Nereid , also known as
Neptune II, is a
moonA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
of
NeptuneNeptune is the eighth planet from the Sun in our Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and...
.
Nereid was discovered on May 1, 1949 by Gerard P. Kuiper, who proposed the name in the report of his discovery. It is named after the
NereidsIn Greek mythology, the Nereids are sea nymphs, the fifty daughters of Nereus and Doris. They often accompany Poseidon and are always friendly and helpful towards sailors fighting perilous storms. They are particularly associated with the Aegean Sea, where they dwelt with their father in the...
, sea-nymphs of
Greek mythologyGreek mythology is the body of myths and legends belonging to the ancient Greeks concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
.
Nereid was the outermost known moon of Neptune from its discovery until 2002. Nereid's diameter is 340 km, making it the third-largest moon in the Neptune system.
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Nereid , also known as
Neptune II, is a
moonA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
of
NeptuneNeptune is the eighth planet from the Sun in our Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and...
.
Nereid was discovered on May 1, 1949 by Gerard P. Kuiper, who proposed the name in the report of his discovery. It is named after the
NereidsIn Greek mythology, the Nereids are sea nymphs, the fifty daughters of Nereus and Doris. They often accompany Poseidon and are always friendly and helpful towards sailors fighting perilous storms. They are particularly associated with the Aegean Sea, where they dwelt with their father in the...
, sea-nymphs of
Greek mythologyGreek mythology is the body of myths and legends belonging to the ancient Greeks concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
.
Nereid was the outermost known moon of Neptune from its discovery until 2002. Nereid's diameter is 340 km, making it the third-largest moon in the Neptune system. Its
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
averages 5,513,400 km in radius, but is highly eccentric and varies from 1,372,000 to 9,655,000 kilometres. This is the highest mean eccentricity of any known satellite in the
solar systemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
. But it may lose this title briefly to an outer irregular moon of a
gas giantA gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune...
, such as
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
's Bestla and
UranusUranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus the father of Kronos and grandfather of Zeus...
's
MargaretMargaret is the only prograde irregular satellite of Uranus. It was discovered by Scott S. Sheppard, et al. in 2003 and given the provisional designation S/2003 U 3....
, because of the
Kozai mechanismIn celestial mechanics, the Kozai mechanism causes a periodic exchange between the inclination and eccentricity of an orbit. That is, libration occurs in the argument of pericenter....
(a
periodic exchangeFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
between the
inclinationInclination in general is the angle between a reference plane and another plane or axis of direction.- Orbits :The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
and eccentricity). The unusual orbit suggests that it may be a captured
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
or
Kuiper beltThe Kuiper belt , sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 times as massive...
object, or that it was perturbed during the capture of Neptune's largest moon
TritonTriton is the largest moon of the planet Neptune, discovered on October 10, 1846 by William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2700 km in diameter, it is the seventh-largest...
.
Halimede may be a fragment of Nereid that was broken off during a collision.
Nereid was too far away from
Voyager 2The Voyager 2 spacecraft is an unmanned interplanetary space probe launched on August 20, 1977. Identical in form to its sister Voyager program craft Voyager 1, Voyager 2 followed a slower trajectory that allowed it to be kept in the ecliptic so that it could be sent to Uranus and Neptune by means...
to be properly imaged when the spacecraft visited the Neptune system in 1989. Photos sent back show only its highly irregular shape, and no surface features could be seen at the resolution available.
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