Psamathe (SAM-uh-thee) is a small irregular satellite (a satellite with a distant, often highly inclined, eccentric or retrograde orbit) of Neptune. It has one of the largest orbits of any known moon, orbiting Neptune at an average distance of just under 30 million miles. However, its orbit is relatively eccentric, being only 19 million miles from Neptune at perineptune and 41 million miles from Neptune at aponeptune. The orbit is also highly inclined, making an angle of about 55 degrees with the rotational plane of the planet, and is 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 Psamathe's brightness corresponds to an object about 25 miles in diameter. If it were assumed to have a density between 1 and 2 times that of water, its mass would be 40 to 80 trillion tons (4 to 8 times 10 to the 13th 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.
Psamathe's orbital elements are similar to those of Neso
, and it has been suggested that the two moons owe their origin to the break-up of a former larger moon.