Sao (SAY-oh) is a small irregular satellite (a satellite with a distant, often highly inclined, eccentric or retrograde orbit) of Neptune. Its average distance from Neptune is about 14 million miles, but it has a slightly eccentric orbit, placing it 12 million miles from Neptune at perineptune, and 16 million miles from Neptune at aponeptune. The orbit is highly inclined, making an angle of 53 degrees with the rotational plane of the planet. However, it does orbit in the same direction as the rotation of the planet, unlike some of the other outer moons of Neptune. 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 Sao's brightness corresponds to an object just under 30 miles in diameter. If it were assumed to have a density between 1 and 2 times that of water, its mass would be 50 to 100 trillion tons (5 to 10 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.
Sao's orbital eccentricity and inclination place it in what is called a Kozai resonance. This means that over time perturbations can cause its orbit to become more circular but more highly inclined, or less circular but with a lower inclination to the plane of Neptune's rotation.
|Data for Sao|
Discovered by T. Grav et al at Cerro Tololo, August 14, 2002 (as S/2002 N2; named in 2007)
Named after one of the Nereids
Orbital size 22 million km (about 14 million miles)
Orbital eccentricity 14%
Orbital inclination 53 degrees
Orbital period 2913 days = 7.97 years
Rotational period unknown
Diameter about 45 km (about 28 miles) (based on assumed albedo)
Albedo (reflectivity) 4% (assumed)