inner moon · moon of Neptune
Proteus
Neptune's second-largest moon, a dark, box-shaped body that stayed hidden in the planet's glare until Voyager 2 arrived.
- Distance from Earth today
- 4.35 billion km29.08 AU · light: 4 h 2 min
- Distance from the Sun today
- 4.47 billion km29.88 AU
- As of
- 2 November 2026position from NASA/JPL orbital elements
Where it is today
Top view of the plane of Earth’s orbit, compressed distance scale. Open on the big map with the time slider →
Facts
- Diameter
- 416 km
- Mass
- 38.7 × 10¹⁸ kg
- Density
- 1,027 kg/m³
- Mean distance from the planet
- 117,700 km
- Orbital period
- 26.9 h
- Orbital eccentricity
- 0.0010
- Orbital inclination
- 0.6°
Source of the figures: JPL SSD – Planetary Satellites (public domain).
Discovery: 1989 – Voyager science team
Description
Proteus is the second-largest moon of Neptune, yet it was unknown until Voyager 2 photographed it in 1989: it is very dark and orbits close enough to Neptune to be lost in the planet's glare from Earth. It has an irregular, somewhat box-like shape and is thought to be close to the size at which a body of its kind would be pulled into a round shape by its own gravity. Its surface is heavily cratered, with one particularly large impact basin. Proteus probably formed from debris left after Triton's capture disrupted Neptune's original moons.