Diagram of orbital parameters for several of Neptune's distant moons.

Orbital parameters for several of Neptune's more distant moons. Image Credit: NASA


Sao was discovered Aug. 14, 2002 by Tommy Grav, Matthew J. Holman, John J. Kavelaars, Wesley C. Fraser, and Dan Milisavljevic using images taken by the 4.0-m Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile and the 3.6-m Canada-France-Hawaii Telescope in Hawaii. Multiple images were digitally combined until stars appeared as streaks, while the moons appeared as points of light.

Sao, Laomedeia and Halimede were discovered through the use of Earth-based telescopes -- this was the first time a Neptunian moon discovery had occurred by telescope since the discovery of Nereid in 1949 by Gerard Kuiper.


Sao is one of three tiny moons (ranging in size from 18 to 24 miles—or 30 to 40 km) of Neptune discovered in 2002 using innovative ground-based telescope techniques. (The other moons discovered were Laomedeia and Halimede.) The moons are so distant and so small they are about 100 million times fainter than can be seen with the unaided eye. The moons were missed by the Voyager 2 spacecraft in 1989 because they are so faint and distant from Neptune.

Sao is considered an irregular satellite because of its distant, eccentric orbit around Neptune. Like most irregular satellites of the giant planets in our outer solar system, Sao most likely formed after a collision between a larger moon and a comet or an asteroid. Sao and Laomedeia have prograde orbits, which means they orbit in the same direction as Neptune's rotation. Halimede has a retrograde orbit, which means Halimede orbits in the opposite direction of Neptune's rotation.

Very little is known about Sao. Scientists are trying to learn more about it and its irregular sisters because they offer a glimpse of the conditions at the time the planets in our solar system were forming billions of years ago.

How Sao Got its Name

Moons of Neptune are named for characters from Greek or Roman mythology associated with Neptune or Poseidon, or the oceans. Irregular satellites are named for the Nereids, daughters of Nereus and Doris and the attendants of Neptune.

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