Neso (moon)

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Neso (Template:IPAc-en), also known as Neptune XIII and previously as S/2002 N 4, is the second-outermost known moon of Neptune, behind S/2021 N 1. It was discovered on 14 August 2002 by Matthew Holman, JJ Kavelaars, Tommy Grav, Wesley Fraser, and Dan Milisavljevic at Cerro Tololo Inter-American Observatory in Chile. Named after one of the Nereids from Greek mythology, Neso orbits Neptune at an average distance of Template:Convert—almost as far as Mercury's orbital distance from the Sun—and takes about 26.8 Earth years to complete one orbit. It is classified as an irregular moon because it follows a distant, highly eccentric, and highly inclined orbit.

Due to Neso's great distance from Neptune, its orbit is significantly influenced by the Sun's gravity. As a result, the moon experiences the Kozai resonance, which causes large, synchronized variations in its orbital eccentricity and inclination over a 600-year period. Neso shares similar orbital characteristics with two other Neptunian irregular moons, Psamathe and S/2021 N 1, which suggests they originated from the collisional breakup of a once-larger moon of Neptune billions of years ago. Most of Neso's physical properties are unknown, though telescope observations suggest it has a reddish color and a diameter between Template:Cvt.

Discovery

Neso was discovered by astronomers Matthew Holman, JJ Kavelaars, Tommy Grav, Wesley Fraser, and Dan Milisavljevic during a search for distant moons of Neptune.[1] Led by Holman, the search began in 2001 and employed large optical telescopes on Earth to obtain numerous long-exposure images of the sky around Neptune.[2][3]Template:Rp The team used the shift-and-add technique to align and combine the images according to Neptune's motion across the sky, which enhanced the faint moons as points of light.[2][3]Template:Rp Through this technique,[3] they discovered Neso alongside two other Neptunian moons—Halimede and Sao—in images taken on 14 August 2002 by the 4-meter Víctor M. Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile.[4][5]

To determine Neso's orbit around Neptune, astronomers conducted follow-up observations at several observatories.[3]Template:Rp A team led by Brett Gladman observed Neso on 3 September 2002 with the 8.2-m Very Large Telescope at Cerro Paranal Observatory, while Holman's team observed it later that day with the 2.5-m Nordic Optical Telescope at La Palma Observatory.[4][6] Neso was next recovered by Holman on 19 August 2003 with the 4-m Blanco Telescope, followed by additional observations with Las Campanas Observatory's 6.5-m Magellan Clay Telescope from 28 to 30 September.[4][6] After more than a year of observations, the Minor Planet Center announced Neso's discovery on 30 September 2003.[4][6] It was the fifth and last moon of Neptune announced in 2003,Template:Efn bringing the planet's count of known moons to 13.[7]

Name

When the discovery of Neso was announced, it was given the temporary provisional designation "S/2002 N 4" by the Minor Planet Center.[4] It was later named and given the Roman numeral designation Neptune XIII by the International Astronomical Union's (IAU's) Working Group for Planetary System Nomenclature on 3 February 2007.[8] In accordance with the IAU's naming convention for Neptunian irregular moons, it was named after Neso (Νησώ),[9] one of the fifty Nereids or daughters of Nereus and Doris from Greek mythology.[1][5]Template:Efn

Orbit

Neso is an irregular moon of Neptune, meaning it follows a distant, highly eccentric, and highly inclined orbit around the planet.[10]Template:Efn Like the majority of irregular moons, Neso's orbit is retrograde, meaning it revolves around Neptune in the opposite direction to the planet's orbit around the Sun.[11]Template:Rp Due to its great distance from Neptune, the moon is strongly affected by the Sun's gravity, which causes substantial long-term variations in its orbit.[10]Template:Rp For this reason, Neso's orbit is better described by proper orbital elements, which are calculated by averaging out its perturbed orbit over an extended period of time.[11]Template:Rp

Over a 10,000-year time span, Neso's semi-major axis from Neptune varies from Template:Convert, averaging about Template:Convert.Template:Efn At such distances, Neptune would appear as a tiny, barely-visible disc from Neso[12] with an average angular diameter of roughly 3 arcminutes.Template:Efn The scale of Neso's orbit is comparable to that of Mercury, which orbits the Sun at a semi-major axis of Template:Convert.[13][14] In terms of average semi-major axis, Neso is the second-outermost known moon of Neptune, behind S/2021 N 1.[15] At its average distance, Neso occupies 43% of Neptune's Hill radius,Template:Efn placing it just over halfway to the outer limit at which retrograde moons can stably orbit the planet.Template:Efn

Approximate dates and distances of
Neso's periapsis and apoapsis
Periapsis Apoapsis
DateTemplate:Efn
(UTC) 		Distance DateTemplate:Efn
(UTC) 		Distance
May 1983[16] Template:Convert Mar 1996[17] Template:Convert
Oct 2010[18] Template:Convert Jul 2025[19] Template:Convert
Jan 2038[20] Template:Convert Jul 2050[21] Template:Convert

On average, Neso takes about Template:Convert to complete one orbit around Neptune, though perturbations by the Sun can vary the orbital period from Template:Convert.Template:Efn Due to its long orbital period, it is unlikely to collide with other irregular moons.[22] While Neso's orbit has an average eccentricity of 0.46 and an average inclination of 128° with respect to the ecliptic, both values vary substantially due to the Sun's perturbations.[11] The moon's eccentricity ranges from 0.14 to 0.88, while its inclination ranges from 117° to 145°.Template:Efn

Since Neso follows an eccentric orbit, its distance from Neptune varies by several tens of millions of km as it moves from periapsis to apoapsis in its orbit.[23]Template:Rp However, because of its variable semi-major axis and eccentricity, its periapsis and apoapsis distances can change.[24]Template:Efn For example, in October 2010, Neso passed periapsis at a distance of Template:Convert,[18] whereas in January 2038, Neso will pass periapsis at a closer distance of Template:Convert.[20] Likewise, in July 2025, Neso passed apoapsis at a distance Template:Convert,[18] whereas in July 2050, Neso will pass apoapsis at a further distance of Template:Convert.[20]

A graphic showing the orbit of Neso (displayed as red) and other irregular moons of Neptune (displayed as gray) as seen from three different views
The orbit of Neso (red) and other irregular moons of Neptune (gray), as seen from three different views. These moons orbit far beyond Triton and Nereid, Neptune's largest moons (colored magenta). Neso's orbit does not form a closed ellipse because it is highly perturbed.

Kozai resonance

Neso's high inclination places it in the Kozai resonance, a dynamical phenomenon in which perturbations from the Sun cause periodic exchanges between orbital eccentricity and inclination.[25][10]Template:Rp First recognized by Matija Ćuk and Joseph A. Burns in 2004,[26][10]Template:Rp the Kozai resonance causes Neso's eccentricity and inclination to oscillate in phase over a period of about 600 years,Template:Efn with eccentricity reaching maximum at maximum inclination and vice versa.[11]Template:Rp Because Neso's orbit is retrograde (its inclination exceeds 90°), maximum inclination corresponds to the smallest displacement from the ecliptic rather than the greatest.[11][23]Template:Rp If Neso had an inclination below 120° and a semi-major axis greater than 60% of Neptune's Hill radius (Template:ConvertTemplate:Efn), the Kozai resonance would destabilize its orbit.[27] While Neso's orbit exhibits nodal precession with a period of 1,116 years,[15] the Kozai resonance prevents it from apsidally precessing.[11]Template:Rp Instead of circulating from 0° to 360°, Neso's argument of pericenter librates around 90° with a period of about 600 years.[11]Template:Rp This means the moon always comes to periapsis above the ecliptic.[11]Template:RpTemplate:Efn

  • Three plots showing periodic oscillations in Neso's eccentricity (e), inclination (i), and argument of pericenter (ω) over 5,000 years.
    Plots showing periodic oscillations in Neso's eccentricity (e), inclination (i), and argument of pericenter (ω) over 5,000 years
  • A sped-up simulation of Neso's orbit around Neptune, viewed at an oblique angle. The time starts at year 1600 and ends at 2400. The simulation shows Neso's orbit twirling around due to nodal precession. At the same time, Neso's orbit warps and changes shape due to the Kozai resonance changing its inclination and eccentricity.
    Sped-up simulation of Neso's orbit around Neptune, viewed at an oblique angle. While Neso's orbit nodally precesses, it also changes shape due to the Kozai resonance.
  • A sped-up simulation of Neso's orbit around Neptune, viewed from above the ecliptic plane. The time starts at year 1600 and ends at 2400. The simulation shows Neso's orbit spiraling due to nodal precession. At the same time, Neso's orbit warps and changes shape due to the Kozai resonance changing its inclination and eccentricity.
    Sped-up simulation of Neso's orbit around Neptune, viewed from above the ecliptic plane. The time starts at year 1600 and ends at 2400.

Group and origin

Irregular moons of all four giant planets, plotted by average distance from their planet (semi-major axis in Hill radii) and orbital inclination (degrees with respect to ecliptic). The Neso group is the cluster of blue diamonds on the upper right, labeled in blue. Data is as of February 2024.
Irregular moons of all four giant planets, plotted by average distance from their planet (semi-major axis in Hill radii) and orbital inclination (degrees with respect to ecliptic). The Neso group is the cluster of retrograde moons labeled in blue.

Neso shares similar orbital characteristics with two other Neptunian irregular moons, Psamathe and S/2021 N 1.[23] Together, they form the Neso group, a family of retrograde irregular moons of which Neso is the largest member.[23] The Neso group was first suspected by astronomers in 2004 based on the similar orbits of Neso and Psamathe,[3]Template:Rp but it was not confirmed until the discovery of S/2021 N 1 as a third member in 2024.[23][28]Template:Efn The group is predicted to contain many more smaller members, though they are too faint to be detected with current technology.[28][23]Template:Rp

Because of their similar orbits, the moons of the Neso group have low mutual dispersion velocities of around Template:Val.[23]Template:Rp The dispersion velocity is the speed needed to eject a fragment from a parent body onto its current orbit.[23]Template:Rp For fragments produced in collisions, their dispersion velocities are typically below Template:Val, comparable to the parent body's escape velocity.[23]Template:Rp This suggests that the Neso group is a collisional family,[23]Template:Rp formed by the destruction of a once-larger parent body by a collision with an asteroid or comet billions of years ago.[28][1] The parent body was probably an irregular moon that was gravitationally captured by Neptune during the Solar System's early history, during or shortly after the formation of the planets.[23]Template:Rp

If the Neso group is a collisional family, then its members are expected to share similar surface characteristics, such as color. However, this remains yet to be verified, as Neso is the only member with a known surface color Template:Asof.[11]Template:Rp

Physical characteristics

Little is known about Neso's physical characteristics.[12][1] Viewed from Earth, Neso has an apparent magnitude of 25.6 in visible light (V band)[29] and an apparent magnitude between 24.7 and 25.3 in red light (R band).[30] With apparent magnitudes this dim, Neso and most of Neptune's irregular moons can only be observed through long-exposure imaging with large optical telescopes.[3][11]Template:Rp

The diameter of Neso has not been directly measured,[12] although it can be indirectly estimated from its brightness and an assumed geometric albedo.[31]Template:Rp Assuming a geometric albedo of 0.04, Neso's diameter would be about Template:Cvt.[31]Template:Rp For a higher albedo of 0.06, Neso would have a diameter of Template:Cvt.[3]Template:Rp At these sizes, Neso's surface gravity is expected to be very low.[12] Planetary scientist Erich Karkoschka speculated that on Neso, it would take roughly 30 seconds for an object to fall to the ground when dropped from human height.[12] The rotation period of Neso is unknown.[12][29]

The surface of Neso is presumably very cold and dimly illuminated, due to its great distance from the Sun.[12] Its surface temperature is estimated to be comparable to that of Neptune's largest moon Triton, whose surface temperature is approximately Template:Convert.[12] A 2018 analysis of Very Large Telescope observations found that the color of Neso's surface is similar to (if not slightly redder than) the typical colors of trans-Neptunian objects and centaurs, but not those of resonant trans-Neptunian objects.[29] The colors of Neso and other Neptunian irregular moons indicate that they lack the "ultrared" material commonly seen on the surfaces of Kuiper belt objects, suggesting that their surfaces have been altered by collisions or sublimation of volatiles.[30]

See also

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Notes

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References

  1. ↑ Template:Cite web Cite error: <ref> tag with name "JPL-disc" defined in <references> is not used in prior text.
  2. ↑ Template:Cite web Cite error: <ref> tag with name "MPC-Explorer" defined in <references> is not used in prior text.
  3. ↑ Template:Cite web Check "Just the following Neptunian outer irregular satellites", then check "Neptune 13 (Neso)", then go to "Output Format(s) Required" and check "I require Orbital Elements", and then click the gray "Get Information" button. Cite error: <ref> tag with name "MPC-NatSats" defined in <references> is not used in prior text.
  4. ↑ Template:Cite web Cite error: <ref> tag with name "solsys-phys" defined in <references> is not used in prior text.
  5. ↑ Template:Cite web Cite error: <ref> tag with name "angular_diameter" defined in <references> is not used in prior text.
  6. ↑ Template:Cite web Cite error: <ref> tag with name "logeion" defined in <references> is not used in prior text.
  7. ↑ Template:Cite web Cite error: <ref> tag with name "Sheppard-IAUC" defined in <references> is not used in prior text.
  8. ↑ Template:Cite book Cite error: <ref> tag with name "Nicholson2008" defined in <references> is not used in prior text.
  9. ↑ Template:Cite web Cite error: <ref> tag with name "apsis-eq" defined in <references> is not used in prior text.
  10. ↑ Template:Cite journal Cite error: <ref> tag with name "Grishin2024" defined in <references> is not used in prior text.
  11. ↑ 1.0 1.1 1.2 1.3 Template:Cite web
  12. ↑ 2.0 2.1 Template:Cite web
  13. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Template:Cite journal
  14. ↑ 4.0 4.1 4.2 4.3 4.4 Template:Cite journal
  15. ↑ 5.0 5.1 Template:Cite web
  16. ↑ 6.0 6.1 6.2 Template:Cite journal
  17. ↑ Template:Cite web
  18. ↑ Template:Cite journal
  19. ↑ Template:Cite book
  20. ↑ 10.0 10.1 10.2 10.3 Template:Cite journal
  21. ↑ 11.00 11.01 11.02 11.03 11.04 11.05 11.06 11.07 11.08 11.09 Template:Cite journal
  22. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 12.6 12.7 Template:Cite web
  23. ↑ Template:Cite web
  24. ↑ Template:Cite web
  25. ↑ 15.0 15.1 Template:Cite web
  26. ↑ Template:Cite web Periapsis occurs when Neso's radial velocity (deldot) changes from negative to positive (changes from moving toward to away from Neptune). Neso's radial distance (delta) from Neptune is expressed in astronomical units (AU).
  27. ↑ Template:Cite web Apoapsis occurs when Neso's radial velocity (deldot) changes from positive to negative (changes from moving away to moving toward Neptune). Neso's radial distance (delta) from Neptune is expressed in astronomical units (AU).
  28. ↑ 18.0 18.1 18.2 Template:Cite web Periapsis occurs when Neso's radial velocity (deldot) changes from negative to positive (changes from moving toward to away from Neptune). Neso's radial distance (delta) from Neptune is expressed in astronomical units (AU).
  29. ↑ Template:Cite web Apoapsis occurs when Neso's radial velocity (deldot) changes from positive to negative (changes from moving away to moving toward Neptune). Neso's radial distance (delta) from Neptune is expressed in astronomical units (AU).
  30. ↑ 20.0 20.1 20.2 Template:Cite web Periapsis occurs when Neso's radial velocity (deldot) changes from negative to positive (changes from moving toward to away from Neptune). Neso's radial distance (delta) from Neptune is expressed in astronomical units (AU).
  31. ↑ Template:Cite web Apoapsis occurs when Neso's radial velocity (deldot) changes from positive to negative (changes from moving away to moving toward Neptune). Neso's radial distance (delta) from Neptune is expressed in astronomical units (AU).
  32. ↑ Template:Cite web
  33. ↑ 23.00 23.01 23.02 23.03 23.04 23.05 23.06 23.07 23.08 23.09 23.10 Template:Cite journal
  34. ↑ Template:Cite web Ephemeris Type: Osculating Elements, Center: 500@8 (Neptune Barycenter), Timestep: 1 year, Units: km and days.
    Key: A = semi-major axis (km), PR = orbital period ("period of revolution"; days), EC = eccentricity, IN = inclination with respect to ecliptic (°), QR = periapsis distance (km), AD = apoapsis distance (km)
  35. ↑ Template:Cite journal
  36. ↑ Template:Cite journal
  37. ↑ Template:Cite journal
  38. ↑ 28.0 28.1 28.2 Template:Cite web
  39. ↑ 29.0 29.1 29.2 Template:Cite journal
  40. ↑ 30.0 30.1 Template:Cite journal
  41. ↑ 31.0 31.1 Template:Cite journal

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