Nereid (moon)

Source: Wikipedia, the free encyclopedia.
Nereid
Synodic rotation period
11.594±0.017 h[5]
Albedo0.24[5]
Temperature≈50 K (mean estimate)
19.2[citation needed]
4.4[7]

Nereid, or Neptune II, is the third-largest moon of Neptune. It has the most eccentric orbit of all known moons in the Solar System.[4] It was the second moon of Neptune to be discovered, by Gerard Kuiper in 1949.

Discovery and naming

Gerard P. Kuiper, discoverer of Nereid

Nereid was discovered on 1 May 1949 by

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).[8]

Physical characteristics

Nereid is third-largest of Neptune's satellites, and has a mean radius of about 180 kilometres (110 mi).[5] It is by far the largest normal irregular satellite known,[9] having about two-thirds the mass of all irregular moons combined.[10] (Triton is much larger, but is very unusual for an irregular satellite.) Nereid also accounts for about 98% of the mass of Neptune's entire irregular satellite system altogether (excluding Triton), which is similar to the situation of Phoebe at Saturn (the second-largest normal irregular moon in the Solar System).[10]

The shape of Nereid is unknown.

Mimas, though Nereid is denser. 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 a maximum aspect ratio of 1.3:1, which disfavors forced precession of the rotation.[5] The thermal model also indicates that the surface roughness of Nereid is very high, likely similar to the Saturnian moon Hyperion.[5]

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

eccentricity of 0.749 takes it as close as 1,381,500 km (858,400 mi) and as far as 9,626,500 km (5,981,600 mi).[4]

regular satellites.[14]

In 1991, a rotation period of Nereid of about 13.6 hours was determined by an analysis of its light curve.

Kepler space telescope.[5]

Exploration

The only spacecraft to visit Nereid was Voyager 2, which passed it at a distance of 4,700,000 km (2,900,000 mi)[16] between 20 April and 19 August 1989.[17] Voyager 2 obtained 83 images with observation accuracies of 70 km (43 mi) to 800 km (500 mi).[17] 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.[18] 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.[8]

If selected, The Arcanum mission[19] would do a flyby of Nereid before its primary purposes of orbiting Neptune and observing Triton.[20]

See also

Notes

  1. ^ Flattening derived from the maximum aspect ratio (a/c): , where (a/c) ≤ 1.3.[5]

References

  1. ^ a b Kuiper, G. P. (August 1949). "The Second Satellite of Neptune". Publications of the Astronomical Society of the Pacific. 61 (361): 175–176.
    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. ^ a b c d 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.
  5. ^
    doi:10.1093/mnras/stw081
    .
  6. ^ "Planetary Satellite Physical Parameters".
    JPL (Solar System Dynamics). Archived
    from the original on 2010-05-27. Retrieved 2011-10-26.
  7. ^ "M.P.C. 115892" (PDF). Minor Planet Circular. Minor Planet Center. 27 August 2019.
  8. ^ a b 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.
    S2CID 45403579
    .
  9. ^ a b Grav, T.; M. Holman; J. J. Kavelaars (2003). "The Short Rotation Period of Nereid". The Astrophysical Journal. 591 (1): 71–74.
    S2CID 8869351
    .
  10. ^ a b Denk, Tilmann (2024). "Outer Moons of Saturn". tilmanndenk.de. Tilmann Denk. Retrieved 25 February 2024.
  11. ^ a b
    S2CID 119267757
    .
  12. ^ a b Grav, Tommy;
    S2CID 15706906
    .
  13. ^ a b c d Brown, Michael E.; Koresko, Christopher D.; Blake, Geoffrey A. (December 1998). "Detection of Water Ice on Nereid".
    S2CID 17451226
    .
  14. S2CID 204960799
    .
  15. ^ Williams, I.P.; Jones, D.H.P.; Taylor, D.B. (1991). "The rotation period of Nereid".
    doi:10.1093/mnras/250.1.1p
    .
  16. ^ Jones, Brian (1991). Exploring the Planets. Italy: W.H. Smith. pp. 59.
    ISBN 978-0-8317-6975-8
    .
  17. ^ a b 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
    .
  18. ^ "PIA00054: Nereid". NASA. 1996-01-29. Retrieved 2009-11-08.
  19. ^ https://www.cambridge.org/core/journals/aeronautical-journal/article/concept-of-operations-for-the-neptune-system-mission-arcanum/5B8B0E048C520C06BEE74675AD6781C6
  20. ^ https://arxiv.org/pdf/2106.09409.pdf
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