Sunday 18 December 2016

Janus & Epimetheus

Jaysus Janus! I work hard to understand, so that you don't have to. Today is the 50th anniversary of the 18th Dec 1966 discovery of Epimetheus by Richard Walker. Epimetheus is a moon of Saturn whizzing around the giant planet with an orbital radius of about 150,000km. That's from the centre! Saturn itself has a radius a tad under 60,000km, so Epimetheus is a bare 90Mm above the cloud tops. The moon is tiny compared to Saturn but still a substantive chunk of territory 130 x 155 x 105 km so bigger than Corsica and smaller than Sardinia. Walker reported his observation and was able to calculate the orbital radius from the photograph. Trouble is that 3 days earlier on 15th Dec 1966, Audouin Dolphus, working from the Paris Observatory Vive La France! had snapped another picture of a moving object that was 90,000 km up from the surface of Saturn. He called his chunk Janus and because of the astronomical rules of priority [mighty slag-fest prev] the name Janus stuck to both slabs. Janus is bigger: 203 × 185 × 150 km or about the size of Corsica and Sardinia combined. You should know that Dolphus won an academic argument with Gerald "Kuiper Belt" Kuiper [bloboprev] about the geological composition of the surface of Mars. Dolphus was vindicated by the Viking Landers in his iron oxide theory for why it is the Red Planet.

This-all caused a certain amount of consternation among those who care about the moons of other planets because the two sets of observations were very difficult to reconcile into an orbit that could refer to a real object in physical space. As physics-trained people, it must have made them scratch their heads and think of an electron bi-locating - being in two separate places at more or less the same time. Further observations and a creative Aha! moment by Stephen Larson and John Fountain in October 1978 fell the data into place as two separate bodies occupying the same orbit. Actually these boffins can calculate to a precision that one orbit is about 50km wider than the other, that's a level of resolution rather less than the diameter of either body. That's the same orbit in my book. The existence of two slabs of rock was confirmed two years later by pictures when Voyager 1 got up close and personal in its fly-by past Saturn. The picture [R] was taken much later by Cassini: Janus [R.R] is 40,000km further from the camera than Epimetheus [R.L].

How does it happen? How do two chunks of rock dance round the ringed planet without ever (so far) bumping into each other? "So far" because this pas de deux has clearly been going on for much much longer than 50 years. If you try to investigate the how and why [let's start at Wikipedia, folks] you are presented with a highly authoritative but deeply confusing diagram that implies the two moons oscillate back and forth in their orbit. This is emphasised by looking at moving-image evidence on youtube which seems to show Janus and Epimetheus coming to a screeching halt every 4 years and going into reverse. It certainly does not help that these orbital relationships are called horseshoe orbits. This is bloody nonsense as explained by the patient people on Quora. The inner moon travels faster than the outer one and eventually catches up with its partner. As they get close the force of gravity heaves them together so that they swap orbits, the inny becoming outy and outy going all inny. The inner moon travels faster and several years later catches up with its tag-partner and they swap places again.

Horseshoe orbits are shown much more clearly by a pair of MOVies in the Wikipedia entry for Cruinthe, a small body in orbital resonance with Earth which is billed by some people from the Arts Block as being our second moon. As one of the QI panellists said at the time "Who comes up with this shit?".

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