page_title: Phoebe (moon)
section_title: Observation and exploration
hierarchical_section_title: Phoebe (moon) / Observation and exploration
caption_attribution_description: English: 4 minute exposure of Saturn's moon Phoebe with a 24" telescope. Phoebe is apparent magnitude 16.3 in this image taken at 2010-03-09 11:30 UT. The star to the left of Phoebe is magnitude 17.2. Also visible is the galaxy NGC 4179 at apmag 12. During this photo the moon was 32% full and only 97 degrees away. JPL Solar System Simulator for Saturn at 2010-03-09 11:30 UT.
context_page_description: Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km. It was discovered by William Henry Pickering on March 18, 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.
  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.
context_section_description: Unlike Saturn's other moons, Phoebe was not favorably placed for the Voyager probes. Voyager 2 observed Phoebe for a few hours in September 1981. In the images, taken from a distance of 2.2 million kilometres at low phase angle, the size of Phoebe was approximately 11 pixels and showed bright spots on the otherwise dark surface.
  Cassini passed 2,068 kilometres (1,285 mi) from Phoebe on 11 June 2004, returning many high-resolution images, which revealed a scarred surface. Because Voyager 2 had not been able to produce any high quality images of Phoebe, obtaining them was a priority for the Cassini mission and its flight path was deliberately designed to take it close by; otherwise, Cassini would likely not have returned images much better than Voyager's. Because of Phoebe's short rotation period of approximately 9 hours, 17 minutes, Cassini was able to map virtually the entire surface of Phoebe. The close fly-by enabled the mass of Phoebe to be determined with an uncertainty of only 1 in 500.
caption: Phoebe (with NGC 4179 in the lower right corner) as imaged with a 24" telescope
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