Hippocamp.space

Inner moon of Neptune · Inner Neptunian system, just inside Proteus

Hippocamp

Neptune XIV · S/2004 N 1

A 35-km moon hiding in old Hubble images — possibly a chip knocked off Proteus by a comet.

Key factsTelescope picker
Hippocamp at a glance
Object typeSmall inner moon of Neptune[1]
Discovered1 July 2013 by Mark Showalter, in Hubble images from 2004–2009[1]
NamedFebruary 2019, after the mythical sea-horse[1]
Diameter≈ 35 km[1]
Orbital period≈ 23 hours[1]
Mean distance≈ 105,300 km from Neptune[3]
NeighbourOrbits about 12,000 km inside Proteus[1]

Found in the archive

Mark Showalter of the SETI Institute found Hippocamp on 1 July 2013 in archival Hubble images of Neptune.[1] It was too dim for Voyager 2 to detect during its 1989 flyby.[1]

Too close to Proteus

Hippocamp orbits only about 12,000 km inside Proteus, Neptune’s largest inner moon. Normally a big moon would sweep aside or swallow a small neighbour.[1][2]

A chip off Proteus?

Showalter’s team proposed that Hippocamp formed from debris knocked off Proteus by the comet impact that made Proteus’s giant crater Pharos.[1] Proteus is about 400 km across.[4]

Panning for moons

Showalter digitally warped and stacked short Hubble exposures to follow orbital motion, then extended his search beyond the rings and spotted a faint white dot.[1]

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Orbit

Hippocamp circles Neptune in about 23 hours at about 105,300 km.[1][3]

Neptune Proteus (117.7 thousand km)Triton (354.8 thousand km) farthest ≈ 105.4 thousand km closest ≈ 105.2 thousand km
Top-down orbit of Hippocamp to scale, computed from JPL orbital elements (a ≈ 105.3 thousand km, e ≈ 0.001).[3] Proteus and Triton orbits shown for comparison.[3] Mean orbital elements; Hippocamp’s orbit is nearly circular and close to Neptune’s equator.

The sea-horse

It is named after the hippocampus, a mythical creature with a horse’s upper body and a fish’s tail, symbol of Poseidon and Neptune.[1]

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The moon anyone could have found

How public Hubble images revealed Hippocamp.

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Born from a crater?

Hippocamp, Proteus and the Pharos impact.

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Neptune’s 16 moons

Where Hippocamp fits in a lopsided system.

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Sources

  1. Hippocamp (moon) — Wikipedia
  2. Hippocamp — NASA Science
  3. Planetary Satellite Mean Elements — NASA JPL Solar System Dynamics
  4. Proteus (moon) — Wikipedia
  5. Mark R. Showalter — Wikipedia
  6. Hubble Space Telescope — Wikipedia
  7. Neptune — Wikipedia

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