Phoebe is a dark, battered world circling Saturn backwards at nearly 13 million km - ten times farther out than Iapetus. Almost everything about it is wrong for a homegrown moon and right for a captured one: it is most likely a centaur, a wanderer from the Kuiper-belt region that strayed too close to Saturn long ago and never escaped.
It orbits Saturn backwards, taking about 18 months per revolution.
Dust knocked off Phoebe forms a vast, faint ring and slowly blackens Iapetus’ leading face.
Cassini flew past it in June 2004, eleven days before entering orbit around Saturn.
How to spot a captured moon
Moons that form alongside their planet orbit in the planet’s spin direction, close to its equatorial plane. Phoebe breaks every rule: it orbits backwards, on a tilted, stretched-out path, and its density and composition resemble Pluto more than its icy neighbours. That is the fingerprint of gravitational capture - Saturn did not build Phoebe, it caught her.
Physical characteristics
Diameter
213 km
Mass
8.300e+18 kg
Surface gravity
0.05 m/s²
Density
1.64 g/cm³
Rotation period
9.3 h
Mean temperature
-198 °C
Geometric albedo
0.06
Atmosphere and composition
Atmosphere
None.
Composition
Ice and carbon-rich rock, more like a Kuiper-belt body than a Saturnian moon.
Discovery
Discovered by
William Pickering
Year
1899
How
Photographic plates - the first moon discovered photographically
Missions
Voyager 2NASA · Flyby probe · 1977 · active
Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.
Cassini orbited Saturn for 13 years, delivering ESA’s Huygens probe to the surface of Titan in January 2005 - the first landing in the outer Solar System. It ended with a deliberate plunge into Saturn in 2017 to protect the potentially habitable moons Enceladus and Titan.