5 Facts about Saturn’s Mystery Moon, Phoebe

Phoebe's true nature is revealed in startling clarity in this mosaic of two images taken during Cassini's flyby on June 11, 2004. Image Credit: NASA/JPL

Saturn is the most glamorous planet in our Solar System, with its strings of softly gleaming colors, its icy gossamer rings system, and its shimmering entourage of iced moons. Saturn’s myriad moons are a diverse collection, extending from the gigantic hydrocarbon-slashed moon Titan–which is larger than planet Mercury–to the tiniest of shimmering icy moonlets. In all, Saturn is circled by an extraordinary following of 82 known moons with sanctioned orbits. But, out of this brilliant collection, the small moon Phoebe stands out in the organization as an interesting mystery-moon. 

Phoebe is one of Saturn’s most attractive moons, revolving at a distance of 12,952,000 kilometers ( 8,049,668 miles ) from the host planet, almost four times the distance from Saturn than its nearest next-door-neighbor, the moon Iapetus. Iapetus and Phoebe are the only knowns moons in the Saturnian system that do not revolve closely to Saturn’s equator plane.

Here are five facts about Saturn’s Mystery Moon – Phoebe:

  1. Phoebe was discovered by the American astronomer William Henry Pickering (1858 CE-1938 CE) on March 18, 1899 CE from an examination of photographic plates that had been obtained beginning on August 16, 1898 CE at the Boyden Observatory near Arequipa, Peru, by the American astronomer DeLisle Stewart (1870 CE-1941 CE). Indeed, Phoebe has the distinction of being the first moon to be observed photographically.
  2. Phoebe is approximately spherical and has a radius of about 106.5 kilometers (66.2 miles), about one-sixteenth of Earth’s Moon’s radius. Phoebe rotates on its irregular axis every nine hours *roughly), and it makes a complete orbit around Saturn in about 1.5 Earth Years (18 Earth months). Its elliptical, irregular orbit is inclined about 175 degrees to Saturn’s equator. Phoebe’s orbit is also reversed, which means it goes around its planet, Saturn, in the opposite direction than other moons.
  3. Unlike most moons revolving around Saturn, Phoebe is exceptionally dark and reflects barely 6 percent of the sunlight it gets. Its darkness and retrograde, irregular orbit suggest Phoebe is most likely a seized object. A captured object is a celestial body trapped by the gravitational pull of a much more giant landmass, usually a planet. Phoebe’s mystery, in particular, hints that the small moon comes from the outer solar system, a region where there are lots of dark material.
  4. Phoebe is another name for the goddess that the Romans called Diana, and the Greeks called Artemia. She was the young goddess of Earth’s forests, moons, hunting, and wild animals. Sworn to independence and chastity, she never married and was closely recognized with her brother Apollo.​
  5. Phoebe was the first celestial object encountered upon the Cassini spacecraft’s arrival in the Saturn system in 2004 CE and is thus remarkably well-studied for an unusual moon of its size. Cassini’s trajectory to Saturn and arrival time was explicitly chosen to permit this brilliant flyby.

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