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Irregular moon of Neptune (retrograde) · Outer Neptunian system

Halimede

Neptune IX · S/2002 N 1

A grey moon on a wild, wandering orbit — with a 41% chance of hitting Nereid in 4.5 billion years.

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Halimede at a glance
Object typeIrregular, retrograde moon of Neptune[1]
Discovered14 Aug. 2002, Cerro Tololo, Chile[1]
Named3 Feb. 2007, after one of the Nereids[1]
Diameter≈ 42–62 km[1]
ColourGrey, similar to Nereid[1]
BrightnessApparent magnitude ≈ 24[1]
Mean distance≈ 16.6 million km from Neptune[3]
Orbital period≈ 1,879 days (5.1 years)[3]
Inclination≈ 120° (retrograde)[3]
Eccentricity≈ 0.52 (varies 0.19–0.90)[1]

Explore Halimede in 3D

See where Halimede is among the moons of Neptune on an interactive 3D map built from JPL orbital data. Drag to rotate, scroll or pinch to zoom.

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Science2 research papers in plain English, grouped by topic.Read the research →FactbookHalimede at a glance: every number sourced or labeled.Open the factbook →Business3 ways to make money, each with an editable calculator.Run the numbers →

Neptune’s second retrograde moon

Halimede is the second-innermost known retrograde moon of Neptune, after Triton. It takes about 5.1 years to orbit.[1]

A wandering orbit

The Sun’s gravity changes Halimede’s orbit substantially over thousands of years: its eccentricity ranges from 0.19 to 0.90 and its inclination from 111° to 145°.[1]

On a collision course?

A 2004 study estimated a 41% chance that Halimede hits Nereid within 4.5 billion years.[1] Its colour is similar to Nereid’s, so it might even be a fragment of Nereid.[1]

Found from the ground

Matthew Holman’s team found Halimede on 14 August 2002 with the 4-m Blanco Telescope, using shift-and-add imaging. It was among the first Neptunian moons found from the ground since Nereid in 1949.[1][2]

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Orbit

JPL’s mean elements give an average distance of about 16.6 million km, ranging from about 7.9 to 25.2 million km.[3]

Neptune Triton (0.35 million km)Nereid (5.51 million km) farthest ≈ 25.2 million km closest ≈ 7.95 million km
Top-down orbit of Halimede to scale, computed from JPL orbital elements (a ≈ 16.59 million km, e ≈ 0.521).[3] Triton and Nereid orbits shown for comparison.[3] Mean orbital elements; the real orbit is tilted steeply and shifts over time, so this top-down view flattens it onto one plane.

The name

Halimede is named after one of the fifty Nereids, daughters of Nereus and Doris. The “e” ending marks its retrograde orbit.[1]

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Halimede’s wandering orbit

How the Sun reshapes a distant moon’s path.

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Will Halimede hit Nereid?

A 41% chance over 4.5 billion years.

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Shift-and-add: finding faint moons

The technique behind Halimede’s 2002 discovery.

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Sources

  1. Halimede (moon) — Wikipedia
  2. Halimede — NASA Science
  3. Planetary Satellite Mean Elements — NASA JPL Solar System Dynamics
  4. Nereid (moon) — Wikipedia
  5. Neptune — Wikipedia

Figures are rounded; where sources differ we note it. Spotted an error? Tell us.