Monday, May 02, 2005

Today's Tethys Non-Targeted Encounter

While we are entering a lull in satellite encounters for Cassini with the beginning of the ring occultation sequence of the Cassini Tour, this doesn't mean that there is nothing fun to do. On this orbit, Cassini will come within 50,000 km of Tethys. Tethys was seen on orbits 0A, which provided an excellent view of the freak crater Penelope, and on orbit 4, which provided great views of a region southeast of Odysseus basin and of the central portion of Ithaca Chasma.

On this orbit, the view near close approach, as simulated by JPL's Solar System Simulator, will show the south polar region of Tethys, a region very poorly seen by Voyager and Cassini. One interesting target in this region only seen as far as I know in Cassini images as I don't seen it in any Voyager maps, is the so-called "South Polar crater". This crater is not at the south polar but is in the far southern latitudes. This crater can be found in several released and raw images (1, 2 (near the left limb), and 3). Also, the southern terminus of Ithaca Chasma could be in view, but I can't be certain.

Closest approach is at 3pm MST today. Given the inability of the JPL Raw images page to have ALL the images returned, I can't guarantee when or if you will see them, though I would suspect sometime tomorrow morning.

UPDATE: 05/02/2005 1:15 pm: Turns out the south polar crater has a name, Melanthius, a large crater at 62S, 210W. I didn't realize this since it never shows up on any Voyager era maps and the map with names I do have doesn't have Melanthius on it, even though this is an approved name. Another crater that should show up, at 60S, 270W is Antinous, again this crater doesn't show up on the maps I have. I want to thank Phil for the info on this. I am mildly disturbed that there are feature names mentioned in the atlas for these moons that aren't on the maps within said atlas ("The Compact Nasa Atlas of the Solar System").


Anonymous Phil Stooke said...

Another word or two on Tethys names... I took them from the USGS's Atlas of Six Saturnian Satellites (A NASA Special publication assembled by USGS in the days when there was money for this sort of thing), which is a Voyager-era publication. The Compact NASA Atlas is a bit too compact to have all the names on it. Actually the Compact Atlas and its earlier version, just "The NASA Atlas of the Solar System", were put together by USGS to be another NASA Special Publication, but the money ran out and they had to scramble for a commercial publisher. This was in the time (later 80s, early 90s) when Washington was seriously considering cancelling not only planetary exploration but even the USGS itself.


5/02/2005 04:29:00 PM  
Blogger Jason said...

The issue with not having all the names in that publication appeared to be more of an issue for Mars where the "nomenclature resolution" is higher than the map resolution used in the book. For Venus, it is downright useless. For outer solar system bodies, it is at least up-to-date as of Voyager. the gazateer in the back of "The Compact Nasa Atlas of the Solar System" actually has Melanthius and Antinous listed, but again, the map does not show these, pointed out as a feature or shown on the map.

5/02/2005 05:07:00 PM  
Anonymous Phil Stooke said...

The map problem goes back to the original source maps from USGS. It's very interesting to compare different editions of the maps. For Saturn's moons, USGS made a set of preliminary maps right after the Voyager encounters - in fact a set after each encounter. After Voyager 2 they were published in the USGS I-series as "preliminary Pictorial Maps" of each moon. These are in the NASA SP "Atlas of six Saturnian Satellites". (Jason - I'm sure Maria Schuchardt has it) (say Hi to her from me!). Later, after improved control was calculated, new maps were produced and published in the USGS I-series. The second set are in the NASA Atlas of the Solar System. But.. look at the differences! For Dione, the best Voyager 2 image, filling an important longitude gap, was never used in the second version. For Tethys, the south polar craters were shown on the older maps but not the newer maps. There are other differences too. The moral of this story... consult *all* the sources! Of course, as a historian of planetary cartography, I've gone over these things more than most people ever do. (PS I'm writing this away from my office, I hope I'm not making a mistake with it...)

5/02/2005 05:50:00 PM  
Anonymous Bjorn Jonsson said...

Is there always a lot of the images returned missing from the JPL raw images page or do they show up later ? I have been a bit surprised there haven't been more images recently. Also any news on when the Cassini images start showing up on the PDS ? At least there nothing should be missing.

Regarding maps, several years ago I was surprised at how bad a pre-Galileo USGS map of Ganymede was. For example, a large fuzzy area near the south pole could be greatly improved using a single wide angle image.

5/03/2005 06:52:00 AM  
Blogger Jason said...

It definitely looks like a lot of images lately are not ending up on the JPL Raw images page. The recent Mimas image sequence had 36 images in total, but only 4 are in the JPL raw images page.

5/03/2005 11:59:00 AM  

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