|Discovered by||M. R. Showalter, I. de Pater, J. J. Lissauer, R. S. French|
|Discovery date||July 1, 2013|
|S/2004 N 1|
Average orbital speed
|Albedo||assumed low (≈0.09)|
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. 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.
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. After deciding "on a whim" to expand the search area to radii well beyond the rings, he found the "fairly obvious dot" that represented the new moon. 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. Given that the relevant images have long been available to the public, the discovery could have been made by anyone.
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. 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.
Another hypothesis suggests that Hippocamp may have formed specifically out of debris from the nearest moon of Neptune, the much larger Proteus. 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.
Hippocamp is assumed to resemble Neptune's other inner satellites in having a surface as dark as "dirty asphalt". Their geometrical albedos range from 0.07 to 0.10. 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. 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. 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, 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).
Hippocamp completes one revolution around Neptune every 22 hours and 28.1 minutes (0.9362 days), 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. 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.
The periods of Larissa and Hippocamp are within about one percent of a 3:5 orbital resonance, while Hippocamp and Proteus are within 0.1% of a 5:6 resonance.[Note 1] Larissa and Proteus are thought to have passed through a 1:2 mean-motion resonance a few hundred million years ago. Proteus 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) and are thus being tidally accelerated, while Larissa is within and is being tidally decelerated.
The moon is named after the Hippocamp, a mythological creature that is half horse and half fish in Greek mythology. 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, consistent with the naming of other moons of Neptune. Among the names considered was Polyphemus, the gigantic one-eyed son of Poseidon and Thoosa. 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. In 2019, the IAU accepted the name Hippocamp.
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