Hyperion is the eighth-largest moon of Saturn. It is distinguished by its highly irregular shape, chaotic rotation, low density, and unusual sponge-like appearance. It was the first non-rounded moon to be discovered.
Discovery and naming
Hyperion was independently discovered by William Cranch Bond and his son George Phillips Bond in the United States, and William Lassell in the United Kingdom in September 1848.
The moon is named after the Titan Hyperion, the god of watchfulness and observation, and the elder brother of Cronus (the Greek equivalent of the Roman god Saturn). It is also designated Saturn VII. The adjectival form of the name is Hyperionian.
Hyperion's discovery came shortly after John Herschel 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. He also beat Bond to publication.
Physical characteristics
thumb|400px|Hyperion compared to [[Ceres (dwarf planet)|Ceres and the Moon 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 in the atmosphere of Titan.
Static charge
Hyperion's surface is electrically charged and was the first discovered to be so other than the Moon's surface.
Orbit and rotation
thumb|right|Animation of Hyperion's orbit.<br />
The Voyager 2 images and subsequent ground-based photometry 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 asteroids. 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 because of its highly irregular shape, fairly eccentric orbit, and proximity to the 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 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
right|thumb|350px|Image of Hyperion processed to bring out details. It was taken by the [[Cassini–Huygens|Cassini space probe.]]
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. Cassinis last flyby was on 31 May 2015 at a distance of about .
See also
- Chaotic rotation
- Hyperion in fiction
- Moons of Saturn
- List of geological features on Hyperion
Notes
References
External links
- Cassini mission Hyperion page
- [https://web.archive.org/web/20190713072219/https://solarsystem.nasa.gov/moons/saturn-moons/hyperion/in-depth/] at NASA's Solar System Exploration site
- The Planetary Society: Hyperion
- NASA: Saturn's Hyperion, A Moon With Odd Craters
- Cassini images of Hyperion
- Images of Hyperion at JPL's Planetary Photojournal
- Hyperion nomenclature from the USGS planetary nomenclature page