Pioneer 11's 'near miss' at Saturn Options

[Ian R]

According to the American Geophysical Union's publication entitled Pioneer Saturn, here are the details regarding the object(s) discovered orbiting Saturn by Pioneer 11:

1979 S1 - imaged by the photopolarimeter.
1979 S2 - discovered by the charged-particle experiment.
These were later determined to be the same object, also later observed from Earth and called 1980 S3. This was later named Epimetheus.
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1979 S4 - This is a different object, co-orbital to S1/S2, also later seen from Earth and called 1980 S1. This went on to be offically labelled Janus.
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Some of the information on the page that Phil linked to seems to contradict what I've stated above. However, since the paper in the Pioneer Saturn journal was written by Brian Marsden, I'm pretty confident of its accuracy.

Ian.

[scalbers]

The details might be elsewhere in this forum & the internet, but I believe that Janus was also photographed from Earth in 1966 during an edgewise ring presentation. Did it simply take until 1980 to confirm its orbit? I recall the name Janus being used from the 1966 timeframe.

[nprev]

I recall the name Janus being used from the 1966 timeframe.

True. The famous French astronomer Audouin Dollfus observed a moon during 1966 when the rings were edge-on which he named Janus. It's thought that this was in fact the Janus (he might have also seen Epimetheus), but confirmation had to wait until Voyager 1.

[Ian R]

Yes, this is the discovery image from 1966:



http://66.249.91.104/translate_c?hl=en&...an/decjanus.htm

[Phil Stooke]

The problem with 'Janus' as seen in 1966 was that the various observations of this tiny object at the edge of the rings could not be fitted to a good orbit. Now we know that this was caused by having observations of two objects in essentially the same orbit, but that unexpected and unprecedented situation was not considered. As a reliable orbit could not be defined the situation remained unclear until the spacecraft data began to come in. The near-ring area could not be seen after ring-plane crossing because of glare, so there were no observations between 1966 and 1979. In retrospect, Dollfus observed both Janus and Epimetheus.

Phil

[Ian R]

Using Mark Showalter's superlative Saturn Viewer, we can now see that Dollfus did indeed discover Janus with the image I posted earlier:

[Ian R]

We can also confirm that Pioneer 11 was the first spaceprobe to image Epimetheus, with photo F-12E taken 00:29 hrs, 1979-SEP-01. The F-ring and Tethys are also visible:



The Saturn Viewer produces a plot of the same situation albeit as seen from Earth; here, the view is from beneath the ring plane (Pioneer was above it). Adding to the confusion is Dione, which is somewhat in the foreground and out of Pioneer's field-of-view in the image above. But the issue is seemingly settled: 1979 S1 was indeed Epimetheus: