Halimede (moon)

Halimede 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.^{[15][16]: 865} 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.^{[15][16]: 865–866} Through this technique,^[16] they discovered Halimede alongside two other Neptunian moons—Sao and Neso—in images taken on 14 August 2002 by the 4-meter Víctor M. Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile.^[3] Halimede was among the first Neptunian moons discovered from a ground-based telescope since Nereid in 1949.^[1][15]

To determine Halimede'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 Halimede on 3 and 4 September 2002 using the 8.2-m Very Large Telescope at Cerro Paranal Observatory,^[17] 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.^[18][15] Earlier observations of Halimede were found by Holman in Blanco Telescope images from 10 August 2001.^[17] The discovery of Halimede, alongside the Neptunian moons Sao and Laomedeia, was announced by the Minor Planet Center on 13 January 2003.^[17][18] The announcement raised Neptune's number of known moons to 11.^[19]

Following the announcement of Halimede's discovery, its discoverers conducted additional observations in 2003 and 2004 to refine calculations of the moon's orbit.^[16]: 866 During this work, they identified precovery images of Halimede from 18 July 1999,^[7] taken by an earlier survey for Neptunian moons with the Canada–France–Hawaii Telescope at Maunakea Observatory.^[16]: 866 These constitute the earliest known observations of the moon.^[7] The additional observations extended Halimede's observation arc to cover almost its entire orbital period, which allowed the discoverers to accurately determine the moon's orbit at an early stage.^[16]: 866 As of 2026^[update], Halimede's observation arc spans more than 22 years, covering over four times its orbital period.^[7]

• 
  The Víctor M. Blanco Telescope was used for the discovery of Halimede in 2002.
• 
  Discovery images of Halimede from the Blanco Telescope. The images were shifted-and-added to reveal Halimede as a single dot against trailed background stars.

When the discovery of Halimede was announced, it was given the temporary provisional designation "S/2002 N 1" by the Minor Planet Center.^[17] It was later named and given the Roman numeral designation Neptune IX by the International Astronomical Union's (IAU's) Working Group for Planetary System Nomenclature on 3 February 2007.^[20] In accordance with the IAU's naming convention for Neptunian irregular moons, it was named after Halimede, one of the fifty Nereids or daughters of Nereus and Doris from Greek mythology.^[1][21] 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.^[22]: 414

Halimede is an irregular moon of Neptune, meaning it follows a distant, highly eccentric, and highly inclined orbit around the planet.^[23][d] Like the majority of irregular moons, Halimede's orbit is retrograde, revolving around Neptune in the opposite direction to the planet's orbit around the Sun.^[24]: 1 Due to its great distance from Neptune, Halimede is strongly affected by the Sun's gravity, which causes gradual but substantial variations in its orbit over thousands of years.^[24]: 4 For this reason, Halimede's orbit is better described by proper orbital elements, which are calculated by averaging out its perturbed orbit over an extended period of time.^[24]: 4

Over a 10,000-year time span, Halimede's semi-major axis from Neptune varies from 16.55 to 16.63 million km (10.28 to 10.33 million mi; 0.1106 to 0.1112 AU), averaging about 16.59 million km (10.31 million mi; 0.1109 AU).^[b] In terms of average semi-major axis, Halimede is the second-innermost known retrograde moon of Neptune, after Triton.^[8] On average, Halimede takes about 5.14 years (1,879 days) to complete one orbit around Neptune,^[8] though perturbations by the Sun can vary the orbital period from 5.13 to 5.16 years (1,873 to 1,886 days).^[b] While Halimede's orbit has an average eccentricity of 0.52 and an average inclination of 120° with respect to the ecliptic, both values vary substantially due to the Sun's perturbations.^[24] The moon's eccentricity ranges from 0.19 to 0.90, while its inclination ranges from 111° to 145°.^[b]

Among all known retrograde irregular moons in the Solar System as of 2026^[update], Halimede has the lowest value for its average inclination.^[25] Since retrograde orbits are defined as having inclinations greater than 90°,^[13] Halimede's low inclination value means its orbit is actually highly inclined.^[23]: 7 When disregarding Halimede's orbital direction, the moon's average orbit is inclined 60° from the ecliptic.^[e] For comparison, the most inclined known prograde irregular moon, Margaret, has an average inclination of 60.5° with respect to the ecliptic.^[25]

Halimede does not appear to experience any orbital resonances,^[23] nor does it share orbit similarities with other Neptunian irregular moons.^[6]: 9 Its orbit exhibits both apsidal and nodal precession with periods of 5,395 and 5,156 years, respectively.^[8] Due to apsidal precession, the moon's argument of pericenter circulates from 0° to 360°.^[23]: 7 Halimede's longitude of pericenter circulates regularly in the direction of its retrograde orbit.^[23]: 7

Little is known about Halimede's physical characteristics.^[1] Like most of Neptune's irregular moons, Halimede is extremely faint and difficult to observe due to its small size and great distance from the Sun.^[15] In visible light (V band), Halimede has an apparent magnitude of about 24.^[10]: L78 With apparent magnitudes this dim, Halimede and most of Neptune's irregular moons can only be observed through long-exposure imaging with large optical telescopes.^{[15][24]: 12}

The diameter of Halimede has not been directly measured, although it can be indirectly estimated from its brightness and an assumed geometric albedo.^{[9]: 173–174} Assuming a low geometric albedo of 0.04, Halimede's diameter would be about 62 km (39 mi).^[9]: 174 For an albedo of 0.06, Halimede would have a diameter of 54 km (34 mi).^[16]: 866 For an albedo of 0.1, Halimede would have a diameter of 42 km (26 mi).^[6]: 15 Halimede appears to be similar in size to the parent bodies that formed the Sao and Neso groups of Neptunian irregular moons.^[6]: 12

Telescope observations from 2003 found that Halimede changes brightness significantly between days, with its V-band absolute magnitude dropping from 9.01±0.07 to 9.74±0.08 between July 27–28 and August 1.^[10] This brightness change suggests that Halimede is rotating with a non-spherical shape.^[10] The observations also showed that Halimede has a neutral (gray^[11]) color similar to that of Nereid, suggesting that they share a surface composition rich in water ice.^[10] The colors of Halimede and other Neptunian irregular moons indicate that they lack the "ultrared" material commonly seen on the surfaces of Kuiper belt objects, suggesting that their surfaces have been altered by collisions or sublimation of volatiles.^[12]

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