Phoebe (moon)
Phoebe (/ˈfiːbi/ FEE-bee) is the most massive irregular satellite of Saturn with a mean diameter of 213 km (132 mi). It was discovered by William Henry Pickering on 18 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru, by DeLisle Stewart. It was the first satellite to be discovered photographically.
Cassini image of Phoebe | |
| Discovery | |
|---|---|
| Discovered by | W.H. Pickering |
| Discovery date | 18 March 1899 (from photos taken 16 August 1898) |
| Designations | |
Designation | Saturn IX |
| Pronunciation | /ˈfiːbi/ FEE-bee |
Named after | Φοίβη Phoíbē |
| Adjectives | Phoebean /fiːˈbiːən/ fee-BEE-ən |
| Orbital characteristics | |
| 12,960,000 km | |
| Eccentricity | 0.1562415 |
| 550.564636 d | |
| Inclination | 173.04° (to the ecliptic) 151.78° (to Saturn's equator) |
| Satellite of | Saturn |
| Group | Norse group |
| Physical characteristics | |
| Dimensions | (218.8±2.8) × (217.0±1.2) × (203.6±0.6) km |
Mean radius | 106.5±0.7 km |
| Mass | (8.3123±0.0162)×1018 kg |
Mean density | 1.6428±0.0326 g/cm3 |
| 0.038–0.050 m/s2 | |
| ≈ 0.10 km/s | |
| 9.2735 h (9 h 16 min 25 s ± 3 s) | |
| 152.14° (to orbit) | |
| Albedo | 0.100±0.005 |
| Temperature | ≈ 73(?) K |
| |
Phoebe was the first target encountered upon the arrival of the Cassini spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an irregular satellite of its size. Cassini's trajectory to Saturn and time of arrival were specifically chosen to permit this flyby. After the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus.
Phoebe is roughly spherical and has a differentiated interior. It was spherical and hot early in its history and was battered out of roundness by repeated impacts. It is believed to be a captured centaur that originated in the Kuiper belt. Phoebe is the second largest retrograde satellite in the Solar System after Triton.