Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Saturn: Hyperion Link for sharing this page on Facebook
(mostly written in 2004, before the Cassini spacecraft reached Saturn)

Hyperion (approximately true color) as viewed by Voyager 1 in 1980 (left),
and (black/white) by Voyager 2 in 1981 (right), from a slightly different angle.

      Hyperion holds the distinction of being the largest irregularly-shaped object in the Solar System. It is an approximately disk-shaped oval with diameters which vary from less than 150 miles to more than 250 miles. Usually, large objects are essentially spherical as a result of gravity compressing them into a regular shape, and it is hard to see how Hyperion could have ended up with its current shape unless it was broken off a somewhat larger object, or had large amounts of material blasted off of it, and it simply ended up with too small a size and too little gravity to be forced into a rounder shape. It certainly is an old chunk of some sort as it is covered, despite its small size, with a large number of craters of various sizes, including one that is nearly 80 miles in diameter and several miles deep, which is a very large fraction of Hyperion's own size.
      Hyperion has a moderately low density and must, like most of Saturn's moons, be composed mostly of water ice and a lesser amount of rock. Its surface is a relatively dark reddish color, and it is thought that dark debris coats most of the surface. The color of the debris closely matches that of Iapetus, which is the next moon out from Saturn, suggesting that whatever has caused Iapetus to have its odd appearance may also be responsible for the color of Hyperion.
      Hyperion rotates chaotically, meaning that it occasionally tumbles erratically, and it is not possible to predict exactly how its surface will be turned in space at any given time. It is believed that a combination of three factors allows it to have such a strange rotation. One of these is its irregular shape, another is its very eccentric orbit, which allows it to pass relatively close to Saturn's largest moon Titan at times, and the third is that its orbit is locked to that of Titan by a commensurability relationship of 3 to 4, so that the times and places where they pass relatively close are fairly stable, allowing Titan to exert an even greater influence on its orbit and rotation than would usually be possible.
     (Added without any alteration of the previous material, 07/14/2005) A closer inspection of the moon by the Cassini spacecraft reveals that it is much less dense than previously estimated -- only about 60% as dense as water ice -- suggesting that it is not only made almost exclusively of water ice but is very porous, being not a solid body but a loose conglomeration of icy chunks not unlike a very loosely compacted snowball. It is this low density that allows it to keep its irregular shape despite its size; being larger than usual for an object of its mass, it has a lower gravity and is less compressed than might have been expected; this low compression prevents the ices from being squeezed into a rounder, more solid structure.

The porous nature of Hyperion is suggested by its spongy appearance in this still image, taken at close encounter (distance about 110,000 miles) with the Cassini spacecraft on June 10, 2005. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod050726) An animation of the close encounter, between June 9 and June 11, is also available. (Cassini Imaging Team, JPL, ESA, NASA, SSI)

An even closer view of Hyperion, taken when the Cassini spacecraft was less than 40,000 miles from the moon on September 26, 2005 shows the reason for its unusually dark surface. Many of the craters scattered across the spongy (low-density) moon are filled with dark material perhaps a few meters thick. In natural color, Hyperion is very reddish; in this false-color image, the reddish color is toned down and other hues enhanced, to emphasize small differences in coloration. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod)

Data for Hyperion

Discovered by William Bond (and others) in 1848
Named after one of the sons of Uranus and Gaea
Orbital size 1,481,000 km (approximately 920,000 miles)
Orbital eccentricity 10%
Orbital inclination 0.4 degrees
Orbital period 21.2766 days
Rotation period chaotic
Diameter 360 x 280 x 220 km (225 x 170 x 135 miles) (very irregular)
Mass 1/500,000 of Earth's
Density 0.7 times density of water (Composition probably mostly ice and open space)
Surface gravity 0.3% of Earth's (varies from place to place because so irregular)
Albedo (reflectivity) 30%
Largest non-round object in Solar System