Halimede.space

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]

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.