Phoebe (moon)
Cassini image of Phoebe. Jason crater occupies much of the upper image | |||||||||
| 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 | |||||||||
| 12 929 400 km | |||||||||
| Eccentricity | 0.164 | ||||||||
| 550.303910 d | |||||||||
Average orbital speed | 1.71 km/s | ||||||||
| Inclination | 175.2° (to the Laplace plane) 151.64° (to Saturn's equator) | ||||||||
| Satellite of | Saturn | ||||||||
| Group | Norse group | ||||||||
| Physical characteristics | |||||||||
| Dimensions | 218.6 × 216.8 × 203.6 km (± 1.8 × 0.8 × 0.4 km) | ||||||||
| 106.4±0.4 km | |||||||||
| Circumference | ~685 km (equatorial) | ||||||||
| Volume | (5.06±0.20)×106 km3 | ||||||||
| Mass | (8.3123±0.0162)×1018 kg | ||||||||
Mean density | 1.6428±0.0326 g/cm3 | ||||||||
| 0.038–0.050 m/s2 (~1/200 to ~1/260 g) | |||||||||
| 89–108 m/s | |||||||||
| 9.27365±0.00002 h (9 h 16 min 25 s) | |||||||||
| 23.95° (to orbit) | |||||||||
North pole right ascension | 356.6°±0.3° | ||||||||
North pole declination | 78.0°±0.1° | ||||||||
| Albedo | 0.0856±0.0023 (visual geometric) 0.023±0.007 (Bond) | ||||||||
| |||||||||
| 16 | |||||||||
| 6.6 6.59±0.02 6.71 | |||||||||
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 by DeLisle Stewart starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru. It was the first natural satellite to be discovered photographically.
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 moon of its size. Cassini's trajectory to Saturn and time of arrival were chosen to permit this flyby. After the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus.
Phoebe has a moderately eccentric, retrograde orbit, making it part of Saturn's Norse group of satellites. It is the second-largest retrograde satellite in the Solar System after Triton. 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. There is some evidence that it may be a captured centaur that originated in the Kuiper belt.