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Neptune IX

Proper NameHalimede
DesignationNeptune IX
Primary PlanetNeptune
Family / ClassRetrograde irregular moons
Orbital Period5 years, 53 days
Distance from NeptuneSemi-Major Axis: 16,681,000 km (0.11 AU)
Periapsis: 11,828,000 km (0.08 AU)
Apoapsis: 21,534,000 km (0.14 AU)
Rotation PeriodNot known
Mean Diameter62 km
39 miles

A small grey moon of Neptune, Halimede is no more than about sixty kilometres in diameter, and orbits Neptune at a distance of nearly seventeen million kilometres. Its name, like those of many of Neptune's moons, comes from a daughter of Nereus (that is, a Nereid) in Greek mythology, and is pronounced 'halimeedee'.

This mythological link with Nereus is an appropriate one, because available evidence suggests that Halimede was originally a part of the larger moon Nereid. Broken away from that larger moon, presumably by some cataclysmic impact, Halimede now pursues its own distant and eccentric orbit around Neptune. Its violent origin also resulted in a retrograde orbit; that is, the moon travels around Neptune in the opposite direction to the planet's spin on its axis (whereas regular moons follow a prograde orbital path, matching the planet's rotation).


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