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Hippocamp, originally known as S/2004 N 1, is a small moon of Neptune, about 35 km (20 mi) in diameter, which orbits the planet in just under one Earth day. Its discovery on 1 July 2013 increased the number of Neptune's known satellites to fourteen.[7] The moon is so dim that it was not observed when the Voyager 2 space probe flew by Neptune and its moons in 1989. Mark Showalter of the SETI Institute found it by analyzing archived Neptune photographs the Hubble Space Telescope captured between 2004 and 2009.[8]

S-2004 N1 Hubble montage.jpg
Composite of Hubble images from 2009 showing Neptune, its rings, Hippocamp, and other inner moons. The relative brightness of the moons is exaggerated.
Discovered byM. R. Showalter, I. de Pater, J. J. Lissauer, R. S. French[1]
Discovery dateJuly 1, 2013
Named after
S/2004 N 1
Orbital characteristics[4]
105,284 km
0.9362 d[2]
8.9408 km/s[3]
Satellite ofNeptune
Physical characteristics
Mean radius
17.4±2.0 km[4]
Mass≈5×1016 kg[6]
Albedoassumed low (≈0.09[4])

The moon was formally numbered Neptune XIV (14) on 25 September 2018 in Minor Planet Circular 111804, and named Hippocamp in February 2019.


Mark Showalter discovered Hippocamp on 1 July 2013 while examining Hubble Space Telescope (HST) images of Neptune's ring arcs from 2009. He used a technique similar to panning to compensate for orbital motion and allow stacking of multiple images to bring out faint details.[7][9][10] After deciding "on a whim" to expand the search area to radii well beyond the rings,[11] he found the "fairly obvious dot" that represented the new moon.[2] He then found it repeatedly in other archival HST images going back to 2004. Voyager 2, which had observed all of Neptune's other inner satellites, did not detect it during its 1989 flyby, due to its dimness.[7] Given that the relevant images have long been available to the public, the discovery could have been made by anyone.[2]

Hippocamp is the first moon of Neptune to be discovered since September 2003,[10][12] when Psamathe was discovered.


Neptune's largest moon, Triton, has a retrograde and inclined orbit. It is hypothesized that Neptune captured it from the Kuiper belt well after Neptune's original satellite system formed. The pre-existing moons' orbits would have been shifted by this event, leading to the ejection of some moons and the collisional destruction of others.[13][14] At least some of Neptune's present inner satellites are thought to have then accreted from the resulting rubble after Triton's orbit was circularized by tidal deceleration.[15]

Another hypothesis suggests that Hippocamp may have formed specifically out of debris from the nearest moon of Neptune, the much larger Proteus.[4][6] The debris was possibly ejected by the comet impact that formed its largest crater Pharos; this ejecta would have had about 50 times greater volume than Hippocamp. Hippocamp orbits relatively close to Proteus; their semi-major axes differ by about 12,000 km. Hippocamp probably formed within a few thousand kilometres from Proteus; Proteus would then have gradually receded from it due to its much stronger tidal interaction with Neptune.[4]

Physical propertiesEdit

Neptune and its smallest moon Hippocamp (artist's impression).[16]

Hippocamp is assumed to resemble Neptune's other inner satellites in having a surface as dark as "dirty asphalt".[9] Their geometrical albedos range from 0.07 to 0.10.[17] Derived from Hippocamp's apparent magnitude of 26.5, its diameter was initially thought to be around 16 to 20 km, making it the smallest of Neptune's known moons. More recent observations of Neptune's moons have shown that Hippocamp is almost twice as large as previously thought, giving it a diameter of 34.8 km.[4] However, it remains by a wide margin the smallest of Neptune's inner, regular, satellites.

The near-infrared spectra of Neptune's rings and inner moons have been examined with the HST NICMOS instrument.[18][19] Similar dark, reddish material, characteristic of small outer Solar System bodies, appears to be present on all their surfaces. The data is consistent with organic compounds containing C−H and/or C≡N bonds,[19] but spectral resolution was inadequate to identify the molecules. Water ice, abundant in the outer Solar System, is believed to be present, but its spectral signature could not be observed (unlike the case of small Uranian moons).[19]

Orbital propertiesEdit

Diagram of the orbits of Neptune's moons out to Triton, with Hippocamp's orbit highlighted.

Hippocamp completes one revolution around Neptune every 22 hours and 28.1 minutes (0.9362 days),[2] implying a semi-major axis, or orbital distance of 105,283 km (65,420 mi), just over a quarter that of Earth's Moon, and roughly twice the average radius of Neptune's rings. Both its inclination and eccentricity are close to zero.[2] It orbits between Larissa and Proteus, making it the second outermost of Neptune's regular satellites. Its small size at this location runs counter to a trend among the other regular Neptunian satellites of increasing diameter with increasing distance from the primary.[5]

The periods of Larissa and Hippocamp are within about one percent of a 3:5 orbital resonance,[Note 1] while Hippocamp and Proteus are nearly in a 11:13 resonance.[20] Larissa and Proteus are thought to have passed through a 1:2 mean-motion resonance a few hundred million years ago.[21][22] Proteus[21] and Hippocamp have drifted away from Larissa since then because the former two are outside Neptune-synchronous orbit (Neptune's rotational period is 0.6713 day[23]) and are thus being tidally accelerated, while Larissa is within and is being tidally decelerated.[21]


The moon is named after the Hippocamp, a mythological creature that is half horse and half fish in Greek mythology.[24] When the moon was numbered as Neptune XIV, it remained without an official name until February 2019.

The discovery team decided to submit a name proposal to the International Astronomical Union based on a figure from Greco-Roman mythology with a relationship to Poseidon or Neptune, the god of the sea,[11] consistent with the naming of other moons of Neptune.[25] Among the names considered was Polyphemus, the gigantic one-eyed son of Poseidon and Thoosa.[26] Showalter, the lead of the discovery team, chose the name Hippocamp partly in acknowledgement of the seahorse genus, Hippocampus, and the mythological creature that is aptly part horse and part fish.[3] In 2019, the IAU accepted the name Hippocamp.[27]


  1. ^ Given the moons' respective periods of 0.55465 and 0.93618 days, the actual ratios are 2.962:5.000.


  1. ^ Yeomans, D. K.; Chamberlin, A. B. (2013-07-15). "Planetary Satellite Discovery Circumstances". JPL Solar System Dynamics web site. Jet Propulsion Lab. Retrieved 2013-07-17.
  2. ^ a b c d e f Kelly Beatty (15 July 2013). "Neptune's Newest Moon". Sky & Telescope. Retrieved 12 June 2017.
  3. ^ a b Ian Sample (2019-02-20). "'Breakneck speed' mini moon hurtles around Neptune at 20,000mph". The Guardian. Retrieved 2019-02-20.
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  5. ^ a b Editors of Sky & Telescope. "A Guide to Planetary Satellites". Sky & Telescope web site. Sky & Telescope. Retrieved 2013-07-17.CS1 maint: extra text: authors list (link)
  6. ^ a b Karl Hille (2019-02-20). "Tiny Neptune Moon Spotted by Hubble May Have Broken from Larger Moon". NASA. Retrieved 2019-02-20.
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  8. ^ "Nasa's Hubble telescope discovers new Neptune moon". BBC News. 2013-07-15. Retrieved 2013-07-16.
  9. ^ a b Showalter, M. R. (2013-07-15). "How to Photograph a Racehorse ...and how this relates to a tiny moon of Neptune". Mark Showalter's blog. Retrieved 2013-07-16.
  10. ^ a b Grossman, L. (2013-07-15). "Neptune's strange new moon is first found in a decade". New Scientist space web site. New Scientist. Retrieved 2013-07-18.
  11. ^ a b Klotz, I. (2013-07-15). "Astronomer finds new moon orbiting Neptune". Reuters. Retrieved 2013-07-16.
  12. ^ Marsden, Brian G.; S/2002 N 4, MPEC 2003-S107 (30 September 2003)
  13. ^ Goldreich, P.; Murray, N.; Longaretti, P. Y.; Banfield, D. (1989). "Neptune's story". Science. 245 (4917): 500–504. Bibcode:1989Sci...245..500G. doi:10.1126/science.245.4917.500. PMID 17750259.
  14. ^ Agnor, C. B.; Hamilton, D. P. (2006-05-11). "Neptune's capture of its moon Triton in a binary–planet gravitational encounter". Nature. 441 (7090): 192–194. Bibcode:2006Natur.441..192A. doi:10.1038/nature04792. PMID 16688170.
  15. ^ Banfield, Don; Murray, Norm (October 1992). "A dynamical history of the inner Neptunian satellites". Icarus. 99 (2): 390–401. Bibcode:1992Icar...99..390B. doi:10.1016/0019-1035(92)90155-Z.
  16. ^ "Hubble helps uncover origin of Neptune's smallest moon Hippocamp". Retrieved 21 February 2019.
  17. ^ Karkoschka, Erich (2003). "Sizes, shapes, and albedos of the inner satellites of Neptune". Icarus. 162 (2): 400–407. Bibcode:2003Icar..162..400K. doi:10.1016/S0019-1035(03)00002-2.
  18. ^ Dumas, C.; Terrile, R. J.; Smith, B. A.; Schneider, G. (March 2002). "Astrometry and Near-Infrared Photometry of Neptune's Inner Satellites and Ring Arcs". The Astronomical Journal. 123 (3): 1776–1783. Bibcode:2002AJ....123.1776D. doi:10.1086/339022. ISSN 0004-6256.
  19. ^ a b c Dumas, C.; Smith, B. A.; Terrile, R. J. (August 2003). "Hubble Space Telescope NICMOS Multiband Photometry of Proteus and Puck". The Astronomical Journal. 126 (2): 1080–1085. Bibcode:2003AJ....126.1080D. doi:10.1086/375909. ISSN 0004-6256.
  20. ^ Brozović, M.; Showalter, M. R.; Jacobson, R. A.; French, R. S.; Lissauer, J. J.; de Pater, I. (March 2020). "Orbits and resonances of the regular moons of Neptune". Icarus. 338: 113462. arXiv:1910.13612. doi:10.1016/j.icarus.2019.113462.
  21. ^ a b c Zhang, K.; Hamilton, D. P. (June 2007). "Orbital resonances in the inner neptunian system: I. The 2:1 Proteus–Larissa mean-motion resonance". Icarus. 188 (2): 386–399. Bibcode:2007Icar..188..386Z. doi:10.1016/j.icarus.2006.12.002. ISSN 0019-1035.
  22. ^ Zhang, K.; Hamilton, D. P. (January 2008). "Orbital resonances in the inner neptunian system: II. Resonant history of Proteus, Larissa, Galatea, and Despina". Icarus. 193 (1): 267–282. Bibcode:2008Icar..193..267Z. doi:10.1016/j.icarus.2007.08.024. ISSN 0019-1035.
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  24. ^ Timmer, John (20 February 2019). "Hubble images show a Neptune moon that may have been repeatedly reborn". Ars Technica. Retrieved 21 February 2019.
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External linksEdit

  Media related to Hippocamp (moon) at Wikimedia Commons