Proteus
Moon· orbits Neptune
Proteus is Neptune's second-largest moon — a dark, irregular, heavily cratered body about as large as a moon can be without being pulled into a sphere.
Open in 3D viewSee Proteus in the live, interactive 3D map
Key facts
Mass (est.)
3.87 × 10¹⁹ kg
0.000527 Moon masses
Radius
208 km
12% of our Moon's radius
Surface gravity
0.0597 m/s²
1% of Earth's gravity
Day length
26.94 hours
Orbit around Neptune
1.12 Earth days
Science
Physical
- Mass (estimated)
- 3.87 × 10¹⁹ kg · 0.000527 Moon masses
- Estimated from its size and an assumed density (~1.0 g/cm3); the GM is not dynamically constrained, so this is not a measurement. Source: JPL SSD Planetary Satellite Physical Parameters (GM NEP097).
- Radius
- 208 km · 12% of our Moon's radius
- JPL SSD Planetary Satellite Physical Parameters (mean radius)
- Surface gravity
- 0.0597 m/s² · 1% of Earth's gravity
- Estimated from the estimated mass and radius
- Escape velocity
- 158 m/s · 1% of Earth's escape velocity
- Estimated from the estimated mass and radius
Orbit around Neptune
- Distance from Neptune
- 117,600 km
- From JPL orbital elements
- Orbital period
- 26.94 hours
- From JPL orbital elements
- Eccentricity
- 0
- From JPL orbital elements
- Inclination to Neptune's Laplace plane
- 0.00 °
- From JPL orbital elements · The Laplace plane is the one a moon's orbit precesses about: close in it is the planet's equatorial plane, the plane of the rings, and further out it tilts toward the planet's own orbit.
Rotation
- Rotation period
- 26.94 hours
- Tidal locking
- Tidally locked to its parent
Temperature
- Mean
- 51 K (-222 °C)
Composition
- Bulk composition
- A dark mixture of water ice and carbon-rich rock.
Radiation & temperature
- Insolation
- 0.00112 S⊕ · 1/893 of the sunlight Earth receives
- Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
- Geometric albedo
- 0.10
- Geometric (V-band) albedo — a dark surface; Bond albedo not directly measured.
- Equilibrium temperature
- 50 K (-224 °C)
- Computed from albedo + distance from the Sun (airless black body); uses the geometric albedo as an estimate
- Measured mean temperature
- 51 K (-222 °C)
Discovery & history
- By
- Voyager 2 (Stephen P. Synnott)
- Name
- Proteus, the shape-shifting sea-god of Greek myth.
Proteus was found by Voyager 2 in 1989 during its Neptune flyby; it had hidden from Earth's telescopes because it is one of the darkest objects in the Solar System, reflecting only a few percent of the light it receives. Almost as large as a body can be without gravity pulling it round, it is instead a battered, boxy lump.
Images
Images from open archives · credit shown per image