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Neptune's natural satellites

Neptune has 13 known moons.

The largest by far is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself.

It took a hundred years to discover the second, Nereid.

Triton orbits Neptune on a circular but retrograde orbit.

While retrograde orbits are common among distant irregular satellites, Triton is a unique case of retrograde moon so close to its planet.

The third largest moon of Neptune, Nereid follows a prograde but the most eccentric orbit among the moons of the solar system, being at its apocenter more than 7 times further from the planet than at its pericenter.

Two natural satellites discovered in 2002 and 2003, Psamathe and S/2002 N 4, have the largest orbits of any natural satellites discovered in the Solar system to date.

They take 25 years to orbit Neptune at an average of 125 times the distance between Earth and the Moon.

It is likely that Neptune's inner satellites are not the original bodies that formed with Neptune but accreted rubble from the havoc that was wreaked after Triton's capture.

Triton's original captured orbit would have been highly eccentric, and caused chaotic perturbations in the orbits of the original inner Neptunian satellites, caussing them to collide and become reduced to a rubble disc.

Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites.

The mechanism of the Triton’s capture have been the subject of a few theories over the years.

The most recent postulates that Triton was captured in a three body encounter.

In this scenario, Triton is the surviving member of a binary object1 disrupted by the encounter with Neptune.

Note:   The above text is excerpted from the Wikipedia article "Neptune's natural satellites", which has been released under the GNU Free Documentation License.
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May 29, 2015

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