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Full Version: Rev 114 - Jul 3-18, 2009 and onwards - Astrometric observations
Unmanned Spaceflight.com > Outer Solar System > Saturn > Cassini Huygens > Cassini's ongoing mission and raw images
Vultur
The ciclops "looking ahead" article for Rev 114 says that Cassini will take astrometric observations of "several of Saturn's small satellites including Pandora, Atlas, Janus, Methone, and Pallene". It doesn't say how close Cassini will come to these moons - will it get any data about the moons themselves (like brightness or albedo observations), or just orbital data?
volcanopele
I don't recall the distances but they are usually on the order of 1 million km. In other words, these observations are mostly used to better constrain the orbital motion of these satellites, with less emphasis on better understanding their physical nature due to the low resolution of the observations.
Vultur
OK, thanks.

Will there be closer-up observations at a later date, to get a better handle on their composition, shape, surface features etc.? Or are they considered too low priority, and if so why? Or are the orbits just inconvenient for Cassini?
volcanopele
Absolutely, there will be closer passes later in the XM and in the proposed extended-extended mission to many of these rocks which will allow for a study of the physical aspects of these small satellites. These astrometric observations are just a case where we can still get scientific results from observations taken closer to apoapsis.

For example, we will be encountering the Dione trojan, Helene, in early March 2010 at a distance of 1800 km, which will provide an opportunity to get high resolution images of a rock.
Vultur
Ah, thanks.

NickF
A rather nice Janus image has appeared in Cassini's raw image gallery, viz-

http://saturn.jpl.nasa.gov/photos/raw/rawi...?imageID=198273
Floyd
I believe this was part of a whole series that came across around July 28th. This one was retransmitted today? because of dropout? Definitely is a nice image of Janus.
volcanopele
That image had some data dropouts. Images have to have in final form before they are posted in the JPL raw images page, and images with data dropouts usually go through a few passes (to make sure that missing data isn't on the ground but skipped for some reason) before they are in final form.
jasedm
I've just come across this image, which states in the caption that the target is Methone. If it is, then judging from the apparent diameter, it looks like it was taken at around 100,000km, and shows a discernible disc - something I didn't think Cassini had yet achieved for this little 'rock'.

I'm just wondering if anybody can tell me if the resolved disc is actually Methone, or another small moon which just happened to be in the field of view at the time.

The reason I ask is that the largest object in the image is not frame-centred, and thus might not be the intended target.

Would be grateful if anyone is able to definitively pin this down (image was taken 17th September 2010, and the relevant 'looking-ahead' article for that orbit states that Methone was a planned astrometric target on that date)

Cheers!

Jase
machi
This looks as classic astrometric image. Methone is probably somewhere in the middle of the image and is very small. "Large moon" (with discernible disc) is possibly Epimetheus or so.
But we have images of Methone as discernible disc and we have one close flyby in future (~1900 km, 20.5.2012).
jasedm
Agreed Machi- that's what I suspected. The Wikipedia page for Methone is incorrect then wink.gif , as it displays a version of one of the astrometric Methone raws in question, but cropped around the 'interloper' moon (whichever one that may be!) taken on 17th September last year.

"But we have images of Methone as discernible disc"

Could you give me a reference for these? - I don't remember seeing any images of Methone as anything other than a pixel or two across.

Thanks

Jase
machi
Now I'm wrong, it looks, that we haven't good image of Methone. I mistake Methone with Pallene.
Best to date image of Methone is probably N1496907282 from distance 224 063 km (resolution 1.34 km/pix).
So Methone is only 2 - 3 pixels wide.
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