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
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