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Cassini's doing Iapetus encounter science now

Posted by Emily Lakdawalla

09-09-2007 17:18 CDT

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I've been watching the Cassini raw images website all weekend, waiting for more Iapetus images to come down. The ones that are on the ground now are not actually part of the encounter science observation plan; they're all from fairly large distances, and were mostly captured as "ride-alongs" to observations being made by the CIRS instrument. As such, the pointing wasn't always perfect; apparently CIRS occasionally looks away from its target to black space for calibration, and some of the attempts that Cassini's camera made to photograph Iapetus were actually taken while the spacecraft's optical remote sensing instruments were pointed at black space. But there were plenty enough images to fudge together color composites from all the observations so far. There are five:

Cassini approaches Iapetus, September 2007

NASA / JPL / SSI / color composites by Emily Lakdawalla

Cassini approaches Iapetus, September 2007
Cassini's closest-ever flyby to Iapetus will take place on September 10, 2007. As Cassini approached for the flyby from September 3 to September 8, Iapetus showed a crescent phase (phase angle of about 150 degrees), with only a sliver of the leading hemisphere lit by sunlight. The leading hemisphere is covered by a dark reddish-brown material, but the pole is not, so it appears much brighter white. Beginning in the second image, the "belly band" of mountains is visible on the eastern limb. Most of the color composites are made of images captured through ultraviolet, green, and infrared filters, overlaid on an image captured through a clear filter, though for some of these observations some of the images were missing and so had to be simulated from other available images.
According to my Iapetus flyby timeline, they should just have completed the "far-encounter" downlink, emptying the solid-state recorders for the flyby data. So, if we keep watching the website this afternoon, we should see the first images where Iapetus fills more than one Cassini narrow-angle camera field of view. Those will still be high-phase crescent views; we'll have to wait until after the flyby is over to see nearly-full-phase views from the outbound leg.

The Refresh button on my browser is going to get quite a lot of exercise over the next couple of days! Stay tuned, and I'll post images as I find them...

 
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