Hyperion (moon)

Hyperion, also known as Saturn VII, is the eighth-largest moon of Saturn. It distinguished by its highly irregular shape, chaotic rotation, low density, and its unusual sponge-like appearance. It was the first non-rounded moon to be discovered.

Discovery
Hyperion was independently discovered by William Cranch Bond and his son George Phillips Bond in the United States, and William Lassell in the United Kingdom in September 1848.

Name
The moon is named after Titan Hyperion, the god of watchfulness and observation, and the elder brother of Cronus (the Greek equivalent of the Roman god Saturn). It is also designated Saturn VII. The adjectival form of the name is Hyperionian.

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. William Lassell, who saw Hyperion two days after William Bond, had already endorsed Herschel's naming scheme and suggested the name Hyperion in accordance with it. He also beat Bond to publication.

Shape
Hyperion is one of the 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. Hyperion has about 15% of the mass of Mimas, the least massive known ellipsoidal body. The largest crater on Hyperion is approximately 121.57 km in diameter and 10.2 km deep. A possible explanation for the irregular shape is that Hyperion is a fragment of a larger body that was broken up by a large impact in the distant past. A proto-Hyperion could have been 350 – in diameter (which ranges from a little below the size of Mimas to a little below the size of Tethys). Over about 1,000 years, ejecta from a presumed Hyperion breakup would have impacted Titan at low speeds, building up volatiles in the atmosphere of Titan.

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.

Hyperion has a porosity of about 0.46. Although Hyperion is the eighth-largest moon of Saturn, it is only the ninth-most massive. Phoebe has a smaller radius, but it is more massive than Hyperion and thus denser.

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 Hyperion's surface. Early images from the Cassini orbiter suggested an unusual appearance, but it was not until Cassini's first targeted flyby of Hyperion on 25 September 2005 that Hyperion's oddness was revealed in full.

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. Scientists attribute Hyperion's unusual, sponge-like appearance to the fact that it has an unusually low density for such a large object. Its low density makes Hyperion quite porous, with a weak surface gravity. These characteristics mean impactors tend to compress the surface, rather than excavating it, and most material that is blown off the surface never returns.

The latest analyses of data obtained by Cassini during its flybys of Hyperion in 2005 and 2006 show that about 40 percent of it 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.

Static charge
Hyperion's surface is electrically charged and was the first discovered to be so other than the Moon's surface.

Orbit and rotation
The Voyager 2 images and subsequent ground-based photometry indicated that Hyperion's rotation is chaotic, that is, its axis of rotation wobbles so much that its orientation in space is unpredictable. Its Lyapunov time is around 30 days. Hyperion, together with Pluto's moons Nix and Hydra, is among only a few moons in the Solar System known to rotate chaotically, although it is expected to be common in binary asteroids. It is also the only regular planetary natural satellite in the Solar System known to not be tidally locked.

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. The fact that its rotation is not locked probably accounts for the relative uniformity of Hyperion's surface, in contrast to many of Saturn's other moons, which have contrasting trailing and leading hemispheres.

Exploration
Hyperion has been imaged several times from moderate distances by the Cassini orbiter. The first close targeted flyby occurred at a distance of 500 km on 26 September 2005. Cassini made another close approach to Hyperion on 25 August 2011 when it passed 25000 km from Hyperion, and third close approach was on 16 September 2011, with closest approach of 58000 km. Cassini's last flyby was on 31 May 2015 at a distance of about 34000 km.