September 16 – Discovery of Saturn’s Moon, Hyperion (1848)

Hyperion is an unusual moon of Saturn.  It was discovered on September 16th 1848 by three astronomers: William Lassell of Liverpool (on his own), and Americans William Cranch Bond and his son George Phillips Bond (together).  The Bonds spotted the moon first, but Lassell got published first, so all three are credited.  They are honored together in the form of the only named feature on Hyperion, a prominent ridge called the Bond-Lassell Dorsum.

Hyperion (image credit: NASA/JPL/Space Science Institute)
Hyperion (image credit: NASA/JPL/Space Science Institute)

It’s an odd place, Hyperion.  You can tell from the photograph that it isn’t a regular shape, suggesting it may be the remnant of a larger body destroyed by an impact at some point long ago, and appears to be a bit like a bath sponge (i.e. mostly holes) or the papery wasp’s nest I extracted from our bird box last autumn following a Summer spent creeping up to it with an insect spray and running away as fast as possible after giving them a quick blast in the entrance.  Latest calculations suggest that as much as 40% of the moon’s volume is empty space, and what solids there are may turn out on closer inspection to be mostly water ice, with a sprinkling of frozen methane and carbon dioxide, and only a small percentage of rock.  Hyperion is the second largest moon in the Solar System to have an irregular shape, and is the largest with a “chaotic” (wobbly) rotation.

Hyperion was first visited by Voyager 2 in August 1981, where its unusual shape and rotation were the subject of much head-scratching.  Voyager only got to within about 300,000 miles though, so it was left to the Cassini mission to reveal the full oddness of the moon’s surface when it got to a mere 628 miles (1010 km) in September 2005.  Cassini took today’s  first photograph.

In Greek mythology, Hyperion was a Titan.  There were twelve of them, and they were the children of the old gods Gaia and Uranus.  Hyperion married Theia, his sister (as usual) and had three children with her: Helios (the Sun), Selene (the Moon) and Eos (the Dawn).  So today’s second photograph (aren’t you lucky?) is the Horse of Selene from the east pediment of the Parthenon (now in the British Museum).  I first saw this sculpture when I was seventeen, on a sixth form trip to London.  If I’d paid more attention to my teacher, Mr Perry, at the time I could probably tell you all sorts of facts about it, but alas.

The Horse of Selene.
The Horse of Selene.

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ALSO TODAY  . . . . 

C-type main belt asteroid 105 Artemis was discovered on September 16th 1868 by J C Watson, and named after the Greek goddess of the hunt.

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