The moon of Neptune – Sao (XI) – was previously temporarily designated as S/2002 N2. It was discovered in August 2002 by the following team of astronomers: M. Holman, J.J. Kavelaars, T. Grav, W. Fraser and D. Milisavljevic.
    This discovery was achieved by the 4-m telescope Blanco w Cerro Tollolo (Chile). You can read additional details in The International Astronomical Union Circular
    Sao is one of three new satellites discovered since the →Voyager-2 Neptune system fly-by which occured 13 years ago (1989). With this moon, the team discovered also: Halimede and Laomedeia.
    The name of that moon is of mythological origin. Sao was one of the nymphs, the daughter of Poseidon and Demeter...
    The moon is in elliptical prograde orbit (→eccentricity e
 = 0.2931) with a →semimajor axis a = 22,422,000 km. At pericenter (closest to the planet) Sao is separated from the Neptune a distance of q = 15,850,112 km. At apocenter (furthest from the planet) this moon is separated from the Neptune a distance of Q = 28,993,888 km. In future these parameters may vary due to a large orbital distance from Neptune and the orbital motion being disturbed by the Sun and other factors. Perhaps it is a captured asteroid (with a diminutive mass: ~8.9920×1016 kg – it equals 12 mountains like Giewont!), and in future will possibly return to heliocentric orbit.

    The orbital period is almost 8 earthly years, which means that the average speed of its celestial motion is only 0°07'25'' per day. When this motion was detected and compared to the changes in Neptune's position, this body was classified as a true planetary satellite (and not a distant celestial body with its position projected near the planet).
Other obstacle in detecting this moon earlier was its brightness of only 24.5m (about 25.1 million times fainter than the faintest stars visible to naked eye). For comparison: Neptune's brightness is about 7.8m, so Sao is shining fainter by about 16.7m. The corresponding difference in brightness is almost 4.79 million times!

I have given some crucial data of Sao below. Table one contains the basic information. Whereas table two gives more detailed parametrs of its orbit (calculated using the following formulae).

Translated by Karol Pankowski


Mean distance from the planet  [×103 km] 22,422.0
Mean distance from the planet  [planetary R] 905.4
Orbital period  [days] 2,914.07
Orbital eccentricity  [e] 0.2931
Orbit inclination  [degrees] 48.5
Mean diameter  [km] 44.0
Main discoverer and year of the discovery M. Holman   2002
Visual magnitude  [mag] 24.5
Mass  [kg] ~8.9920 × 1016

Orbital Parameters

Distance from the planet  [×103 km] 15,850.1 28,993.9
Distance from the planet  [planetary R] 640.0 1,170.8
Angular size of the moon's orbit observed from the Earth*  [degrees] 0°12'32.03'' 0°22'55.67''
Angular diameter of the planet's disc as observed from the moon  [degrees] 0°10'44.53'' 0°05'52.35''
Brightness of the planet as observed from the moon**  [mag] –4.4 –3.1
Diameter of the moon's disc as observed from the planet's "surface"  [degrees] 0°00'00.57'' 0°00'00.31''
Brightness of the moon as observed from the planet's "surface"**  [mag] 12.3 13.6
Orbital velocity  [km/sec] 0.75 0.41
  * This value is calculated for Neptune at opposition (distance 4.4 billion km = 29.1 AU)
** The given value of magnitude is not corrected for some decreasing factors (e.g. the changing phase of illumination)

See other related links:
AstroNEWS – "Three moons more has ... Neptune"
Monde des MonatsSao
Discovery of Three Irregular Neptunian Moons
The Astronomy Workshop – Satellite Viewer
Planetary Satellite Mean Orbital Parameters
Natural Satellite Physical Parameters

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