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.