Distant moon of NeptuneA moon on a very strange path

Nereid

Nereid follows one of the most eccentric orbits of any major planetary moon, suggesting a history of capture or disruption.

Moon of Neptune · about 340 km across · 360-day eccentric orbit · discovered in 1949

Host: Neptune
Type: Small irregular moon
Mean diameter: About 340 km
Mean orbital radius: 5.51 million km
Orbital period: About 360 days
Orbital eccentricity: About 0.75
Rotation: Possibly irregular
Discovered: 1949 · Gerard Kuiper

Nereid up close

NASA imagery

Voyager 2 view of Neptune's distant moon Nereid, a small pale irregular body — **Neptune's distant moon Nereid** Nereid was observed by Voyager 2 as a small, irregular world travelling through Neptune's outer moon system. Image: NASA/JPL.

An eccentric orbit

Nereid's distance from Neptune varies by a factor of several during each orbit. At its closest it is roughly 1.4 million kilometres from the planet; at its farthest it can be more than 9 million kilometres away.

The large eccentricity is unusual for a regular moon and points to a dramatic past. Nereid may have been captured or scattered by Triton during Triton's arrival in the Neptunian system.

A difficult target

Voyager 2 passed Neptune during a rapidly changing sequence in 1989. Nereid was then far from the spacecraft, so most images show only a small disk at low resolution.

The moon rotates irregularly and may have formed from the fragments of a disrupted moon. Its surface, composition and exact shape remain poorly known without a dedicated mission.

Worth knowing

Nereid was discovered by Gerard Kuiper in 1949, decades before Voyager 2 reached Neptune.
It has one of the most eccentric orbits of any planetary moon.
Nereid is named after the sea nymphs of Greek mythology.

Watch and explore

Research & further reading

Mission imagery and measurements are interpreted alongside orbital data. Follow the source links for the current NASA overview and precise satellite parameters.

Mission reference NASA · Nereid ↗ NASA's overview of Nereid and its exploration. Orbital data NASA/JPL satellite elements ↗ Mean orbital elements and current satellite catalogue data.