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I was using Celestia to see what will be visible during Cassini's upcoming flyby of Titan and Enceladus, and I noticed an interesting effect: there will be a moment when Enceladus will pass directly in front of Tethys, as seen from Cassini.

See the following CEL (I'm using Celestia 1.4pre6 and Jestr's Cassini xyz):

cel://Follow/Sol:Cassini/2005-02-16T18: ... 5&lm=49152

Does anyone know if we will really get to see this, or are the orbits not accurate enough to be certain?

Good observing! The Solar SYstem Simulator (at http://space.jpl.nasa.gov/) has a similar view for precisely the time you describe. If the Solar System Simulator models this accurately, I wonder if Cassini will be programmed to view the event...


Here is a link to the visualization for 18:45:

http://space.jpl.nasa.gov/cgi-bin/wspac ... -1&bfov=30

The event is fleeting - here's a link for 18:44 (also showing identifying labels for both moons):

http://space.jpl.nasa.gov/cgi-bin/wspac ... -1&bfov=30

It's encouraging that both simulators show this. The occulation is an aesthetically pleasing effect but I wonder if there would be any scientific value in photographing it?

I thought this image might be something to do with the occultation, but then noticed the date was different. I think the other moon seen there is Tethys though:

cel://PhaseLock/Sol:Cassini/Sol:Saturn: ... 5&lm=49156

There's a long article on the Planetary Society website about the Enceladus encounter.

http://www.planetary.org/news/2005/cass ... _0211.html

It seems that after the Titan encounter the exact orbit of Cassini will be uncertain owing to the drag of Titan's atmosphere. Because of this, the Enceladus encounter is untargeted; they're just going to see what they manage to photograph. The Cassini team doesn't seem confident of being able to accurately point the camera at Enceladus even during the closest approach, so I guess it'll be a fluke if they are able to capture the occultation of Tethys. Hey ho.

The Singing Badger wrote:The occulation is an aesthetically pleasing effect but I wonder if there would be any scientific value in photographing it?

With this periapsis, the Occultation Sequence of orbits begins. Since Cassini is now travelling close to the ring plane, occultations involving the moons and the rings (and of course Saturn itself) are more likely to occur than before. I suppose this particular occultation between Enceladus and Tethys is of limited interest (perhaps it can be used to verify the accuracy of known orbital data and of Cassini's own trajectory), but having the rings obscure various bodies with known spectral properties (including the Sun) could possibly lead to new insights in the composition and structure of the rings.

The Singing Badger wrote:The Cassini team doesn't seem confident of being able to accurately point the camera at Enceladus even during the closest approach, so I guess it'll be a fluke if they are able to capture the occultation of Tethys.

The pointing problem during closest approach is due to Cassini not being able to turn at the precise moment with sufficient accuracy. However, the Tethys occultation happens when Cassini is still more than 150,000 km away from Enceladus, almost nine hours before the encounter, and there isn't much angular movement involved. While this is not a targeted flyby, Cassini's trajectory is most likely still known well enough to avoid a collision with Enceladus, in which case Cassini can probably aim its narrow-angle camera towards the moon as well.

If anything prevents Cassini from capturing the occultation, I think it's more likely to be a more important observation with a different instrument requiring Cassini's attention, than uncertainties in the probe's trajectory.

Images from the Enceladus encounter are being added to the raw image archive now, but appearantly so far none of them taken between 02/16/2005 16:41 UTC (N00028176, from far away) and 02/17/2005 02:16 UTC (N00028177, close-up detail), both using the narrow angle camera. I find it very unlikely that no observations at all were made by Cassini during that time, so hopefully we will be seeing more images added later. Simply enter those two timestamps in the search form to find out yourself.

Thanks for the advice - by the way the closeup shots of Enceladus are incredible!

I suppose that these occultations are going to become more common as Cassini moves down into the ring plane. From September through mid-2006 it will be orbiting right in the plane of the rings and of most of the icy moons, so there will be no pretty ring pictures for a while, but lots of moon encounters, and probably many occultation photo-ops too.

Images of a Dione-Tethys occultation are on another thread:

http://www.celestiaproject.net/forum/viewtopic.php?t=6842