Moons Of Neptune: Triton, Nereid

There are 14 moons of planet Neptune as of today. These moons named after water deities in Greek mythology. William Lassell discovered the largest moon of Neptune, Triton only after 17 days of discovery of planet Neptune. After a century the second Neptune Moon Nereid was discovered. The outermost Neptune’s moon is Neso. So, in this article, I will tell you about some of the moons of Neptune.



The moons of Neptune have two groups. The “Regular Moons” and the “Irregular Moons”. The regular moons are the inner moons and follow a circular orbit. But, the irregular moons are the outer moons that have inclined eccentric orbits far from the planet Neptune. And sometimes, irregular moons have retrograde orbits.


Regular Moons Of Neptune


The regular moons of Neptune include Larissa, Naiad, Despina, Thalassa, Hippocamp, Galatea, and Proteus in order of distance from Neptune.




Neptune-III or Naiad. The previous name was S/1989 N 6. It is nearest to the center of the planet and about 48,224 kilometers far from the center of Neptune.


Also, It is the innermost moon of planet Neptune, but Naiad is tidally locked to the planet Neptune. The moon was discovered sometime before mid of 1989. It is irregular in shape.




Neptune VII or Larissa. It is the fifth inner moon from the planet Neptune. In Greek Mythology, Larissa is a lover of Neptune. Discovered on May 24, 1981, it has a surface area of about 118236 kilometers. The only probe who took photos of this Moon was Voyager 2.




Neptune V or Despina. The third inner moon from the planet Neptune. Astronomers discovered this moon by observing the images sent by Voyager 2 probe. It has a diameter of 152 kilometers. Despina has a very irregular shape and has no signs of geographical activities present.




Neptune IV or Thalassa. In Greek mythology “Thalassa” is a sea goddess. It has an irregular shape. Discovered in September 1989, and named in September 1991. The mean radius of this moon is 40.7±2.8 kilometers.


Irregular Moons


The irregular moons of Neptune with respect to distance from Neptune are Triton, Nereid, Halimede, Sao, Laomedeia, Psamathe, and Neso. These moons have both types of orbits i.e retrograde and prograde orbits.


Nereid and Triton, both are unusual irregular satellites of Neptune. They are also the two largest irregular moons of our Solar system.




The largest natural moon of the planet Neptune is Triton. Also, it was the first Neptunian moon that Astronomers discovered. In 1846, William Lassell an English Astronomer discovered it. It has a retrograde orbit. Its orbit is opposite Neptune in direction. Triton has a diameter of about 1,680 miles or 2,710 kilometers.


Among the big moons in the Solar system, Triton is the seventh-largest. Its size is enough to make it a hydrostatic equilibrium. Before it was thought that Triton was a dwarf planet in the Kuiper belt that captured by Neptune through its Gravitational force. It is larger than Pluto in size and its composition and orbit are also like Pluto.


Triton has a frozen surface. Mostly water-ice crust and frozen nitrogen. It has an icy mantle and the core consists of metal and rock. The density of Triton is 2.061 g/cm3. According to Voyager 2 observations, the temperature on Triton is about -235 degrees Celcius.


Triton is also geologically active like some other giant moons of the solar system. These include Europa, Io of Jupiter, and Titan, Enceladus of Saturn. The surface of Triton seems pretty young with a few impact craters.




The third-largest Moon of planet Neptune is Nereid. The orbit of the Nereid is prograde and eccentric. Water-ice is present on this moon’s surface. It is famous to have a very eccentric orbit compared to all other moons. It is the third-largest satellite of Neptune. The radius of Nereid is 110 miles (180 kilometers).


The distance of Nereid from its planet is about 5,513,400 kilometers. Due to its highly eccentric orbit, it is sometimes considered a captured asteroid or a dwarf planet.


Voyager 2 was the only spacecraft to observe Nereid from a distance of about 47,00,000 kilometers. Voyager 2 spacecraft took 83 images of this moon. The accuracy of these images was between 70 to 800 kilometers or 43 to 500 miles.




Sao is a satellite of the planet Neptune. Discovered on the 14th of August in 2002. Matthew J. Holman discovered this moon. The moon is irregular and is very far from its planet, about 22.4 million kilometers away from Neptune.


The diameter of Sao is 44 kilometers. The orbit is inclined and moderately eccentric. The previous name for this Neptunian Moon was S/2002 N 2.




Neso or Neptune XIII. It is the outermost natural moon of planet Neptune. Discovered by Matthew J. Holman on the 14th of August in 2002. The moon is an irregular moon. Being an outermost moon, Neso is farthest from Neptune at a distance of about 48 million kilometers.


Neso completes its one rotation around its planet in 26.67 years. The orbit is retrograde and highly eccentric. The diameter of this moon is about 37 miles or 60 kilometers. The assumed mean density of this moon is 1.5 g/cm3


Updated: April 1, 2021 — 8:52 pm

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