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By Emily Lakdawalla

Which other moon is that?

Mar. 6, 2006 | 13:15 EST | 18:15 UTC

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Cassini has been capturing some neat "mutual event" observations, in which it watches one moon pass in front of another in order to refine their orbits. They are neat because Cassini typically captures around a dozen shots of the two moons, which you can then animate into a little movie. Whenever I do this, I always have to find an answer to the question: which other moon is in the picture? Because the Cassini raw images website lists only one "target" by name for each photo. For example, these two pictures, taken over the last couple of days, were targeting Enceladus, but they didn't say what the larger moon was in each of the images, and it can be really hard to tell the difference between Tethys, Dione, and Rhea.

Click to enlarge >

Enceladus, Rhea, and Rings
Cassini captured this view of Enceladus occulting Rhea on March 2, 2006.

Click to enlarge >

Enceladus, Dione, and rings
Cassini captured this view of Enceladus occulting Dione on March 3, 2006. These two views show primarily the night sides of the moons. Cassini is also on the north or unlit side of the rings, so the most prominent stripe is the F ring, which scatters sunlight that passes through it, while the main rings are dark. These are raw images, not yet processed by the Cassini imaging team to remove artifacts (such as the vertical striping and the white spots and streaks from cosmic ray hits). Credit: NASA / JPL / Space Science Institute

If I am are lucky the photo captures some telltale geographic feature that makes it obvious, like Ithaca Chasma on Tethys, but most of the time I need some help identifying the moons.

For that help, I turn to the Saturn Viewer at the Rings Node of the Planetary Data System. This is a simple form that you can fill out to generate a synthetic view of the Saturn system for any time or date on the Cassini mission (or the Voyager missions, for that matter). You can elect to have a wide- or narrow-angle view, or a wide field of view. From the raw images website you can find out what day the image in question was captured. Then it helps to have a little bit of knowledge about what time of day to begin with, but if I don't know anything, I just begin with a wide view and noon time of day, and try making guesses an hour forward or backward in time until I see the moons begin to line up. Then I'll switch to a narrow field of view and increment it by a few minutes until I capture a good facsimilie of the point of view that Cassini got. Here are the two Saturn Viewer simulations for the views above:


Click to enlarge > Simulated Cassini view of Enceladus and Rhea Mutual Event, March 2, 2006 Credit: Saturn Viewer at the PDS Rings Node	Click to enlarge > Simulated Cassini view of Enceladus and Dione Mutual Event, March 3, 2006 Credit: Saturn Viewer at the PDS Rings Node

The agreement between the Saturn Viewer simulations and the Cassini images is pretty amazing; it shows you how precisely the positions of all of these moving bodies are already known. On the other hand, it's not amazing at all, because of course they have to have that kind of precision in order to be able to figure out when and where to point Cassini's cameras to capture these kinds of pictures in the first place!

While I'm talking about mutual events, I have to post this spectacular triple mutual event from January 27. The biggest moon is Rhea. The smaller two are Enceladus (brighter) and Mimas (smaller).

Click to enlarge >

Mutual event of Enceladus, Rhea, and Mimas
Cassini had the rare opportunity to witness a "mutual event" of three of Saturn's large moons at the same time on January 27, 2006. Credit: NASA / JPL / SSI