Neptunes Moons
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Neptune's natural satellites

Neptune has thirteen known moons. The largest by far is Triton, discovered by William Lassell just seventeen days after the discovery of Neptune itself. It took about one hundred years to discover the second, Nereid.

Unusual orbits

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 seven times further from the planet than at its pericenter.

Two natural satellites discovered in 2002 and 2003, Psamathe and Neso, 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.

Theories of origin

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, causing them to collide and become reduced to a rubble disc. Only after Triton's orbit became circularized did some of the rubble disc re-accrete into the present-day satellites .

The mechanism of the Tritonís 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..

Numerical simulations show that another moon discovered in 2002, Halimede has had a high probability of collision with Nereid during the lifespan of the system. As both moons appear to have similar (grey) colors, the satellite could be a fragment of Nereid

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