Phoebe

Moon· orbits Saturn

Phoebe is a dark, irregular outer moon of Saturn on a retrograde orbit — most likely a captured Kuiper-belt object or centaur rather than a body that formed at Saturn.

Open in 3D viewSee Phoebe in the live, interactive 3D map

Key facts

Mass

8.29 × 10¹⁸ kg

0.000113 Moon masses

Radius

106 km

6% of our Moon's radius

Surface gravity

0.0488 m/s²

0% of Earth's gravity

Day length

9.27 hours

Orbit around Saturn

550 Earth days

1.51 Earth years

Science

Physical

Mass: 8.29 × 10¹⁸ kg · 0.000113 Moon masses; JPL / IAU mean radii; spacecraft-tracked GM where measured
Radius: 106 km · 6% of our Moon's radius; JPL / IAU mean radii; spacecraft-tracked GM where measured
Density: 1,639 kg/m³ · 1.6× water; Computed from mass and radius
Surface gravity: 0.0488 m/s² · 0% of Earth's gravity; Computed from mass and radius
Escape velocity: 102 m/s · 1% of Earth's escape velocity; Computed from mass and radius

Orbit around Saturn

Distance from Saturn: 12,929,400 km; From JPL orbital elements
Orbital period: 550.3 days; From JPL orbital elements
Eccentricity: 0.164; From JPL orbital elements
Inclination to Saturn's Laplace plane: 175.20 °; 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: 9.27 hours
Rotation: Spins once every ~9.3 hours — NOT tidally locked, unlike Saturn's regular moons (Cassini). Its short spin is unrelated to its ~1.5-year retrograde orbit.

Temperature

Mean: 110 K (-163 °C)

Composition

Bulk composition: A dark mix of water ice, rock and carbon-rich material, resembling an outer Solar System body.

Radiation & temperature

Insolation: 0.0112 S⊕ · 1% of the sunlight Earth receives; Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
Geometric albedo: 0.06; Geometric (V-band) albedo — a dark surface; Bond albedo not directly measured.
Equilibrium temperature: 89 K (-184 °C); Computed from albedo + distance from the Sun (airless black body); uses the geometric albedo as an estimate
Measured mean temperature: 110 K (-163 °C)
Greenhouse warming: +21 K; Measured temperature minus the airless equilibrium — the greenhouse effect

Discovery & history

By: William Henry Pickering
Method: Photographic plates
Name: Phoebe, a Titaness of Greek myth.

Phoebe was the first moon discovered photographically, found by William Pickering in 1899 on plates from Harvard's southern station. It circles Saturn backwards, far out and steeply tilted — a captured visitor from the outer Solar System that Cassini revealed as a battered, primitive body.