Phoebe: Saturn’s Odd Fascinating Moon
15 June 2014

This is a Cassini spacecraft image of Phoebe, a small bizarre moon of Saturn.
Credit: NASA/JPL-Caltech/Space Science Institute

Phoebe is a bizarre and intriguing moon of Saturn. It is termed an irregular satellite, as it orbits Saturn in a markedly elliptical, highly inclined orbit, in a clockwise direction, oppositely to the planet’s rotation and the motion of the “regular” moons.

Interestingly, Phoebe was discovered in 1899, in photographic plates, obtained at the Boyden Observatory, to become the first moon discovered photographically.

Phoebe is roughly spherical in shape, with a diameter of 213 km. For comparison, Earth's Moon is 3,476 km across, or about 16 times larger than Phoebe in diameter. Phoebe rotates on its axis every nine hours, and lasts about 18 months to orbit Saturn once, at an average distance of almost 13 million km. It is thought to be composed of rocks and ice, in equal proportions. Its surface is dark and very densely cratered, with a temperature of nearly −200 °C.

For several reasons, including its orbit and surface characteristics, Phoebe is believed to be a captured moon that did not form originally in the Saturn system. Rather, it may have formed far beyond the orbit of Saturn, in a large zone populated by numerous small icy objects, named the Kuiper Belt, after the Dutch-American Astronomer Gerard Kuiper. The Kuiper Belts stretches from the orbit of Neptune, located 4.5 billion km from the Sun, out to 7.5 billion km away from the Sun. Phoebe might thus be a small relative of dwarf planet Pluto (2,300 km across), the largest Kuiper Belt object.

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Aymen Mohamed Ibrahem
Senior Astronomy Specialist
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