APOD: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

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APOD: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by APOD Robot » Mon Feb 01, 2010 4:57 am

Image Shepherd Moon Prometheus from Cassini

Explanation: Another moon of Saturn has been imaged in detail by the Cassini spacecraft. Orbiting Saturn since 2004, the robotic Cassini got its closest look yet at Saturn's small moon Prometheus last week. Visible above in an unprocessed image from 36,000 kilometers away, Prometheus' 100-km long surface was revealed to have an interesting system of bulges, ridges, and craters. These features, together with the moon's oblong shape and high reflectivity, are now being studied to help better understand the history of Prometheus and Saturn's rings. Prometheus is one of the few shepherd satellites known, as its gravity, along with its companion moon Pandora, confines many smaller ice chucks into Saturn's F Ring. Cassini's next major targeted flyby is of the moon Rhea on March 2.

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by BikerMike » Mon Feb 01, 2010 6:51 am

Is it my imagination or does that moon look transparent? :o I'm assuming the speckled look is because it is an unprocessed photo.

On second thought, could it be reflective? :lol:
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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by Javachip » Mon Feb 01, 2010 9:25 am

I wondered the same thing. Are the speckles just artifact, or are they actual particles reflecting light? Was the photo taken through a ring?

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by neufer » Mon Feb 01, 2010 9:54 am

"That's one small step for gyork, one giant leap for gyorkkind"
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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by neufer » Mon Feb 01, 2010 10:20 am

Javachip wrote:
BikerMike wrote:I'm assuming the speckled look is because it is an unprocessed photo.
I wondered the same thing. Are the speckles just artifact, or are they actual particles reflecting light? Was the photo taken through a ring?
  • _______ Images of Art Neuendorffer
    .................................................................
    An unprocessed image __|__ A processed image

    ImageImage
    (speckles just an Artifact)
----------------------------------------
Frequently Unanswered Cassini Questions - Raw Images
http://saturn.jpl.nasa.gov/faq/FAQRawImages/ wrote: ..............................................
What are those streaks I see in some images?

There are high-energy particles that fly though space called cosmic rays. When one of these particles hit the camera's sensor, it causes a bright spot. When one of the particles hit the camera's sensor edge-on, it can leave a trail across the image. Exposures shorter than a second will not have many of these spots or trails. However, long exposures, like those from a minute to 20 minutes will contain many of these trails.
..............................................
What are those dark donut shapes?

Small donut-like dark spots in images are actually out of focus dust specks on the filter wheels, lenses or other parts of the optics of the cameras. Because there is no way to clean the cameras in space, more of these spots may appear as the Cassini mission progresses.
..............................................
What is that horizontal waviness in the picture?

There is a low level source of noise in the camera's signal as it comes out of the sensor and gets converted to numbers. This noise adds and subtracts a small amount to the signal in a cycle. When the data is put into an image, one can see it as bright and dark bands in the image. The amount of noise is very small and is not noticeable in most images. Images that are of black sky or very dark can show this noise. The camera records the baseline of the signal for each line so this noise can be removed in later processing. Both cameras are affected by this noise but the Narrow Angle Camera is worse.
..............................................
Why does the image look bizarre/psychedelic?

As in the previous question, the other way the camera can send back less data (by sending pixels with values from zero to 255 instead of zero to 4095) is to send back only the lower binary digits of the number. This is like having a list of amounts of money and only recording the amount of cents for each one and assigning the brightness in an image to the amount of leftover cents. Pixels with brightness values just under 255, like amounts just under a dollar, will appear almost white, while pixel values just over 255, like amounts just over a dollar with not many cents, will appear dark. The ideal use of this mode is for image scenes that are dark with almost all of the pixel values less than 255. If the scene is simple with gradual increases in brightness, then even if the original values get over 255 and go dark again, the scientists can figure out what the real value was. If the scene is very complicated or the original values are much brighter than 255, the image can have many bright and dark transitions with strange contours. In this case, the image will look very bizarre but not have much scientific value.
..............................................
Why is the image overexposed?

Cassini's cameras have 63 different exposure settings, from 5 milliseconds to 20 minutes. Scientists planning an observation must choose the exposure for each image taken. That can be tough if you're taking a picture of something you've never seen before. Thus, incomplete information on how bright something can be can lead to an underexposed or overexposed image.

Images can be overexposed on purpose too. If the scientist is looking for something dim next to something bright, the bright thing may be overexposed. Finally, Optical Navigation personnel use images to see where Cassini is relative to Saturn and its moons. Often they overexpose images because they need to see where these moons are in relation to the stars in the background sky.
--------------------------------------------
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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by geckzilla » Mon Feb 01, 2010 3:11 pm

Yes, the photo is totally unprocessed. The decision to run an unprocessed image may have been caused by a few discussions we had here recently. Sometimes the image processing results in something confusing to some people. You can see a processed "color" image of this rock in neufer's thread here: http://asterisk.apod.com/vie ... =8&t=18094
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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by ColoradoSky » Mon Feb 01, 2010 4:53 pm

Looks like Starship Prometheus de-cloaking :)

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by NoelC » Mon Feb 01, 2010 5:35 pm

Cool photo. Does bring to mind a question:

Cassini has all these scheduled flybys...

Is NASA actively controlling its flight path using stored fuel, or have they just planned SO far ahead with orbit insertion (and done it with SUCH precision) that the trajectory (including all these scheduled flybys) was completely planned well in advance?

If the latter, I can't think of anything mankind has ever accomplished to that level of precision before.

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by Erebus » Mon Feb 01, 2010 6:15 pm

In any other location I'd guess I was looking at a typical asteroid but it's circular orbit pretty much rules out being captured after planet formation (unless the rings have a stabilizing influence we're not aware of yet).
A piece of one of the other moons, perhaps? Or the remains of something that was eaten by the planet?

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by bystander » Mon Feb 01, 2010 6:29 pm

NoelC wrote:
Cassini has all these scheduled flybys...

Is NASA actively controlling its flight path using stored fuel, or have they just planned SO far ahead with orbit insertion (and done it with SUCH precision) that the trajectory (including all these scheduled flybys) was completely planned well in advance?
Cassini Equinox Mission: Flybys

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by Erebus » Mon Feb 01, 2010 6:47 pm

And about those speckles... Honestly, the first thing I thought of was something from the movie 2010, the sequel to 2001: A Space Odessey - the recording of Dave's last words as he approached the monolith.
"It's full of stars!"

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by NoelC » Mon Feb 01, 2010 8:25 pm

Clearly the shepherd is herding flies, hence the use of the term "flybys" and thus the speckles! :) Just kidding!

Amazing that the expected image artifacts from running a digital camera in space result in so much interest.

So the thing has thrusters, and it's steered gently so that flybys amplify the positioning and cause it to slingshot to new destinations. VERY cool! I'm guessing it has some BIG tanks of fuel for those thrusters, given that it's a multi-year mission. I take it there's no attempt to gather molecules along the way to serve as additional fuel. That would be even more impressive!

I wish we could get an RTG power supply like Cassini has in AA size. :)

-Noel

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by neufer » Tue Feb 02, 2010 2:10 am

http://en.wikipedia.org/wiki/Prometheus wrote:
<<In Greek mythology, Prometheus (Προμηθεύς, "forethought") is a Titan, the son of Iapetus and Themis. He was a champion of human-kind known for his wily intelligence, who stole fire from Zeus and gave it to mortals. Zeus then punished him for his crime by having him bound to a rock while a great eagle ate his LIVER every day only to have it grow back to be eaten again the next day.>>
________Human Liver____________|_____ Prometheus from Cassini
ImageImage

____ Prometheus (Προμηθεύς, "forethought")
http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens#Trajectory wrote:
<<At the end of the Cassini mission, if NASA decides to crash Cassini into one of Saturn's moons, they will likely choose one of the smaller rocky moon. The heat and radioactivity from its RTG power source would cause melting on an icy moon.>>
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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by geckzilla » Tue Feb 02, 2010 2:16 am

Ravioli in one, liver in another... I hope they don't run an Itokawa APOD while you're playing this image association game, Art. ;)
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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by NoelC » Tue Feb 02, 2010 3:02 am

So, Art, you're thinking aliens told the Greeks that this satellite was liver-shaped, giving rise to the legend? Who named this satellite anyway?

Oh, and for the second quote... What? It's important to crash errant space probes into moons so they don't end up littering the solar system, hindering future missions? Seriously, why would someone WANT to crash it into anything? :-O

-Noel

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by neufer » Tue Feb 02, 2010 3:04 am

geckzilla wrote:Ravioli in one, liver in another...
I hope they don't run an Itokawa APOD while you're playing this image association game, Art. ;)
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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by neufer » Tue Feb 09, 2010 5:54 pm

http://www.areavoices.com/astrobob/?archive=2010-02 wrote:
Image
You can clearly see the waves that the gravity of Prometheus creates within the F ring in this photo.
The arcs at left is a portion of the bright inner rings of the planet. Credit: NASA/JPL/Space Science Institute
Click to play embedded YouTube video.
YOU TUBE:
Image
Art Neuendorffer

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Re: Shepherd Moon Prometheus from Cassini (2010 Feb 01)

Post by BikerMike » Wed Mar 17, 2010 7:59 am

NoelC wrote:Cool photo. Does bring to mind a question:

Cassini has all these scheduled flybys...

Is NASA actively controlling its flight path using stored fuel, or have they just planned SO far ahead with orbit insertion (and done it with SUCH precision) that the trajectory (including all these scheduled flybys) was completely planned well in advance?

If the latter, I can't think of anything mankind has ever accomplished to that level of precision before.

-Noel
See: Cassini Gas Gage
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