December 18 – Epimetheus

Saturn’s moon Epimetheus, discovered today in 1966 by Richard Walker, is a non-identical twin.  It, and its larger brother, Janus, which had been discovered three days previously by Audouin Dollfus, fly round Saturn in almost identical orbits (their mean orbital radii differ by a mere 50km or so).  Obviously, at the time of discovery, with only  the evidence of Earth-based telescopes to go by, this caused a certain amount of confusion among astronomers trying to work out where the supposed new single moon was.  At the time it was thought, also fairly obviously,  highly unlikely that two moons would share the same orbit without hitting each other.

Epimetheus from the Cassini Orbiter (image: NASA/JPL/SSI)

Epimetheus from the Cassini Orbiter (image: NASA/JPL/SSI)

It is possible that at some time in the past Epimetheus and Janus were part of a single larger body, but if that is the case it is likely to have been a very long time ago.  Both are heavily cratered, and their surface features display softening of their edges by the accumulation of dust, suggesting nothing much has changed there for some time.

Epimetheus was, in Greek mythology, a Titan (nowadays the name of a rather larger Saturnian moon), brother of Prometheus (also a moon of Saturn).  The pair were sons of Iapetus (another Saturnian satellite).  Epimethius was, according to some myths, married to Pandora (and guess what she gave her name to?).

Epithemius was given the job, shortly after the creation of the Earth, of finding a positive trait to give to each of the newly designed animals.  Unfortunately he ran out of good characteristics with one animal remaining, which you’ve probably already guessed was man.  That’s when Prometheus came along and gave  man the gift of fire, after which it all went downhill.

Epimetheus (3rd left) meets Pandora for the first time.

Epimetheus (3rd left) meets Pandora for the first time.

As it happens, though, they do share the orbit, and they don’t hit each other.  One moon does catch the other up every four years, but interactions between their gravities causes the inner moon to move faster (which causes it to move to a higher orbit) and the outer moon to slow down, causing its own orbit to drop a little.  So they actually exchange places, which is why I’m not going to call them the fifth and sixth moons, because they are each neither and both.


Launch, in 1958, of SCORE, the world’s first communications satellite (Signal Communications by Orbiting Relay Equipment).  The launch, from Cape Canaveral, was via an Atlas-B 10B rocket.