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Exploring Neptune's natural satellites:
A Baker's Dozen of Big and Little Moons
Neptune has 13 natural satellites. Two major moons of Neptune, Triton and Nereid, have been known for a long time:
As it passed by the planet, Voyager 2 discovered six additional natural satellites, giving Neptune a total of eight moons.
- English astronomer William Lassell detected Neptune's largest natural satellite, Triton, less than a month after the planet was discovered in 1846. Triton has an unusual orbit. It moves in a backward (retrograde) direction around Neptune, unlike any other of the Solar System's large satellites. Triton has a diameter of 1,681 miles, which is slightly smaller than Earth's Moon. The most interesting feature of this unusually interesting world are its ice volcanoes. Liquid nitrogen, dust or methane compounds erupt from beneath the surface. One of Voyager's photographs shows a plume rising almost five miles above the surface and extending some 80 miles downwind. Triton, Jupiter's moon Io and the planet Venus are the only bodies in the Solar System outside of Earth that are known to be volcanically active at the present time. Mars was in the past.
- Dutch-American astronomer Gerard Kuiper discovered Nereid in 1949. Nereid also has an unusual orbit. It has the most eccentric orbit of any moon in the Solar System. Its distance from Neptune varies from 900,000 to 6,000,000 miles. Voyager 2 found the small moon to be only 213 miles in diameter. Nereid reflects about 12 percent of the sunlight that reaches it. Nereid's odd orbit suggests it might be a captured asteroid.
Three small moons were discovered in 2002 and two more in 2003.
- Proteus is the largest Voyager-discovered moon. It has an irregular shape with an average diameter of about 250 miles. That is slightly larger than Nereid, but Proteus is a much darker body, reflecting only about six percent of the sunlight that strikes it. Proteus also is closer to Neptune, which explains why it was undiscovered even as Nereid could be observed from Earth. Proteus orbits Neptune at a distance of about 73,500 miles. Proteus is gray. Voyager saw craters and grooves on its surface. There was no sign of geologic activity. The irregular outline of Proteus suggests it has remained cold and icy throughout much of its history. Proteus is probably about as big as an irregular body can be before its gravity would pull it into a spherical shape.
- Larissa is an irregularly shaped, dark object about 130 miles long by 112 miles wide. Larissa orbits Neptune at a distance of about 46,300 miles. It reflects only five percent of the sunlight that falls on it. Larissa appears to have several craters from 19 to 31 miles across. The irregular outline of Larissa also suggests it has been cold and icy throughout its history.
- Despina circles Neptune at a distance of about 38,000 miles.
- Galatea orbits at a distance of about 32,000 miles from Neptune.
- Thalassa revolves around Neptune every 7.5 hours at a distance of 31,000 miles.
- Naiad is in a 7.1-hour orbit. It has a noticeable inclination, tilted 4.5 degrees to the equatorial plane of Neptune.
- The 2002-2003 discoveries range from 19-31 miles in diameter.
- The moons found by Voyager 2 in 1989 range in diameter from 33 miles to 250 miles.
- Nereid is 210 miles in diameter.
- Triton's diameter is 1,680 miles.
The planet was visited once by Voyager 2, an unmanned robot probe from Earth, which flew within 3,100 miles of Neptune in 1989.
Should we go back for another look? »
NASA Solar System Exploration - Neptune »
Jupiter Icy Moons Orbiter (JIMO) proposed to fly in 2015 »
Neptune interplanetary probe proposed to fly in 2015 »
Read more about the Solar System . . . Star: The Sun Inner Planets: Mercury Venus Earth Mars Outer Planets: Jupiter Saturn Uranus Neptune Pluto Other Bodies: Moons Asteroids Comets Beyond: Pioneers Voyagers
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