Halimede (), also known as Neptune IX and previously as S/2002 N 1, is an irregular moon of Neptune. It was discovered on 14 August 2002 by Matthew Holman, JJ Kavelaars, Tommy Grav, Wesley Fraser, and Dan Milisavljevic at Cerro Tololo Inter-American Observatory in Chile. Named after one of the Nereids from Greek mythology, Halimede follows a distant, highly eccentric, and highly inclined orbit with an average orbital period of about 5.1 Earth years. Like the majority of irregular moons, Halimede's orbit is retrograde, meaning it revolves around Neptune in the opposite direction to the planet's orbit around the Sun. It is the second-innermost known retrograde moon of Neptune, after Triton. The orbit of Halimede varies substantially over thousands of years, because it is significantly perturbed by the Sun's gravity. Over billions of years, Halimede has a high chance of colliding with Neptune's large moon Nereid. Telescope observations indicate that Halimede is a gray, non-spherical object with a diameter between 42 and 62 km (26 and 39 mi). Its color is similar to that of Nereid, which has led some astronomers to hypothesize that Halimede may be a collisional fragment of Nereid.
Discovery Halimede was discovered by astronomers Matthew Holman, JJ Kavelaars, Tommy Grav, 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 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. Halimede was among the first Neptunian moons discovered from a ground-based telescope since Nereid in 1949. 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. 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, 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 Halimede were found by Holman in Blanco Telescope images from 10 August 2001. The discovery of Halimede, alongside the Neptunian moons Sao and Laomedeia, was announced by the Minor Planet Center on 13 January 2003. The announcement raised Neptune's number of known moons to 11. Following the announcement of Halimede's discovery, its discoverers conducted additional observations in 2003 and 2004 to refine calculations of the moon's orbit. During this work, they identified precovery images of Halimede from 18 July 1999, taken by an earlier survey for Neptunian moons with the Canada–France–Hawaii Telescope at Maunakea Observatory. These constitute the earliest known observations of the moon. 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. As of 2026, Halimede's observation arc spans more than 22 years, covering over four times its orbital period.
Name When the discovery of Halimede was announced, it was given the temporary provisional designation "S/2002 N 1" by the Minor Planet Center. 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. 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. 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.
… excerpt ends here. Continue reading the full article.
