Neptune has 13 moons, Triton, Nereid, Naiad, Thalassa, Despina, Larissa, Proteus, and Galatea, plus five smaller, unnamed moons. Triton and Proteus orbit close to Neptune; Nereid is in a distant orbit.
Triton is the only moon in our Solar System with a retrograde orbit (orbiting in the opposite direction than its primary, Neptune, is rotating).
The moon Triton is the coldest measured object in our Solar System, and Nereid is the Solar System object with the most eccentric orbit.
The moons of Neptune in order of distance from Neptune.
|Moon||Photo||Discovery||Distance from the center of Neptune (km)||Diameter (km)||Orbital period (in Earth days)||Notes|
|Naiad||NASA’s Voyager 2 mission, 1989||48,230||58||0.29|
|Thalassa||NASA’s Voyager 2 mission, 1989||50,070||80||0.3|
|Despina||NASA’s Voyager 2 mission, 1989||52,530||148||0.33|
|Galatea||NASA’s Voyager 2 mission, 1989||61,950||158||.43|
|Larissa||NASA’s Voyager 2 mission, 1989||73,550||104 x 89||0.5|
|Proteus||NASA’s Voyager 2 mission, 1989||117,650||218 x 208 x 201||1.1||Neptune’s second largest moon. This dark moon has a circular orbit just over the cloud-tops of Neptune. Proteus is dotted with impact craters and is irregularly shaped.|
|Triton||Wm. Lassell, 1846||354,760||2,704||Synchronous||The largest of the 8 moons of Neptune. Unlike all other large planetary moons, Triton has a retrograde orbit (it rotates opposite to Neptune’s rotation) and is in synchronous orbit. It also has a highly inclined axis.
Triton is the coldest object that has been measured in our Solar System, with a temperature of -235° C (-391° F). This rocky moon has a polar ice cap at its south pole and many other varied geologic features including volcanoes, huge cracks in the surface, and geysers of gaseous nitrogen. It has a very thin, hazy atmosphere (mostly nitrogen) and a windy surface covered with nitrogen ice. Triton is slowly spiraling in towards Neptune.
|Nereid||G. Kuiper, 1949||5,513,400||340||360.1||Neptune’s outermost (and third largest) moon is Nereid, which has an irregular shape. It is the smaller of Neptune’s 2 larger moons. It has a VERY elliptical orbit, going as close as 867,000 miles from Neptune and as far as 6 million miles from it; it may be a captured asteroid. It takes almost one Earth year for Nereid to orbit Neptune in this extreme orbit (360.1 days).
Nereid’s has the most eccentric orbit in the solar system. The distance from Nereid to Neptune varies from about 1,353,600 kilometers (841,100 miles) to over seven times as far, 9,623,700 kilometers (5,980,200 miles).