Nereid (moon)

Last updated
Nereid imaged by Voyager 2 in 1989
Discovery [1]
Discovered by Gerard P. Kuiper
Discovery date1 May 1949
Neptune II
Pronunciation /ˈnɪəriəd/ NEER-ee-əd [2]
Named after
Νηρηΐδες Nērēḯdes
Adjectives Nereidian or Nereidean (both /ˌnɛriˈɪdiən/ NERR-ee-ID-ee-ən) [3]
Orbital characteristics [4]
Epoch 27 April 2019 (JD 2458600.5)
Observation arc 68.21 yr (24,897 d)
5,513,940 km (0.0368584 AU)
Eccentricity 0.7417482
0.987 yr (360.11 d)
0° 59m 58.86s / day
Inclination 5.04909° (to the ecliptic)
7.090° (to local Laplace plane) [5]
Satellite of Neptune
Physical characteristics
Mean diameter
357±13 km [6]
340±50 km [7]
11.594±0.017 h [6]
Albedo 0.24 [6]
0.155 [7]
Temperature ≈50 K (mean estimate)
19.2 [7]
4.4 [4]

    Nereid, or Neptune II, is the third-largest moon of Neptune. Of all known moons in the Solar System, it has the most eccentric orbit. [8] It was the second moon of Neptune to be discovered, by Gerard Kuiper in 1949.


    Discovery and naming

    Nereid was discovered on 1 May 1949 by Gerard P. Kuiper on photographic plates taken with the 82-inch telescope at the McDonald Observatory. He proposed the name in the report of his discovery. It is named after the Nereids, sea-nymphs of Greek mythology and attendants of the god Neptune. [1] It was the second and last moon of Neptune to be discovered before the arrival of Voyager 2 (not counting a single observation of an occultation by Larissa in 1981). [9]

    Physical characteristics

    Nereid is third-largest of Neptune's satellites, and has a mean radius of about 180 kilometres (110 mi). [6] It is rather large for an irregular satellite. [10] The shape of Nereid is unknown. [11]

    Since 1987 some photometric observations of Nereid have detected large (by ~1 of magnitude) variations of its brightness, which can happen over years and months, but sometimes even over a few days. They persist even after a correction for distance and phase effects. On the other hand, not all astronomers who have observed Nereid have noticed such variations. This means that they may be quite chaotic. To date there is no credible explanation of the variations, but, if they exist, they are likely related to the rotation of Nereid. Nereid's rotation could be either in the state of forced precession or even chaotic rotation (like Hyperion) due to its highly elliptical orbit.

    In 2016, extended observations with the Kepler space telescope showed only low-amplitude variations (0.033 magnitudes). Thermal modeling based on infrared observations from the Spitzer and Herschel space telescopes suggest that Nereid is only moderately elongated with an aspect ratio of 1.3:1, which disfavors forced precession of the rotation. [6] The thermal model also indicates that the surface roughness of Nereid is very high, likely similar to the Saturnian moon Hyperion. [6]

    Spectrally, Nereid appears neutral in colour [12] and water ice has been detected on its surface. [13] Its spectrum appears to be intermediate between Uranus's moons Titania and Umbriel, which suggests that Nereid's surface is composed of a mixture of water ice and some spectrally neutral material. [13] The spectrum is markedly different from minor planets of the outer solar system, centaurs Pholus, Chiron and Chariklo, suggesting that Nereid formed around Neptune rather than being a captured body. [13]

    Halimede, which displays a similar gray neutral colour, may be a fragment of Nereid that was broken off during a collision. [12]

    Orbit and rotation

    Nereid orbits Neptune in the prograde direction at an average distance of 5,513,400 km (3,425,900 mi), but its high eccentricity of 0.7507 takes it as close as 1,372,000 km (853,000 mi) and as far as 9,655,000 km (5,999,000 mi). [5] [14]

    Nereid's highly eccentric orbit around Neptune. Nereid's orbit around Neptune.svg
    Nereid's highly eccentric orbit around Neptune.

    The unusual orbit suggests that it may be either a captured asteroid or Kuiper belt object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon Triton. [13] If the latter is true, it may be the only survivor of Neptune's original (pre-Triton capture) set of regular satellites. [15]

    In 1991, a rotation period of Nereid of about 13.6 hours was determined by an analysis of its light curve. [16] In 2003, another rotation period of about 11.52 ± 0.14 hours was measured. [10] However, this determination was later disputed, and other researchers for a time failed to detect any periodic modulation in Nereid's light curve from ground-based observations. [11] In 2016, a clear rotation period of 11.594 ± 0.017 hours was determined based on observations with the Kepler space telescope. [6]


    The only spacecraft to visit Nereid was Voyager 2 , which passed it at a distance of 4,700,000 km (2,900,000 mi) [17] between 20 April and 19 August 1989. [18] Voyager 2 obtained 83 images with observation accuracies of 70 km (43 mi) to 800 km (500 mi). [18] Prior to Voyager 2's arrival, observations of Nereid had been limited to ground-based observations that could only establish its intrinsic brightness and orbital elements. [19] Although the images obtained by Voyager 2 do not have a high enough resolution to allow surface features to be distinguished, Voyager 2 was able to measure the size of Nereid and found that it was grey in colour and had a higher albedo than Neptune's other small satellites. [9]

    See also

    Related Research Articles

    Triton (moon) Largest moon of Neptune

    Triton is the largest natural satellite of the planet Neptune, and was the first Neptunian moon to be discovered, on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet, captured from the Kuiper belt.

    Puck (moon) moon of Uranus

    Puck is an inner moon of Uranus. It was discovered in December 1985 by the Voyager 2 spacecraft. The name Puck follows the convention of naming Uranus's moons after characters from Shakespeare. The orbit of Puck lies between the rings of Uranus and the first of Uranus's large moons, Miranda. Puck is approximately spherical in shape and has diameter of about 162 km. It has a dark, heavily cratered surface, which shows spectral signs of water ice.

    Proteus (moon) Large moon of Neptune

    Proteus, also known as Neptune VIII, is the second-largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology. Proteus orbits Neptune in a nearly equatorial orbit at a distance of about 4.75 times the radius of Neptune's equator.

    Hyperion (moon) Moon of Saturn

    Hyperion, also known as Saturn VII (7), is a moon of Saturn discovered by William Cranch Bond, his son George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance. It was the first non-round moon to be discovered.

    Caliban (moon) moon of Uranus

    Caliban is the second-largest retrograde irregular satellite of Uranus. It was discovered on 6 September 1997 by Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns, and John J. Kavelaars using the 200-inch Hale telescope together with Sycorax and given the temporary designation S/1997 U 1.

    28978 Ixion Plutino

    28978 Ixion, provisional designation 2001 KX76, is a large trans-Neptunian object and a possible dwarf planet. It is located in the Kuiper belt, a region of icy objects orbiting beyond Neptune in the outer Solar System. Ixion is classified as a plutino, a dynamical class of objects in a 2:3 orbital resonance with Neptune. It was discovered in May 2001 by astronomers of the Deep Ecliptic Survey at the Cerro Tololo Inter-American Observatory, and was announced in July 2001. The object is named after the Greek mythological figure Ixion, who was a king of the Lapiths.

    Sycorax (moon) moon of Uranus

    Sycorax is the largest retrograde irregular satellite of Uranus. Sycorax was discovered on 6 September 1997 by Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns, and John J. Kavelaars using the 200-inch Hale telescope, together with Caliban, and given the temporary designation S/1997 U 2.

    Thalassa (moon) Moon of Neptune

    Thalassa, also known as Neptune IV, is the second-innermost satellite of Neptune. Thalassa was named after sea goddess Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".

    Moons of Uranus Natural satellites of the planet Uranus

    Uranus, the seventh planet of the Solar System, has 27 known moons, most of which are named after characters that appear in, or are mentioned in, the works of William Shakespeare and Alexander Pope. Uranus's moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. The inner and major moons all have prograde orbits, while orbits of the irregulars are mostly retrograde. The inner moons are small dark bodies that share common properties and origins with Uranus's rings. The five major moons are ellipsoidal, indicating that they reached hydrostatic equilibrium at some point in their past, and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces. The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, about one-twentieth the mass of the Earth's Moon. The orbits of the regular moons are nearly coplanar with Uranus's equator, which is tilted 97.77° to its orbit. Uranus's irregular moons have elliptical and strongly inclined orbits at large distances from the planet.

    Moons of Neptune Natural satellites of the planet Neptune

    The planet Neptune has 14 known moons, which are named for minor water deities 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. Neptune's outermost moon Neso, which has an orbital period of about 26 Julian years, orbits farther from its planet than any other moon in the Solar System.

    Makemake Dwarf planet in the Outer Solar System

    Makemake is a dwarf planet and perhaps the second-largest Kuiper belt object in the classical population, with a diameter approximately two-thirds that of Pluto. It has one known satellite. Its extremely low average temperature, about 40 K (−230 °C), means its surface is covered with methane, ethane, and possibly nitrogen ices.

    38628 Huya Trans-Neptunian object

    38628 Huya ( hoo-YAH), provisional designation 2000 EB173, is a binary trans-Neptunian object located in the Kuiper belt, a region of icy objects orbiting beyond Neptune in the outer Solar System. Huya is classified as a plutino, a dynamical class of trans-Neptunian objects with orbits in a 3:2 orbital resonance with Neptune. It was discovered by the Quasar Equatorial Survey Team and was identified by Venezuelan astronomer Ignacio Ferrín in March 2000. It is named after Juyá, the mythological rain god of the Wayuu people native to South America.

    Irregular moon Captured satellite following an irregular orbit

    In astronomy, an irregular moon, irregular satellite or irregular natural satellite is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They have been captured by their parent planet, unlike regular satellites, which formed in orbit around them. Irregular moons have a stable orbit, unlike temporary satellites which often have similarly irregular orbits but will eventually depart. The term does not refer to shape as Triton is a round moon, but is considered irregular due to its orbit.

    <span class="nowrap">(208996) 2003 AZ<sub>84</sub></span> Plutino

    (208996) 2003 AZ84 is a trans-Neptunian object with a possible moon from the outer regions of the Solar System. It is approximately 940 kilometers across its longest axis, as it has an elongated shape. It belongs to the plutinos – a group of minor planets named after its largest member Pluto – as it orbits in a 2:3 resonance with Neptune in the Kuiper belt. It is the third-largest known plutino, after Pluto and Orcus. It was discovered on 13 January 2003, by American astronomers Chad Trujillo and Michael Brown during the NEAT survey using the Samuel Oschin telescope at Palomar Observatory.

    15810 Arawn

    15810 Arawn, provisional designation 1994 JR1, is a trans-Neptunian object (TNO) from the inner regions of the Kuiper belt, approximately 133 kilometres (83 mi) in diameter. It belongs to the plutinos, the largest class of resonant TNOs. It was named after Arawn, the ruler of the Celtic underworld, and discovered on 12 May 1994, by astronomers Michael Irwin and Anna Żytkow with the 2.5-metre Isaac Newton Telescope at La Palma Observatory in the Canary Islands, Spain.

    Exploration of Neptune Overview of the exploration of Neptune

    Neptune has been directly explored by only one space probe, Voyager 2, in 1989. As of September 2021, there are no confirmed future missions to visit the Neptunian system, although a tentative Chinese mission has been planned for launch in 2024. NASA, ESA, and independent academic groups have proposed future scientific missions to visit Neptune. Some mission plans are still active, while others have been abandoned or put on hold.

    Neptune Eighth and outermost planet from the Sun

    Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly more massive than its near-twin Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. The planet orbits the Sun once every 164.8 years at an average distance of 30.1 AU. It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune's trident or the Greek letter psi.

    Philip D. Nicholson is an Australian-born professor of astronomy at Cornell University in the Astronomy department specialising in Planetary Sciences. He has been editor-in-chief of the journal Icarus since 1998.

    Hippocamp (moon) Smallest moon of Neptune

    Hippocamp, also designated Neptune XIV, is a small moon of Neptune discovered on 1 July 2013. It was found by astronomer Mark Showalter by analyzing archived Neptune photographs the Hubble Space Telescope captured between 2004 and 2009. The moon is so dim that it was not observed when the Voyager 2 space probe flew by Neptune and its moons in 1989. It is about 35 km (20 mi) in diameter, and orbits Neptune in about 23 hours, just under one Earth day. Due to its unusually close distance to Neptune's largest inner moon Proteus, it has been hypothesized that Hippocamp may have accreted from material ejected by an impact on Proteus several billion years ago. The moon was formerly known by its provisional designation S/2004 N 1 until February 2019, when it was formally named Hippocamp, after the mythological sea-horse symbolizing Poseidon in Greek mythology.


    1. 1 2 Kuiper, G. P. (August 1949). "The Second Satellite of Neptune". Publications of the Astronomical Society of the Pacific. 61 (361): 175–176. Bibcode:1949PASP...61..175K. doi: 10.1086/126166 .
    2. "Nereid" . Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
    3. "nereidian, nereidean" . Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
    4. 1 2 "M.P.C. 115892" (PDF). Minor Planet Circular. Minor Planet Center. 27 August 2019.
    5. 1 2 Jacobson, R. A. — AJ (2009-04-03). "Planetary Satellite Mean Orbital Parameters". JPL satellite ephemeris. JPL (Solar System Dynamics). Archived from the original on October 14, 2011. Retrieved 2011-10-26.
    6. 1 2 3 4 5 6 7 Kiss, C.; Pál, A.; Farkas-Takács, A. I.; Szabó, G. M.; Szabó, R.; Kiss, L. L.; et al. (April 2016). "Nereid from space: Rotation, size and shape analysis from K2, Herschel and Spitzer observations" (PDF). Monthly Notices of the Royal Astronomical Society. 457 (3): 2908–2917. arXiv: 1601.02395 . Bibcode:2016MNRAS.457.2908K. doi:10.1093/mnras/stw081.
    7. 1 2 3 "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). Archived from the original on 2010-05-27. Retrieved 2011-10-26.
    8. "Planetary Satellite Mean Orbital Parameters". NASA. 2013-08-23. Retrieved 2019-05-01.
    9. 1 2 Smith, B. A.; Soderblom, L. A.; Banfield, D.; Barnet, C.; Basilevsky, A. T.; Beebe, R. F.; Bollinger, K.; Boyce, J. M.; Brahic, A. (1989). "Voyager 2 at Neptune: Imaging Science Results". Science. 246 (4936): 1422–1449. Bibcode:1989Sci...246.1422S. doi:10.1126/science.246.4936.1422. PMID   17755997. S2CID   45403579.
    10. 1 2 Grav, T.; M. Holman; J. J. Kavelaars (2003). "The Short Rotation Period of Nereid". The Astrophysical Journal. 591 (1): 71–74. arXiv: astro-ph/0306001 . Bibcode:2003ApJ...591L..71G. doi:10.1086/377067. S2CID   8869351.
    11. 1 2 Schaefer, Bradley E.; Tourtellotte, Suzanne W.; Rabinowitz, David L.; Schaefer, Martha W. (2008). "Nereid: Light curve for 1999–2006 and a scenario for its variations". Icarus . 196 (1): 225–240. arXiv: 0804.2835 . Bibcode:2008Icar..196..225S. doi:10.1016/j.icarus.2008.02.025. S2CID   119267757.
    12. 1 2 Grav, Tommy; Holman, Matthew J.; Fraser, Wesley C. (2004-09-20). "Photometry of Irregular Satellites of Uranus and Neptune". The Astrophysical Journal . 613 (1): L77–L80. arXiv: astro-ph/0405605 . Bibcode:2004ApJ...613L..77G. doi:10.1086/424997. S2CID   15706906.
    13. 1 2 3 4 Brown, Michael E.; Koresko, Christopher D.; Blake, Geoffrey A. (December 1998). "Detection of Water Ice on Nereid". The Astrophysical Journal . 508 (2): L175–L176. Bibcode:1998ApJ...508L.175B. CiteSeerX . doi:10.1086/311741. PMID   11542819.
    14. Jacobson, R. A. (3 April 2009). "The Orbits of the Neptunian Satellites and the Orientation of the Pole of Neptune". The Astronomical Journal . 137 (5): 4322–4329. Bibcode:2009AJ....137.4322J. doi: 10.1088/0004-6256/137/5/4322 .
    15. 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 . Bibcode:2020Icar..33813462B. doi:10.1016/j.icarus.2019.113462. S2CID   204960799.
    16. Williams, I.P.; Jones, D.H.P.; Taylor, D.B. (1991). "The rotation period of Nereid". Monthly Notices of the Royal Astronomical Society . 250: 1P–2P. Bibcode:1991MNRAS.250P...1W. doi: 10.1093/mnras/250.1.1p .
    17. Jones, Brian (1991). Exploring the Planets. Italy: W.H. Smith. pp.  59. ISBN   978-0-8317-6975-8.
    18. 1 2 Jacobson, R.A. (1991). "Triton and Nereid astrographic observations from Voyager 2". Astronomy and Astrophysics Supplement Series. 90 (3): 541–563. Bibcode:1991A&AS...90..541J.
    19. "PIA00054: Nereid". NASA. 1996-01-29. Retrieved 2009-11-08.