Question: Why is Nereid's orbit so strange?

Susan Cartwright answered on 19 Jun 2015:

Nereid’s orbit is “strange” in the sense that it is very elongated, so Nereid’s distance from Neptune varies a great deal, from 1.4 million km to 9.7 million km.

There are two possible reasons for this, both involving gravitational capture. One is that Nereid did not form as a moon of Neptune, but was captured by Neptune from the outer asteroid belt (Kuiper belt). The other is that Nereid did form as a moon of Neptune, in a reasonably circular orbit, but its orbit was distorted during Neptune’s capture of its largest moon, Triton. We *know* Triton was captured: it orbits “backwards”, in the opposite direction to the normal rotation of the solar system (most things orbit counterclockwise as seen from the north, but Triton goes clockwise), and the only way to wind up in a backward, or retrograde, orbit is to be captured. If Nereid was nearby when the capture event happened, Triton’s gravity could have distorted its orbit.

The best way to distinguish between the two possibilities would be to compare Nereid’s chemical composition with that of the other moons of giant planets on the one hand, and the objects of the Kuiper belt on the other. Unfortunately it is difficult to do this with precision, because we’ve never had a really good close-up look – our best effort was a fast flyby at a distance of nearly 3 million miles (12 times the distance of the Moon) by the Voyager 2 spaceprobe. What information we do have suggests that Nereid looks more like the moons of Uranus than like the objects of the Kuiper belt, which would tend to favour the dislodged-by-Triton interpretation over the captured-asteroid theory. But the jury is still out.