ground telescope since Nereid in 1949. To confirm its orbit, follow-up observations were made at several observatories, guided by predictions from Brian G. Marsden and Robert A. Jacobson. A team led by Brett Gladman observed it from September 3 to 5, 2002, using the 8.2-meter Very Large Telescope at Cerro Paranal, while other teams used the 2.5-meter Nordic Optical Telescope at La Palma and the 5-meter Hale Telescope at Palomar. Earlier images from the Blanco Telescope on August 11, 2001, also showed Laomedeia. The discovery, along with those of Halimede and Sao, was announced by the Minor Planet Center on January 13, 2003, raising Neptune’s known moon count to 11.
**Name**
Upon announcement, Laomedeia received the temporary provisional designation S/2002 N 3 from the Minor Planet Center.
- Discovery date
- 13 August 2002
- Discoverers
- Matthew Holman, JJ Kavelaars, Tommy Grav, Wesley Fraser, Dan Milisavljevic
- Discovery site
- Cerro Tololo Inter-American Observatory, Chile
- Provisional designation
- S/2002 N 3
- Permanent designation
- Neptune XII
- Average orbital period
- about 8.7 Earth years
- Estimated diameter range
- 37 to 42 km
Lore & Background
Laomedeia was discovered on 13 August 2002 by a team led by Matthew Holman using the 4-meter Víctor M. Blanco Telescope at Cerro Tololo Inter-American Observatory in Chile. The discovery was announced by the Minor Planet Center on 13 January 2003, raising Neptune's known moon count to 11. It was named on 3 February 2007 by the IAU after Laomedeia, one of the Nereids from Greek mythology, with the name ending in 'a' to indicate its prograde orbit.
Laomedeia follows a distant, highly eccentric, and highly inclined prograde orbit around Neptune, with an average semi-major axis of about 23.5 million km and an average orbital period of about 8.67 years. Its orbit is strongly perturbed by the Sun, causing its eccentricity to range from 0.29 to 0.56 and its inclination from 30° to 44°. It is the outermost-known prograde moon of Neptune, nearly tied with S/2002 N 5. Laomedeia shares similar orbital characteristics with Sao and S/2002 N 5, forming the Sao group, which is believed to have originated from the collisional breakup of a once-larger captured moon.
Little is known about Laomedeia's physical characteristics. Its diameter is estimated between 37 and 42 km based on assumed albedos of 0.06 and 0.04, respectively. It has an apparent magnitude of 25.4 in red light, making it about 100 million times fainter than naked-eye visibility. Laomedeia appears to be roughly the same size as Sao if both have the same albedo.
Reader's Guide
Laomedeia's significance lies in its role as part of a new wave of Neptunian moon discoveries from ground-based telescopes, ending a 53-year drought since Nereid's discovery. Its discovery, alongside Halimede and Sao, demonstrated the effectiveness of the shift-and-add technique using large optical telescopes. As a member of the Sao group, Laomedeia provides evidence for collisional families among irregular moons, suggesting that Neptune's irregular satellite system has been shaped by catastrophic disruptions. The low mutual dispersion velocities between Sao group members (100–200 m/s) support a collisional origin from a once-larger moon captured early in Solar System history. Laomedeia's orbit, strongly perturbed by the Sun, illustrates the complex dynamics of distant irregular moons. Its legacy includes being among the first Neptunian irregular moons to have its orbit characterized through coordinated follow-up observations across multiple observatories, and its naming convention (ending in 'a') set a precedent for prograde Neptunian irregular moons.
Did You Know?
- It was among the first Neptunian moons discovered from a ground-based telescope since Nereid in 1949.
- Its name ends with 'a' to indicate its prograde orbit, following a convention similar to Jupiter's irregular moons.
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