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Moons of Neptune facts for kids

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New Webb Images Capture Rare View of Neptune’s Rings (Labeled)
An annotated picture of some of Neptune's many moons as captured by the James Webb Space Telescope. The bright blue diffraction star is Triton, Neptune's largest moon; while Hippocamp, its smallest regular moon, is too small to be seen.

The planet Neptune has 16 known moons, which are named for minor water deities and a water creature in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, 17 days after the discovery of Neptune itself. Over a century passed before the discovery of the second natural satellite, Nereid, in 1949, and another 40 years passed before Proteus, Neptune's second largest moon, was discovered in 1989.

The moons of Neptune can be divided into two groups: regular and irregular. The first group includes the seven inner moons, which follow circular prograde orbits lying in the equatorial plane of Neptune. The second group consists of all nine other moons including Triton. They generally follow inclined eccentric and often retrograde orbits far from Neptune; the only exception is Triton, which orbits close to the planet following a circular orbit, though retrograde and inclined.

List

The Neptunian moons are listed here by orbital period, from shortest to longest. Irregular (captured) moons are marked by color. The orbits and mean distances of the irregular moons are variable over short timescales due to frequent planetary and solar perturbations, therefore the listed orbital elements of all irregular moons are averaged over a 30,000-year period: these may differ from osculating orbital elements provided by other sources. Otherwise, recently-discovered irregular moons without published proper elements are temporarily listed here with inaccurate osculating orbital elements that are italicized to distinguish them from other irregular moons with proper orbital elements. Their orbital elements are all based on the epoch of 1 January 2020. Triton, the only Neptunian moon massive enough for its surface to have collapsed into a spheroid, is emboldened.

Key
 
Inner moons		 ♠
Major moons		 †
Nereid (ungrouped)		 ‡
Halimede (ungrouped)		 ♦
Sao group		 ♥
Neso group


Neptunian moons
Label
 Name Pronunciation
(key)		 Image Abs.
magn.		 Diameter
(km)		 Mass
(×1016 kg)
 Semi-major axis
(km)		 Orbital period
(d)		 Orbital inclination
(°)		 Eccentricity
 Discovery
year		 Year announced Discoverer
 Group
III Naiad
A smeared white object elongated from the bottom-left to top-right can be seen in the center.
 9.6 60.4
(96 × 60 × 52)		 ≈ 13 48224 +0.2944 4.691 0.0047 1989 1989 Voyager Science Team inner
IV Thalassa
A group of three objects, each circled and labeled by the respective designations. Thalassa is the central object designated 1989 N5.
 8.7 81.4
(108 × 100 × 52)		 ≈ 35 50074 +0.3115 0.135 0.0018 1989 1989 Voyager Science Team inner
V Despina
A white oval shaped object somewhat elongated horizontally is seen in the center. There are a few small dark spots on its surface.
 7.3 156
(180 × 148 × 128)		 ≈ 170 52526 +0.3346 0.068 0.0004 1989 1989 Voyager Science Team inner
VI Galatea
A small white object elongated from the bottom-left to top-right can be seen in the center.
 7.2 174.8
(204 × 184 × 144)		 ≈ 280 61953 +0.4287 0.034 0.0001 1989 1989 Voyager Science Team inner
VII Larissa
An irregularly shaped grey object slightly elongated horizontally occupies almost the whole image. Its surface shows a number of dark and white spots.
 6.8 194
(216 × 204 × 168)		 ≈ 380 73548 +0.5555 0.205 0.0012 1981 1981 Reitsema et al. inner
XIV Hippocamp
Composite of multiple Hubble images of the Neptune system, with the moons appearing as bright white dots. The fainter dot to the upper right is Hippocamp, circled and labeled to distinguish it from other moons in this image.
 10.5 34.8±4.0 ≈ 2.2 105283 +0.9500 0.064 0.0005 2013 2013 Showalter et al. inner
VIII Proteus
A conically shaped object is seen almost fully illuminated from the left. The cone axis looks towards the observer. The outline of the object is a rectangle with rounded corners. The surface is rough with a few large depressions.
 5.0 420
(436 × 416 × 402)		 ≈ 3900 117646 +1.1223 0.075 0.0005 1989 1989 Voyager Science Team inner
I Triton♠
A large spherical object is half-illuminated from the bottom-left. The south pole faces to the light source. Around it in the bottom-left part of the body there is a large white area with a few dozens dark streaks elongated in the pole to equator direction. This polar cap has a slight red tinge. The equatorial region is darker with a tint of cyan. Its surface is rough with a number of craters and intersecting lineaments.
 –1.2 2705.2±4.8
(2709 × 2706 × 2705)		 2139000 354759 −5.8769 156.865 0.0000 1846 1846 Lassell
II Nereid†
A small white smeared body is seen in center.
 4.4 357 ± 13 ≈ 2400 5504000 +360.14 5.8 0.749 1949 1949 Kuiper
IX Halimede‡
N2002n1b.jpg
 10.0 ≈ 62 ≈ 12 16590100 −1879.30 113.1 0.286 2002 2003 Holman et al.
XI Sao♦
Sao VLT-FORS1 2002-09-03 annotated.gif
 11.1 ≈ 44 ≈ 3.4 22239300 +2918.70 53.3 0.148 2002 2003 Holman et al. Sao
S/2002 N 5♦ 11.2 ≈ 38 ≈ 3 23365200 +3141.26 42.1 0.548 2002 2024 Holman et al. Sao
XII Laomedeia♦
Laomedeia VLT-FORS1 2002-09-03.gif
 10.8 ≈ 42 ≈ 3.4 23502300 +3175.65 37.7 0.409 2002 2003 Holman et al. Sao
X Psamathe♥
Psamathe arrow.png
 11.0 ≈ 40 ≈ 2.9 47611900 −9149.87 126.6 0.417 2003 2003 Sheppard et al. Neso
XIII Neso♥
Neso VLT-FORS1 2002-09-03.gif
 10.7 ≈ 60 ≈ 11 49871600 −9796.67 126.9 0.400 2002 2003 Holman et al. Neso
S/2021 N 1♥ 12.1 ≈ 25 ≈ 0.8 50623600 −10017.93 134.5 0.441 2021 2024 Sheppard et al. Neso

Images for kids

  • Orbits of inner moons of Neptune including S 2004 N 1

    Orbit diagram of Neptune's inner moons including Triton, with their names and orbit directions indicated

  • Neptune inner moons size comparison

    Size comparison of Neptune's seven inner moons

  • Neptunemoonsdiagram

    Orbital diagram of the orbital inclination and orbital distances for Neptune's rings and moon system at various scales. Notable moons and rings are individually labeled. Open the image for full resolution.

See also

Kids robot.svg In Spanish: Satélites de Neptuno para niños

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