Hyperion , also known as Saturn VII, is the eighth-largest moon of Saturn. It is distinguished by its highly irregular shape, chaotic rotation, low

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

, 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 William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer.Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...

Hyperion, the god of watchfulness and observation, and the elder brother of

Cronus In ancient Greek religion and Greek mythology, mythology, Cronus, Cronos, or Kronos ( or ; ) was the leader and youngest of the Titans, the children of Gaia (Earth) and Uranus (mythology), Uranus (Sky). He overthrew his father and ruled dur ...

(the Greek equivalent of the

Roman god Roman mythology is the body of myths of ancient Rome as represented in the Latin literature, literature and Roman art, visual arts of the Romans, and is a form of Roman folklore. "Roman mythology" may also refer to the modern study of these ...

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

). It is also designated ''Saturn VII''. The adjectival form of the name is ''Hyperionian''. Hyperion's discovery came shortly after

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor and experimental photographer who invented the blueprint and did botanical work. ...

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 William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer.William Bond, had already endorsed Herschel's naming scheme and suggested the name Hyperion in accordance with it. He also beat Bond to publication.

Physical characteristics

Shape

Hyperion is one of the largest bodies known to be highly irregularly shaped (non-ellipsoidal, i.e. not in

hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...

) in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

. The only larger moon known to be irregular in shape is Neptune's moon

Proteus In Greek mythology, Proteus ( ; ) is an early prophetic sea god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea" (''hálios gérôn''). Some who ascribe a specific domain to Prote ...

. Hyperion has about 15% of the mass of Mimas, the least massive known ellipsoidal body. The largest

crater A crater is a landform consisting of a hole or depression (geology), depression on a planetary surface, usually caused either by an object hitting the surface, or by geological activity on the planet. A crater has classically been described ...

on Hyperion is approximately in diameter and 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 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 Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...

at low speeds, building up

volatiles Volatility or volatile may refer to: Chemistry * Volatility (chemistry), a measuring tendency of a substance or liquid to vaporize easily ** Volatile organic compounds, organic or carbon compounds that can evaporate at normal temperature and pre ...

in the atmosphere of Titan.

Composition

Like most of Saturn's moons, Hyperion's low

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 Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

(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 In Greek mythology, Iapetus (; ; ), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other sources. Iapetus was linked ...

. Hyperion is redder than Phoebe and closely matches the color of the dark material on Iapetus. Hyperion has a

porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...

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 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...

'' 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 ''Cassinis 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

s 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 Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

and

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

and appears very similar to material found on other Saturnian satellites, most notably

. 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

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 The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

's surface.

Orbit and rotation

Animation of Hyperion orbit around Saturn

The ''

'' images and subsequent ground-based

photometry Photometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electr ...

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 asteroid A binary asteroid is a system of two asteroids orbiting their common barycenter. The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993. Since then numerous binary asteroids and several triple a ...

s. 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,

. These factors combine to restrict the set of conditions under which a stable rotation is possible. The 3:4

orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...

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 on 26 September 2005. ''Cassini'' made another close approach to Hyperion on 25 August 2011 when it passed from Hyperion, and third close approach was on 16 September 2011, with closest approach of . ''Cassinis last flyby was on 31 May 2015 at a distance of about .

Notes

References

External links

''Cassini'' mission Hyperion page

a
NASA's Solar System Exploration site

''Cassini'' images of Hyperion

Images of Hyperion at JPL's Planetary Photojournal

Hyperion nomenclature
from th
USGS planetary nomenclature page
{{DEFAULTSORT:Hyperion (Moon) Hyperion (moon), Moons of Saturn Discoveries by William Cranch Bond 18480916 Chaotic maps Moons with a prograde orbit