Hyperion (moon)

**Hyperion**
Discovery
Discovered by	Bond, Bond & Lassell
Discovered on	September 16, 1848
Orbital characteristics
Semimajor axis	1,481,009 km
Eccentricity	0.1230061 ^{^*}
Orbital period	21.27661 d
Inclination	0.43° (to Saturn's equator)^{^*}
Is a satellite of	Saturn
Physical characteristics
Diameter	360×280×225 km ^*
Mass	5.686 kg
Mean density	0.6 g/cm³ ^*
Surface gravity	~0.017 m/s²
Escape velocity	~0.022 km/s
Rotation period	chaotic
Axial tilt	variable
Albedo	0.25
Atmosphere	no atmosphere

Hyperion (hye-peer'-ee-ən, IPA , Greek Ὑπερίων) is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance.

Name

The moon is named after Hyperion, a Titan in Greek mythology. It is also designated Saturn VII.

Hyperion's discovery came shortly after John Herschel had suggested names for the seven previously-known satellites of Saturn in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope (^*). Lassell, who saw Hyperion two days after the Bonds, had already endorsed Herschel's naming scheme and suggested the name Hyperion in accordance with it. He also beat the Bonds to publication.

Physical characteristics

Shape

Hyperion is one of the largest highly irregular (non-spherical) bodies in the solar system (second to Proteus). A possible explanation for this is that Hyperion is a fragment of a larger body that was broken by a large impact in the distant past. The largest crater on Hyperion is approximately 120 km in diameter and 10 km deep.

Composition

Like most of Saturn's moons, 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

Voyager 2 passed through the Saturn system but photographed Hyperion only from a distance. It discerned individual craters and an enormous ridge but was not able to make out the texture of the moon's surface. Early images from the Cassini orbiter suggested an unusual appearance, but it was not until Cassini's sole targeted flyby of Hyperion on September 25, 2005 that the moon's oddness was revealed in full.

Hyperion is entirely saturated with deep, sharp-edged craters that give it the appearance of a giant sponge. Dark material fills the bottom of each crater. Nothing has yet been published to explain these features.

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 known moon in the solar system that rotates chaotically, but simulations suggest that other irregular satellites may have done so in the past. It is unique among the large moons in that it is very irregularly shaped, has a fairly eccentric orbit, and is near another large moon (Titan). These factors combine to restrict the set of conditions under which stable rotation is possible. The 3:4 orbital resonance between Titan and Hyperion may also make chaotic rotation more likely.

The odd rotation probably accounts for the fact that the Hyperionian surface is more or less uniform, in contrast to many of Saturn's other moons which have contrastive leading and trailing hemispheres.

External links

... | Titan | Hyperion | Iapetus | ...

Saturn's moons