Overview

Neso, also known as Neptune XIII or by its provisional designation, S/2002 N 4 is the second-outermost known moon of Neptune and the 10th largest natural satellite of Neptune. Neso follows a rather highly inclined, eccentric, retrograde orbit but it has the longest orbital period of any natural satellite. It is possible due to Neptune's great distance from the Sun, resulting in an immense hill sphere even bigger than Jupiter's. It has an estimated diameter at about 60 km based on the presumed albedo.^[1] It is named after Neso, one of the Nereids in Greek mythology. The similarity between the orbital parameters of Neso and both irregular moons, Psamathe and S/2021 N 1 means that both objects may be a fragment from a much larger moon, formed by an impact event, the collisional family is named after Neso, as its the largest member of the group, possibly being the progenitor.^[1]

Discovery

Neso was observed by Matthew J. Holman, Brett J. Gladman and so forth on 14th of August, 2002, using a 4-m Blanco telescope at Cerro Tololo Observatory in Chile, although, it was overlooked and not recognized as a irregular natural satellite of Neptune until a year later on 2003.^[2][3] Neso received its provisional designation S/2002 N 4 after the discovery and on 3rd of February 2007, the irregular satellite is officially named after Neso, one of the Nereids in Greek mythology.^[4]

Orbit

This moon formerly holds a record for the the longest orbital period of a natural satellite with an orbital period of about 25.665 Earth years, only beaten by an another irregular moon, S/2021 N 1. Its extreme orbit and distance is possible owing to the Neptune's great distance from the Sun, causing it to produce an enormous hill sphere even bigger than Jupiter's hill sphere, Neso's orbit is so large that the apocenter measures at around 0.4834 AU, meaning the apocenter of Neso surpasses the innermost planet of the Solar System, Mercury with an apocenter of about 0.4667 AU. Neso's orbit is influenced by the Lidov-Kozai mechanism, implying that it is currently perturbed by a distant third body, which that by being the Sun. Its orbit is constantly perturbed by the Sun's powerful gravitational pull due to its extreme distance from Neptune, giving astronomers more time and effort to find this little moon as time passes on, over the next 6000 years, the eccentric orbit of Neso is approximated to vary at around 0.13 to 0.88 and the orbital inclination of Neso is estimated to oscillate at a range of about 117° to 146°.^[5]

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