Phoebe (moon)

Phoebe ( 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.

Discovery and naming

Phoebe 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 Observatory near Arequipa, Peru, by DeLisle Stewart. It was the first satellite to be discovered photographically. Phoebe is named after Phoebe, a Titaness in Greek mythology associated with the Moon, who was the sister of Cronus (the Greek equivalent of the Roman god Saturn). It is also designated Saturn IX in some scientific literature. The IAU nomenclature standards have stated that features on Phoebe are to be named after characters in the Greek myth of Jason and the Argonauts. In 2005, the International Astronomical Union officially named 24 craters (see Named surface features). Toby Owen of the University of Hawaiʻi at Mānoa, chairman of the International Astronomical Union Outer Solar System Task Group said:

We picked the legend of the Argonauts for Phoebe as it has some resonance with the exploration of the Saturn system by Cassini–Huygens. We can't say that our participating scientists include heroes like Hercules and Atalanta, but they do represent a wide, international spectrum of outstanding people who were willing to take the risk of joining this voyage to a distant realm in hopes of bringing back a grand prize.

Orbit and orbital interactions

For more than 100 years Phoebe was Saturn's outermost known moon, until the discovery of several smaller moons in 2000. Phoebe is almost 4 times more distant from Saturn than its nearest major neighbor (Iapetus), and is substantially larger than any of the other moons orbiting planets at comparable distances. All of Saturn's regular moons except Iapetus orbit very nearly in the plane of Saturn's equator. The outer irregular satellites, including Phoebe, follow orbits that can be moderately to highly eccentric, and none is expected to rotate synchronously as all the regular moons of Saturn (except for Hyperion) do. Phoebe completes a full orbit around Saturn in about 18 months, and its orbit is retrograde; that is, it orbits Saturn in the opposite direction to Saturn's orbit. This categorizes it in a group of irregular satellites called the Norse group. Phoebe's orbit is significantly different from those of the other irregular moons, and it was the innermost known orbit among all retrograde satellites of Saturn, until the announcement of S/2023 S 50 and S/2023 S 38 in 2025. There are a number of satellites with similar orbits that are speculated to be fragments from past collision events involving Phoebe, such as S/2006 S 20, S/2006 S 9, S/2019 S 2, and S/2007 S 2. Once captured in orbit around Saturn, Phoebe had a significant effect on the irregular moon population of Saturn. Due to occupying a relatively much smaller orbital space, irregular satellites are expected to collide amongst themselves with a frequency four orders of magnitude greater than do main-belt asteroids. Phoebe is expected to be involved in half of these collisions, mostly due to its large cross-section. As any objects that crashed into it would have been eliminated, Phoebe may have cleared its orbital surroundings of other moons in this way, in a sense similar to the action of a major planet. This may have reduced the original irregular population from a value 30% greater than today, and decreased Phoebe's orbital distance from a value 30% larger than its present value. Some two thirds of the irregular moons present today were found to count as potential future impactors. Before the 2004 Cassini flyby, Phoebe was predicted to have a heavily cratered surface, with most of the large craters created from impacts with irregular moons.

Phoebe ring

The Phoebe ring is tilted 27 degrees from Saturn's equatorial plane (and the other rings of Saturn). It extends from at least 128 to 207 times the radius of Saturn; Phoebe orbits the planet at an average distance of 215 Saturn radii. The ring is about 40 times as thick as the diameter of the planet. Because the ring's particles are presumed to have originated from micrometeoroid impacts on Phoebe, they should share its retrograde orbit, which is opposite to the orbital motion of the next major moon inward, Iapetus. Inwardly migrating ring material would thus strike Iapetus's leading hemisphere, contributing to its two-tone coloration. Although very large, the ring is virtually invisible—it was discovered using NASA's infrared Spitzer Space Telescope. Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark areas on the surface of Hyperion, another one of Saturn's moons. Debris from the biggest impacts may be the origin of some of the other moons of the Norse group—almost all of which are less than 10 km in radius.

Physical characteristics

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