Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Neptune: Neso Link for sharing this page on Facebook
     Neso (NEE-soh) is a small irregular satellite (a satellite with a distant, often highly inclined, eccentric or retrograde orbit) of Neptune. It has the largest orbit of any known moon, orbiting Neptune at an average distance of just over 30 million miles. Its orbit is also very eccentric, being only 12 million miles from Neptune at perineptune, and 48 million miles from Neptune at aponeptune, and highly inclined, making an angle of about 45 degrees with the rotational plane of the planet, with a backwards (or retrograde) relative to the rotational direction of the planet. Given its large distance from Neptune, its rotational period cannot have any relationship to its orbital motion, and is therefore unknown.
     All the outer moons of Neptune are assumed to have albedos (or reflectivity) of about 4%. If that is correct Neso's brightness corresponds to an object about 35 miles in diameter. If it were assumed to have a density between 1 and 2 times that of water, its mass would be 100 to 200 trillion tons (1 to 2 times 10 to the 14th power). However, since the albedo could be substantially higher (or even a little lower) than the assumed value, its size and mass are essentially unknown.
     Neso's orbital elements are similar to those of Psamathe, and it has been suggested that the two moons owe their origin to the break-up of a former, larger moon.

Data for Neso

Discovered by M. Holman et al at Cerro Tololo, August 14, 2002
     (unnoticed until 2003, at which time it became S/2002 N4; named in 2007)
Named after one of the Nereids
Orbital size 49 million km (about 30 million miles, or 1/3 AU)
Orbital eccentricity 57%
Orbital inclination 136 degrees = -44 degrees (retrograde revolution)
Orbital period -9740 days = -26.7 years
Rotational period unknown
Diameter about 60 km (35 miles) (based on assumed albedo)
Mass unknown
Albedo (reflectivity) 4% (assumed)