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Posted by: jasedm Nov 22 2007, 12:02 PM

Close encounter of the third (of December) kind approaching fast. There seems to have been little anticipation so far for this flyby of Epimetheus (compared to say Hyperion or Phoebe), although at ~9000km, it will be the closest 'rock' flyby in the whole of the prime mission.

I understand that approach is from the nightside, phase angle is very high at C/A (139 degrees) and spacecraft speed is ~22km/second - these all presumably hamper the usual suite of investigations. Having said that, does anybody here have any detail regarding ISS observations an hour or so either side of C/A?

Jase

Posted by: volcanopele Nov 22 2007, 06:45 PM

Actually, there was an accidental flyby of Polydeuces in February 2005 that was closer.

More info on the Epimetheus encounter will be available next week.

Posted by: volcanopele Nov 27 2007, 11:03 PM

The Looking Ahead article for Rev53 has just been posted:

http://ciclops.org/view.php?id=4020

There are no Epimetheus observations (at least by ISS) until shortly after C/A when Epimetheus is 36,000 km from that satellite. ISS will then commence imaging Epimetheus using nearly its full compliment of filters until Cassini is 43,000 km from the moon.

Posted by: jasedm Nov 28 2007, 09:19 PM

Thanks VP. I suppose the risk of sun damage to the optics prevented a NAC snap of the tiny sliver of crescent at C/A - would have been great to see that lumpy terminator at 70m/pixel though....
It will be very interesting to compare the terrains of Janus and Epimetheus at ranges of less than 40,000km (next year for Janus)- theories of shared histories abound, and they have very similar albedos and of course almost identical orbits.
Cassini images thus far make their terrains appear quite distinct due to the different lighting conditions when images were shuttered, but unfortunately there don't appear to be any opportunities for mid-range observations of Epimetheus at sub-50 degree phase angles (even allowing for the XM).
This is a shame, beause I can't rid myself of the feeling that Janus (and thus maybe Epimetheus) has relatively dark lag deposits at the bottom of some craters.....

Posted by: Ian R Dec 4 2007, 04:24 PM

A very rough and ready colour composite:

Posted by: Ian R Dec 4 2007, 05:04 PM

A stacked and cleaned-up version:

Posted by: Phil Stooke Dec 4 2007, 05:10 PM

And my version of the above, processed to show detail near the terminator. Looks like this is the south polar region in the middle, and only outside the broad southern facet do we see the territory previously imaged by Voyager. Earlier Cassini close-ups were of the area past this limb at upper right.

Phil

Posted by: ngunn Dec 4 2007, 05:13 PM

Very nice. Are we seeing 3 distinct shades of grey in that prominent crater, or is one of those shade boundaries just a change in slope? Do we have suitable data for an anaglyph?

Posted by: ugordan Dec 4 2007, 05:43 PM

R+B color and greatly enhanced:

IR1+GRN+UV3 greatly saturated:


All in all, another one of those really gray rocks.

Posted by: tedstryk Dec 4 2007, 06:02 PM

The grooves of Phobos and the dust of Deimos. I could have mistaken it for either Martian moon (and yes, I know is much larger, but there is still the resemblance). Wow!

Posted by: MarcF Dec 4 2007, 08:33 PM

Really Nice !!
Is it an illusion or is this hemisphere covered by a giant impact scare like the one found on Hyperion (upper right 2/3) ?
If it is really a crater, it confirms that like Hyperion, or asteroid Mathilde, Epimetheus has a rubble-pile structure.
Marc.

Posted by: volcanopele Dec 4 2007, 08:52 PM

Very cool stuff. The depression with a central peak (crater or not) was best seen previously in May 2005: http://photojournal.jpl.nasa.gov/catalog/PIA07531 . Epimetheus' geology certainly shares some similarities to Hyperion. However, unlike moons like Telesto, Epimetheus seems to be only partially dust covered. Might be interesting to see what VIMS can pull out of their data.

Posted by: Phil Stooke Dec 4 2007, 08:52 PM

No, it's not an illusion, this is a big crater.

Here is a mosaic of Voyager images of Epimetheus, projected onto a sphere and viewed from the south pole (azimuthal equidistant projection):

It shows the crater as a shadowed area (it was over the limb in Voyager views, not really full of shadow). Rotate clockwise 120 degrees to fit the new image.

Phil

Posted by: MarcF Dec 4 2007, 09:43 PM

OK, thanks a lot Jason and Phil. Following your Voyager mosaic Phil, the big crater on left of the new Cassini image corresponds to Pollux.
I'm impatient to see a new map of Epimetheus. With the three "relative-close" flybys performed by Cassini, the coverage of the moon's surface must not be too bad.
Marc.

Posted by: Exploitcorporations Dec 4 2007, 09:52 PM

Here's another handy misshapen-lump-'o-ice key, out of scale and with north crudely to the right. Is there an object in this system (excluding Enceladus and Titan) without a grotesquely disproportionate impact structure?

Posted by: MarcF Dec 4 2007, 10:07 PM

Great Exploitcorporations,

Finally, I'm not quite sure about my identification of Pollux in the last Cassini pictures.
I think also that you made a mistake with the blue dot in the Voyager image. I would have put it in the smaller neighbouring crater which is half in the shadow (lower right), called Hilairea.
Marc.

Posted by: Phil Stooke Dec 4 2007, 10:11 PM

MarcF - no, it's more like this:

Phil

Posted by: volcanopele Dec 4 2007, 10:11 PM

Not bad, though I would make one correction (I think...):

Posted by: Exploitcorporations Dec 4 2007, 10:15 PM

Thanks! Those two craters were confusing.

Posted by: MarcF Dec 4 2007, 10:20 PM

Yes Phil, I realized (a little bit too late) that I was wrong. Nice composite. Thanks again.
And I completely agree with Jason's corrections (it's what I tryed to explain in my previous post).
Marc.

Posted by: Stu Dec 4 2007, 11:47 PM

Hmmm... looks a bit like Hyperion if you do a bit of stack'n and tweak'n...

Posted by: alan Dec 5 2007, 12:52 AM

What is the dark arc in the 11 Nov 1980 image in Exploitcorporations's post?

Posted by: nprev Dec 5 2007, 01:31 AM

IIRC, that's a ring shadow. Saturn was still pretty close to edge-on to the Sun in 1980 (the plane crossing was in 1979).

Hey, EC! Good to see ya!

Posted by: belleraphon1 Dec 5 2007, 02:00 AM

Yes, the arc is a ring shadow...

Agree with nprev.... great to see you back EC!

Am I imagining things, or does the edge terrain on the left of the image seem more muted, as if ice blasted by ring particles perhaps?

Craig

Posted by: CAP-Team Dec 5 2007, 09:00 AM

Looks more like comet Tempel-1 to me

Posted by: n1ckdrake Dec 5 2007, 11:56 AM

Here's my attempt at cleaning it up.

Posted by: volcanopele Jan 11 2008, 04:37 PM

And now for the image that has been my desktop background for the last month:

http://photojournal.jpl.nasa.gov/catalog/PIA09813

Posted by: ugordan Jan 11 2008, 04:42 PM

Ahh, it's always nice to see color views produced by you guys where color is available. The lack of a brownish tint in stretched color reminds me of Telesto. Can't remember if Helene fits into this category as well?

Posted by: Phil Stooke Jan 11 2008, 06:13 PM

That is beautiful!

Phil

Posted by: stevesliva Jan 11 2008, 09:25 PM

I thought you liked spheres, Phil?

Posted by: Phil Stooke Jan 12 2008, 04:50 AM

Admittedly a featureless sphere would be more to me present taste, but this is a nice second best.

Phil

Posted by: volcanopele Jan 12 2008, 05:55 AM

Featureless? No, No. The perfect world would have the north pole, south pole, and the equator clearly marked.

Posted by: MarcF Jan 12 2008, 10:20 AM

Could this mean that Iapetus is close to a perfect world ???

Marc.