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Space Topics: Saturn

Phoebe

Captured Moon

Phoebe
Click to enlarge >
Cassini captured this view of Phoebe, which is made of two separate images, on June 11, 2004. The surface is covered with craters, and it appears that the moon was once nearly broken to bits by impacts that left huge gouges (at top and bottom). North is to the top. The center to lower right area looks brighter because the Sun is striking that part of the moon most directly. Credit: NASA / JPL / Space Science Institute

Size: 214 kilometers - 9th largest moon of Saturn
Orbital radius: 12,952,000 kilometers - 215 Saturn radii - an irregular satellite, possibly a captured object
Orbital period: 550 days, retrograde
Discovery: 1898 by William Henry Pickering

Phoebe is the most distant large moon of Saturn and is an oddball among Saturn’s 31 known moons. Most of Saturn’s moons orbit the planet in the same plane and in the same direction as Saturn’s rings, but Phoebe travels in the opposite direction and in a steeply inclined orbit. This unusual orbit, in combination with its dark surface, has caused scientists to suggest that Phoebe originally formed elsewhere in the Solar System and was later captured by Saturn’s gravity.  Cassini observations of Phoebe's surface showed it to be covered not only with water ice -- common on the surfaces of Saturn's moons -- but also significant quantities of carbon dioxide either as an ice or bound together with other organic material.  That observation, together with a higher density than expected, strongly hints at an origin independent from Saturn's, in the outer reaches of the solar system.

Phoebe's unusual and distant orbit lies far beyond the distance to which Cassini ever travels during its Saturn tour.  So the mission planned for Cassini to fly by Phoebe on its way in to the Saturn system, before it entered orbit.

Phoebe's features are named for people and places from the Argonautica (the story of Jason and the Argonauts) by Apollonius Rhodius and Valerius Flaccus.

Flybys of Phoebe

Close view of Phoebe crater
Click to enlarge >
NASA / JPL / SSI

Cassini
June 11, 2004 at 19:34 UTC
“00PH” targeted flyby [P1]
Closest approach altitude 2,071 kilometers (1,287 miles)