Hyperion is second largest bodies known to be highly irregularly shaped (non-ellipsoidal; i.e., not in hydrostatic equilibrium) in the solar system. The only larger moon known to be irregular in shape is Neptune's moon Proteus.
Discovery
William Lassel discovered Hyperion in 1848. The same year William Cranch Bond, with his son George Phillips Bond, independently discovered the moon, and all three men are jointly credited with the discovery.
Hyperion was the first non-round moon to be discovered.
Naming
Hyperion the moon is named after Hyperion, the Titan god of watchfulness and observation – the elder brother of Cronus, the Greek equivalent of Saturn.
In Greek Mythology, Hyperion was one of the twelve Titans of Ancient Greece, the sons and daughters of Gaia (the physical incarnation of Earth) and Ouranos (literally meaning 'the Sky'). He was the brother of Cronus. He was also the lord of light, and the Titan of the east.
Stats
Diameter (mean): 270 km
Semi-major axis: 1,481,010 km
Orbital Period: 21.28 days
Formation
Hyperion is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance.
The largest crater on Hyperion is approximately 121.57 km in diameter and 10.2 km deep. A possible explanation for Hyperion's irregular shape is that it is a fragment of a larger body that was broken by a large impact in the distant past.
Composition
Hyperion's low density indicates that it is composed largely of water ice with only a small amount of rock. It is thought that Hyperion may be similar to a loosely accreted pile of rubble in its physical composition.
However, unlike most of Saturn's moons, Hyperion has a low albedo (0.2–0.3), indicating that it is covered by at least a thin layer of dark material. This may be material from Phoebe (which is much darker) that got past Iapetus. Hyperion is redder than Phoebe and closely matches the color of the dark material on Iapetus.
Surface features
Hyperion's surface is covered with deep, sharp-edged craters that give it the appearance of a giant sponge. Dark material fills the bottom of each crater. The reddish substance contains long chains of carbon and hydrogen and appears very similar to material found on other Saturnian satellites, most notably Iapetus.
The latest analyses of data obtained by NASA's Cassini spacecraft during its flybys of Hyperion in 2005 and 2006 show that about 40 percent of the moon is empty space. It was suggested in July 2007 that this porosity allows craters to remain nearly unchanged over the eons.
The new analyses also confirmed that Hyperion is composed mostly of water ice with very little rock.
Rotation
The Voyager 2 images and subsequent ground based photometry indicate that Hyperion's rotation is chaotic, that is, its axis of rotation wobbles so much that its orientation in space is unpredictable.
Hyperion is the only moon in the Solar System known to rotate chaotically. It is also the only regular natural satellite in the Solar System not to be tidally locked.
The fact that Hyperion's rotation is not locked probably accounts for the relative uniformity of it's surface, in contrast to many of Saturn's other moons which have contrasting trailing and leading hemispheres.
4:3 resonance with Titan
Hyperion is unique among the large moons in that it is very irregularly shaped, has a fairly eccentric orbit, and is near a much larger moon, Titan.
These factors combine to restrict the set of conditions under which a stable rotation is possible. The 3:4 orbital resonance between Titan and Hyperion may also make a chaotic rotation more likely.
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