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Hyperion moon



Orbit: 1,481,000 km from Saturn
Orbital period: 21 days
Gravity: 0.021 m/s2
Diameter: 286 km
Mass:  1.77e19 kg
Hyperion, also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. Hyperion is the largest highly irregular (non-spherical) body in  the solar system.  Proteus is quite a bit larger but is almost spherical.  It seems likely that Hyperion is a fragment of a  larger body that was broken  by a large impact in the distant past. Hyperion’s  density is even lower than most of Saturn’s other moons, not more  than 1/2 that of water.  The indicates that it is  composed of water ice  with only a small amount of rock and considerably porous.  It’s very low density  also seems to allow impacts to form deeper and sharper craters. But unlike most of Saturn’s moons, Hyperion has a low  albedo (.2 – .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.  Casini data from 2007 indicates that this material is rich in organic molecules. Saturn's moon Hyperion
The Voyager images and subsequent ground  based photometry indicate that  Hyperion’s rotation is  chaotic,  i.e. its axis of rotation wobbles so much that its  orientation in space is  completely unpredictable.  There is only one other known body in the solar  system (asteroid 4179 Toutatis) that rotates chaotically but  simulations seem to indicate that other irregular satellites may have done so  in the past. Hyperion is unique in that it is very irregularly shaped,  has a highly 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. Hyperion’s odd rotation probably accounts  for the fact that Hyperion’s  surface  is more or less uniform, in contrast to many of Saturn’s other  moons which  have distinctly different leading and trailing hemispheres.

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