|Discovery date||August 14, 2002|
|S/2002 N 4|
|Adjectives||Nesoan // or Nesoian //|
|Epoch June 10, 2003|
|Periapsis||~21,215,700 km (0.141 AU)|
|Apoapsis||~77,784,500 km (0.518 AU)|
|0.33 AU (~49,500,000 km), 3,220 Neptune radii, 49.285 Gm|
|9740.73 d |
|60 km (for albedo 0.04)|
Neso //, also known as Neptune XIII, is the outermost known natural satellite of Neptune. It is a retrograde irregular moon discovered by Matthew J. Holman, Brett J. Gladman, et al. on August 14, 2002, though it went unnoticed until 2003. Neso orbits Neptune at a distance of more than 48 Gm (million km), making it (as of 2015) the most distant known moon of any planet. At apocenter, the satellite is more than 72 Gm from Neptune. This distance is great enough that it exceeds Mercury's aphelion, which is approximately 70 Gm from the Sun.
Neso is also the moon with the longest orbital period, 26.67 years. It follows a retrograde, highly inclined, and highly eccentric orbit illustrated on the diagram in relation to other irregular satellites of Neptune. The satellites above the horizontal axis are prograde, the satellites beneath it are retrograde. The yellow segments extend from the pericentre to the apocentre, showing the eccentricity.
Neso is named after one of the Nereids. Before it was officially named on February 3, 2007 (IAUC 8802), Neso was known by its provisional designation, S/2002 N 4.
- JPL (2011-07-21). "Planetary Satellite Discovery Circumstances". Jet Propulsion Laboratory. Retrieved 2011-10-24.
- Green, Daniel W. E. (October 1, 2003). "S/2001 U 2 and S/2002 N 4". IAU Circular. 8213. Retrieved 2011-10-24.
- Jacobson, R. A. (2008). "NEP078 – JPL satellite ephemeris". Planetary Satellite Mean Orbital Parameters. Retrieved 2009-09-23.
- Sheppard, Scott S.; Jewitt, David C.; Kleyna, Jan (2006). "A Survey for "Normal" Irregular Satellites around Neptune: Limits to Completeness". The Astronomical Journal. 132 (1): 171–176. arXiv:astro-ph/0604552. Bibcode:2006AJ....132..171S. doi:10.1086/504799. S2CID 154011.
- Holman, M. J.; Kavelaars, J. J.; Grav, T.; et al. (2004). "Discovery of five irregular moons of Neptune" (PDF). Nature. 430 (7002): 865–867. Bibcode:2004Natur.430..865H. doi:10.1038/nature02832. PMID 15318214. S2CID 4412380. Retrieved 24 October 2011.
- Physical parameters from JPL