Hyperion , also known as Saturn VII, is a moon of Saturn discovered by

William Cranch Bond William Cranch Bond (September 9, 1789 – January 29, 1859) was an American astronomer, and the first director of Harvard College Observatory. Upbringing William Cranch Bond was born in Falmouth, Maine (near Portland) on September 9, 1789. ...

, his son George Phillips Bond and

William Lassell William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer.round Round or rounds may refer to: Mathematics and science * The contour of a closed curve or surface with no sharp corners, such as an ellipse, circle, rounded rectangle, cant, or sphere * Rounding, the shortening of a number to reduce the numbe ...

moon to be discovered.

Name

The moon is named after Hyperion, the Titan god of watchfulness and observation – the elder brother of

Cronus In Ancient Greek religion and mythology, Cronus, Cronos, or Kronos ( or , from el, Κρόνος, ''Krónos'') was the leader and youngest of the first generation of Titans, the divine descendants of the primordial Gaia (Mother Earth) an ...

, the Greek equivalent of the Roman god

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 nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

. 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, experimental photographer who invented the blueprint and did botanical wor ...

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 (hydrostatic balance, 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. In the planeta ...

) in the

Solar System The Solar System Capitalization 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 "Solar ...

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

Proteus In Greek mythology, Proteus (; Ancient Greek: Πρωτεύς, ''Prōteus'') 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)'' ...

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

crater Crater may refer to: Landforms *Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet *Explosion crater, a hole formed in the ground produced by an explosion near or below the surfac ...

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 at low speeds, building up

volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances. On planet Earth, the term ...

in the atmosphere of Titan. Hyperion in natural colours

Composition

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

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

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 measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...

(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 (; ; grc, Ἰαπετός, Iapetós), 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 ...

. 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, although 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, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, '' Voyager 1'', on ...

'' 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 and

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxi ...

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.

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 electrom ...

indicated that Hyperion's rotation is

chaotic Chaotic was originally a Danish trading card game. It expanded to an online game in America which then became a television program based on the game. The program was able to be seen on 4Kids TV (Fox affiliates, nationwide), Jetix, The CW4Kids, ...

, that is, its axis of rotation wobbles so much that its orientation in space is unpredictable. Its

Lyapunov time In mathematics, the Lyapunov time is the characteristic timescale on which a dynamical system is chaotic. It is named after the Russian mathematician Aleksandr Lyapunov. It is defined as the inverse of a system's largest Lyapunov exponent. Use ...

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 not to be

tidally locked Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a 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 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 relations ...

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