Hyperion

Saturn VII

   Hyperion ("hi PEER ee en") is the sixteenth of Saturn's known satellites:

        orbit:    1,481,100 km from Saturn
        diameter: 286 km (410 x 260 x 220)
        mass:     1.77e19 kg

   In Greek mythology Hyperion was a Titan, the son of Gaea and Uranus and the father of Helios.

   Discovered by Bond and 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.

   Like most of Saturn's moons, Hyperion's low density indicates that it is composed of water ice with only a small amount of rock.

   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.

   The Voyager images and subsequent ground based photometry show that Hyperion's rotation is chaotic, i.e. it wobbles so much that its orientation in space is completely unpredictable. Hyperion is unique in that it is very irregularly shaped, has a highly eccentric orbit, and is near another large moon (Titan).

   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.

Open Issues

• Will Hyperion's rotation remain chaotic forever? Under what circumstances could it become stable?
• Do any of the other small irregular moons (for which we have little or no data) with eccentric orbits also rotate chaotically?