Most eccentric satellite

The satellite with the greatest orbital eccentricity is Nereid, the third-largest moon of Neptune, which has an mean eccentricity of 0.7507. Its highly elliptic orbit brings it to within 1,372,000 km (852,521 mi) of Neptune before swinging out to a distance of 9,655,000 km (5,999,338 mi). Nereid was discovered on 1 May 1949 by Dutch-American astronomer Gerard Kuiper.

In orbital mechanics, the term "eccentricity" is used to describe the degree to which a body's orbit deviates from a perfect circle. It is typically expressed as a dimensionless parameter where a value of 0 represents a perfectly circular orbit, a value of between 0 and 1 represents an elliptical orbit and a value greater than 1 is a parabolic escape orbit.

It is thought that Nereid was once an inner moon of Neptune, but that it was catapulted into its present-day orbit when a large Kuiper Belt object was pulled into Neptune's gravitational sphere of influence. This object, now known as the moon Triton, fell into a retrograde orbit and its mass scattered or destroyed the planet's existing moons.

Very little is known about Nereid as the Neptune system has never been visited by an orbiting probe (Voyager 2's 1989 fly-by of the Neptune remains the only exploration mission to the planet).