Starship Asterisk*

by **neufer** » Tue Nov 06, 2012 2:20 pm

Chris Peterson wrote:
...we need to consider inclination, as well, which may be different.

I have no inclination to do so.

by **Chris Peterson** » Tue Nov 06, 2012 2:13 pm

neufer wrote:Indeed, most mechanisms that push a body into a different orbit do so by adding or removing angular momentum.

Nevertheless:

Every elliptical orbit with an apoapsis intersecting Dione's circular orbit will have less angular velocity than Dione at collision.

But there are orbits with the same eccentricity. And we need to consider inclination, as well, which may be different.

by **StarCuriousAero** » Mon Nov 05, 2012 10:54 pm

For anyone interested in a more visual/learning/fun aid to understanding orbits a bit better (and velocities at apoapses/periapses), the smart-phone game/app called Osmos is quite a lot of fun. I found myself visualizing the game while reading through these comments at least.

by **neufer** » Mon Nov 05, 2012 10:29 pm

Chris Peterson wrote:
neufer wrote:
Were the inner moons somehow pushed out to the orbit of Dione
they would be expected to have the less angular velocity than Dione.
No, they would have the same angular velocity if they were at the same orbital radius.

Any mechanism that pushes a body into a different orbit does so by adding or removing angular momentum.

Indeed, most mechanisms that push a body into a different orbit do so by adding or removing angular momentum.

Nevertheless:

Every elliptical orbit with an apoapsis intersecting Dione's circular orbit will have less angular velocity than Dione at collision.

And:

Every elliptical orbit with a periapsis less than half of Dione's circular orbit radius will have less angular velocity than Dione at collision.

Hence, in most situations, inner moons (or inner dust) forced to collide with Dione will do so on Dione's leading edge.

[Contrariwise, in most situations, outer moons (or outer dust) forced to collide with Dione will do so on Dione's trailing edge.]

by **dougettinger** » Mon Nov 05, 2012 9:31 pm

neufer wrote:
rudminjd@jmu.edu wrote:It makes perfect sense to me that Dione should be most heavily cratered on its trailing side, because that is the direction from which the vast majority of material comes, including the vast majority of fast impacts. Smaller orbits have a faster angular velocity than larger orbits. They approach the trailing side of an object in a higher orbit.
Smaller orbits have less angular momentum than larger orbits.

Were the inner moons somehow pushed out to the orbit of Dione
they would be expected to have the less angular velocity than Dione.

Since Dione's escape velocity (0.5 km/s) is negligible as compared to its orbital velocity (10 km/s)
Dione would be expected to be most heavily cratered on its leading side on purely kinetic arguments.

How can Dione's escape velocity be so much smaller than its orbital velocity? Escape velocities are larger than orbital velocities.

What is wrong with the following logic? Asteroids being captured by Saturn's gravity field will possibly, while falling, orbit at higher velocities than the orbiting satellites. Hence, asterioids captured by Saturn should more often run into the trailing side of Dione and other satellites. I also like in combination the idea of dust from previous collisions in the same orbital region being swept by the leading side of Dione.

Riding high on satellites,
Doug

by **Chris Peterson** » Mon Nov 05, 2012 6:49 pm

neufer wrote:Were the inner moons somehow pushed out to the orbit of Dione
they would be expected to have the less angular velocity than Dione.

No, they would have the same angular velocity if they were at the same orbital radius. Any mechanism that pushes a body into a different orbit does so by adding or removing angular momentum.

Both the leading and trailing faces will see impacts at the same velocity from debris in a prograde orbit similar to Dione's. The leading edge sees impacts from retrograde debris, and there is a clear asymmetry when considering either debris in solar orbit, or in highly eccentric orbits around Saturn.

by **rudminjd@jmu.edu** » Mon Nov 05, 2012 6:35 pm

Art Neuendorffer said,
"Smaller orbits have less angular momentum than larger orbits. Were the inner moons somehow pushed out to the orbit of Dione they would be expected to have the less angular velocity than Dione."

True. My reasoning was unclear. I agree that Dione will overtake some particles scattered upward, which have just barely enough energy to reach Dione's orbit. But, when two particles scatter from lower in the disk, those that reach Dione's orbit are very likely to have a higher angular momentum than Dione's. Especially the high impact particles will have higher angular momentum.

Joe

by **Guest** » Mon Nov 05, 2012 5:34 pm

BDanielMayfield wrote: Dione is orbiting Saturn with an average velocity of 10.005787 km/s, and that must be added to the average velocity of Saturn’s orbit around the Sun which is 10.04991 km/s. Yes, impactors can fall in from any direction, but the ones that make the biggest craters per impactor size will be the ones that hit the leading side.

Bruce

Thanks for the answer. It is obvious now why the leading face matters. In my ignorance I supposed that the speed of asteroids is hundreds or thousands times bigger than the speed of Dione. So actually a rock traveling with 20 km / sec will hit the leading face with 40 km/sec, while coming from behind speeds are almost the same so the will be barely kind of a free fall.

Now illuminated as I am, I will have a better sleep.

by **FloridaMike** » Mon Nov 05, 2012 5:25 pm

rudminjd@jmu.edu wrote:...Dione should be most heavily cratered on its trailing side, ...

neufer wrote: ...Dione would be expected to be most heavily cratered on its leading side ...

Oh, Snap. Only an Epic Rap Battle of Astonomy can resolve this dispute.....

by **gorade** » Mon Nov 05, 2012 5:16 pm

earendil wrote:While I don't have a problem with a large impactor, could it be possible that the smoothness of the leading side is due to ablation from more smaller impactors? It seems to me that with the ring formations around Saturn, there should be a higher amount of small debris and enough small impacts should smooth the features and add dust layers that further disguise the surface. I don't know much about the atmospheric makeup of Dione, is there enough there to stop small debris?

Yes, I agree. That seems intuitively plausible, doesn't it?

by **neufer** » Mon Nov 05, 2012 5:15 pm

earendil wrote:
While I don't have a problem with a large impactor, could it be possible that the smoothness of the leading side is due to ablation from more smaller impactors? It seems to me that with the ring formations around Saturn, there should be a higher amount of small debris and enough small impacts should smooth the features and add dust layers that further disguise the surface.

Excellent point

Dione would constantly receive rain from the E ring: http://media.web.britannica.com/eb-medi ... 7682B4.jpg

[... possibly focused by the moderately sized leading (L₄ trojan) moon Helene.]

by **earendil** » Mon Nov 05, 2012 5:02 pm

While I don't have a problem with a large impactor, could it be possible that the smoothness of the leading side is due to ablation from more smaller impactors? It seems to me that with the ring formations around Saturn, there should be a higher amount of small debris and enough small impacts should smooth the features and add dust layers that further disguise the surface. I don't know much about the atmospheric makeup of Dione, is there enough there to stop small debris?

by **neufer** » Mon Nov 05, 2012 4:48 pm

rudminjd@jmu.edu wrote:It makes perfect sense to me that Dione should be most heavily cratered on its trailing side, because that is the direction from which the vast majority of material comes, including the vast majority of fast impacts. Smaller orbits have a faster angular velocity than larger orbits. They approach the trailing side of an object in a higher orbit.

Smaller orbits have less angular momentum than larger orbits.

Were the inner moons somehow pushed out to the orbit of Dione
they would be expected to have the less angular velocity than Dione.

Since Dione's escape velocity (0.5 km/s) is negligible as compared to its orbital velocity (10 km/s)
Dione would be expected to be most heavily cratered on its leading side on purely kinetic arguments.

by **rudminjd@jmu.edu** » Mon Nov 05, 2012 4:20 pm

It makes perfect sense to me that Dione should be most heavily cratered on its trailing side, because that is the direction from which the vast majority of material comes, including the vast majority of fast impacts. Smaller orbits have a faster angular velocity than larger orbits. They approach the trailing side of an object in a higher orbit. As one can see from the diagram at the link below, Dione resides very high in Saturn's system of moons and rings. So, the vast majority of material hitting Dione comes from smaller orbits. The faster impacts probably come from objects ejected by lower residing moons, or by errant comets.

http://media.web.britannica.com/eb-medi ... 7682B4.jpg

Joe Rudmin

by **kyokugaisha** » Mon Nov 05, 2012 4:18 pm

Hi guys,

I used images downloaded from NASA's Planetary Data System http://pds.nasa.gov/ where you will find full, calibrated img files with complete metadata from pretty much all the planetary missions since voyager.

You can use their search tool here - http://pds-rings.seti.org/search/ and there's a great starter guide if you fancied doing some yourself here - http://www.planetary.org/explore/space- ... rials.html.

The raw image site is great and more up to date but they are not the highest quality and require more processing to get a decent final result.

- Marc Canale

http://www.flickr.com/photos/76658917@N04

bystander wrote:
RedFishBlueFish wrote:How did the 'dedicated amateur' gain access to NASA's images?
http://saturn.jpl.nasa.gov/photos/raw/

by **bystander** » Mon Nov 05, 2012 2:01 pm

RedFishBlueFish wrote:How did the 'dedicated amateur' gain access to NASA's images?

http://saturn.jpl.nasa.gov/photos/raw/

by **Tszabeau** » Mon Nov 05, 2012 1:57 pm

I've always assumed that photos of other planets and moons are oriented, like we orient maps and depictions of the Earth, with the north pole at the top and the south pole at the bottom. Is that true or false? I know that some planets and moons have been knocked on their sides and thus spin perpendicular relative to most planets and think it might be appropriate to depict them with poles on the right and left of the image. Is there a convention in this regard in astro photography or is it an aesthetic consideration, only?

by **orin stepanek** » Mon Nov 05, 2012 1:40 pm

Here's to Cassini!

This little space probe has given us so much!

http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens http://www.nasa.gov/mission_pages/cassi ... index.html

by **BDanielMayfield** » Mon Nov 05, 2012 1:18 pm

biddie67 wrote:Good Morning. Many of the craters seem to have a little "hill" in the middle of them. Is this a usual effect when some debris hits the surface?

Yes it is, if the impact is forceful enough. If you can, try looking at the (our) moon through a small telescope or even binoculars and you will see many lunar craters with central peaks.

On Mars, the rover Curiosity's mission is to include a climb up a massive central peak insde Gale Crater.

Bruce

by **Boomer12k** » Mon Nov 05, 2012 12:55 pm

http://system.solaire.free.fr/saturnesat2.htm

Did an image search of Dione. Above is an interesting page...video of Dione rotating, and two face view of Dione front and back, I guess...seems to be in French.

I observe that not only are there less craters on one side, but there appears to be less in the bottom part of the pictures. I am not sure if UP IS NORTH in these photos.

Go down the page a little and watch the video of RHEA...it shows a very similar feature....one side pockmarked, and a smoother side...

:---[===] *

by **biddie67** » Mon Nov 05, 2012 12:34 pm

Good Morning. Many of the craters seem to have a little "hill" in the middle of them. Is this a usual effect when some debris hits the surface?

by **BDanielMayfield** » Mon Nov 05, 2012 12:14 pm

To the question raised by Guest and others wondering about why the leading face of a tidally locked moon will have more craters than the trailing face an analogy might be of help. Think of a vehicle driving through rain. Some raindrops will hit the rear window, but many more will hit the front windshield, and these will hit with greater velocity.

Dione is orbiting Saturn with an average velocity of 10.005787 km/s, and that must be added to the average velocity of Saturn’s orbit around the Sun which is 10.04991 km/s. Yes, impactors can fall in from any direction, but the ones that make the biggest craters per impactor size will be the ones that hit the leading side.

This principle is also the same reason why observing meteors here on earth is more productive before dawn than it is after dusk.

The fact that in Dione this crater count is reversed means that a large impact must have rotated the moon, like a car that’s been spun 180 degrees by a collision. Hope this helped.

Bruce

by **RedFishBlueFish** » Mon Nov 05, 2012 11:49 am

...crater-forming impacts were so large they spun Dione, changing the part that suffered the highest impact rate before the moon's spin again became locked...

This would nicely explain the differential cratering at the leading and trailing surfaces.

But, to my mind, this leaves unexplained the large craters on the sides of the moon perpendicular to the path of the orbit - halfway between the leading/trailing parts (on the plane perpendicular to the path) - looking the same as those on the leading portion.

To me it seems that if there are all of these impactors coming from one direction hitting the moon dead-on and leaving the round craters we see, then there should be furrowed craters from glancing strikes along the sides - which is not what is seen on the side shown in the image, as the craters on the side look the same as those on the leading edge.

Dust on the now-leading edge seems attractive, if so, then one should see a smooth shading of the craters on side of the moon between leading/trailing edges. Whilst the most superior of those craters in this image suggests such blurring none of the others do - nor does inspection in this image of the now-leading yellow edge even hint at obscured craters lying below.

The reason that I always start my day with APOD is that the image is often beautiful and the subject always makes one think.

How did the 'dedicated amateur' gain access to NASA's images?

by **gorade** » Mon Nov 05, 2012 11:34 am

FLPhotoCatcher wrote:I don't think that dust falling onto Dione could hide any of the craters except the smallest. And if dust had fallen heavily enough to hide them on one side, an area that got less dust would show evidence of hidden craters underneath, like a chair shows up as a smooth bump after a very heavy snowfall.

To me it looks just like there were hidden craters below snow or something..

by **Ann** » Mon Nov 05, 2012 6:30 am

One hemisphere - the leading one? - is yellower than the other. This might make sense if Dione is collecting a thin coating of reddish dust particles in its path. On the other hand, Dione is highly reflective, with an albedo of 0.998 ± 0.004, meaning that the moon must be very white. The likeliest reason for the white color of Dione might be that it has picked up a coating of snow-white ice particles emanating from Enceladus.

According to a Cassini enhanced-color composite of Dione, it is in fact the trailing hemisphere that is yellower than the other. Perhaps color on Dione is such a very subtle thing that it mostly has to do with changes in illumination.

Ann

Starship Asterisk*

APOD: Saturns Moon Dione in Slight Color (2012 Nov 05)

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