Dark moon of NeptuneA world almost round

Proteus

Proteus is a dark, irregular moon that teeters on the threshold between a lumpy asteroid and a round planetary world.

Moon of Neptune · about 420 km across · 1.12-day orbit · photographed by Voyager 2

Host: Neptune
Type: Large irregular moon
Mean diameter: About 420 km
Mean orbital radius: 117,647 km
Orbital period: 1.12 days
Surface gravity: About 0.07 m/s²
Surface: Dark, cratered, non-spherical
Discovered: 1989 · Voyager 2

Proteus up close

NASA imagery

Voyager 2 image of Neptune's dark, irregular moon Proteus — **Neptune's moon Proteus** Proteus was discovered by Voyager 2 during its 1989 Neptune encounter. Its dark, box-like shape shows that self-gravity has not been strong enough to make it fully round. Image: NASA/JPL.

Almost round

Proteus is large enough to be nearly spherical but retains an irregular, box-like outline. Its shape suggests it formed low in the debris disk around Neptune and never gathered enough additional mass to fully settle into a sphere.

Its dark surface is heavily cratered, including the large impact structure Pharos. The moon is much darker than Neptune's nearby moon Triton.

A survivor of Neptune's history

Proteus is one of the largest inner moons in the Solar System and one of the darkest. Its low inclination and nearly circular orbit indicate that it formed in the regular satellite system rather than being captured.

Triton's backwards orbit and capture history would have disturbed the Neptunian system. Proteus offers a way to study how the surviving regular moons were affected by that event.

Worth knowing

Proteus was named after a shape-shifting sea god in Greek mythology.
It was discovered from Voyager 2 images in 1989 and confirmed shortly afterward.
The moon was almost missed because it is so dark and close to Neptune.

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