Hyperion
  
Sunday marks the fifth anniversary of the flyby of Saturn’s moon Hyperion by NASA’s Cassini spacecraft. The moon Hyperion (pronounced “hi-PEER-e-on”) is one of the known moons of Saturn--53 as of this writing. Hyperion has been imaged several times from moderate distances by NASA’s Cassini spacecraft, but has been studied closely only once, at a fly-by distance of 500 kilometers, on September 26, 2005. Hyperion is best distinguished by its irregular shape, chaotic rotation and sponge-like appearance. No future missions to this small body are currently planned.
  

The image is an approximately true color mosaic of Hyperion. Composed of several narrow-angle frames and processed to match Hyperion’s natural color, the images were taken during Cassini’s flyby of the moon on September 26, 2005. Image Credit: NASA

   
Mythology
    
In Greek mythology Hyperion (Greek, meaning “The High-One”) was one of the twelve Titans--the generation that preceded the more well-known Greek gods. Hyperion was the brother of Cronus (in Roman mythology, Saturn), and Hyperion was also the lord of light. He was the son of Gaia (the physical incarnation or Earth) and Uranus (Greek, meaning "the Sky").
   
Discovery
  
The moon was discovered in 1848 by astronomers William Cranch Bond, George Phillips Bond and William Lassell. The discovery came shortly after astronomer John Herschel had published possible names for the seven previously-known satellites of Saturn in 1847. Lassell saw the new moon two days before the Bonds and was already in favor of Herschel’s naming scheme, and so suggested the name Hyperion in accordance with that scheme and managed to publish ahead of the Bonds.
  
Spacecraft Visits
  
Cassini was the second spacecraft to study Hyperion at a moderate distance. NASA’s Voyager 2 passed through the Saturn system in 1979 and discerned individual craters on Hyperion as well as an enormous ridge. Cassini’s early images suggested that it had an unusual appearance, but it was not until Cassini’s close flyby that the oddness of this moon was fully revealed. The surface is covered with deep, sharp-edged craters that give Hyperion the appearance of a giant sponge. The rough dimensions are 328 km by 260 km by 214 km. There is dark, reddish material in the bottom of each crater. Spectroscopic analysis finds that this material contains carbon and hydrogen and it appears very similar to material found on other Saturn moons. The accumulated data indicate that about 40 percent of Hyperion is empty space. Also, the material that is there is mostly water ice with a very little amount of rock.
  
Density and Coloration
  
The low density of Hyperion indicates that the moon is composed largely of water ice with only a small amount of rock. Astronomers think that the composition of Hyperion may be similar to a loosely accreted pile of rubble. However, unlike most of Saturn’s moons, Hyperion has a low albedo--the ratio of reflected to incident light. Hyperion’s albedo is 0.2-0.3, indicating that it is covered by at least a thin layer of dark material. Two candidate have been suggested for the source of the dark material. One is darker, nearby moon Phoebe. The other is the closer moon Iapetus. Since the dark material has a reddish tint, and since Iapetus is reddish, this would suggest that Iapetus is the more likely source of the two, if at all.
   

Let’s color-compare. The above image is a combination of three moons. Phoebe is on the left, two-toned Iapetus is in the middle and Hyperion is on the right. The moons are not shown in relative scale with each other. All Images Credit: NASA

   
To learn more about Hyperion, the Saturn system, and the spacecraft visitors to Hyperion, check out these sites:
  
NASA Solar System Exploration - Saturn System
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn

NASA World Book - Saturn
http://www.nasa.gov/worldbook/saturn_worldbook.html

NASA Cassini Mission
http://www.nasa.gov/cassini

NASA Voyager - the Interstellar Mission
http://voyager.jpl.nasa.gov/
  
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