Starship Asterisk*

In the Company of Dione

Explanation: That is not our Moon. It's Dione, and it's a moon of Saturn. The robotic Cassini spacecraft took the featured image during a flyby of Saturn's cratered Moon last month. Perhaps what makes this image so interesting, though, is the background. First, the large orb looming behind Dione is Saturn itself, faintly lit by sunlight first reflected from the rings. Next, the thin lines running diagonally across the image are the rings of Saturn themselves. The millions of icy rocks that compose Saturn's spectacular rings all orbit Saturn in the same plane, and so appear surprisingly thin when seen nearly edge-on. Front and center, Dione appears in crescent phase, partially lit by the Sun that is off to the lower left. A careful inspection of the ring plane should also locate the moon Enceladus on the upper right.

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Interesting that the rings are not quite in the plane of the camera. On the right side of the photos the rings show up as two "white" lines - where the the full arc of the rings is visible, whereas on the left, the planet obscures the rings behind the planetallowing only the near half of the rings to be seen.

The accompanying text calls the rings "surprisingly thin". How thick (thin) are the rings?

ceelias wrote:The accompanying text calls the rings "surprisingly thin". How thick (thin) are the rings?

According to Wikipedia the rings' thickness ranges from ten meters to one kilometre.

Very dramatic. Beautiful picture.

Can anyone explain why the upper part of Saturn is illustrated? My bets guess is reflected light from the rings. Is this correct?

G. M.

gmurray618 wrote:Can anyone explain why the upper part of Saturn is illustrated? My bets guess is reflected light from the rings. Is this correct?

APOD Robot wrote:Saturn itself [is] faintly lit by sunlight first reflected from the rings.

Did anyone notice the date is July 7th again? There are two July 7, 2015 on APOD. Something changed in the Matrix.

Markus Schwarz wrote:

ceelias wrote:The accompanying text calls the rings "surprisingly thin". How thick (thin) are the rings?

According to Wikipedia the rings' thickness ranges from ten meters to one kilometre.

Thank you for that I was curious as well

Now, why would the ring be so thin? What force is essentially keeping it in check? Is it gravity? It seems odd because it is such a flat, smooth looking plane.

Any insight is appreciated.

CURRAHEE CHRIS wrote:Now, why would the ring be so thin? What force is essentially keeping it in check? Is it gravity? It seems odd because it is such a flat, smooth looking plane.

Any insight is appreciated.

Because Saturn is so oblate, the only stable orbits around it have near-zero inclination, particularly for bodies close to the planet. Also, a ring system isn't really a disc, because everything has to orbit around a common center. So for a system with non-zero thickness, bodies that are not on the disc plane have to cross it twice each orbit (that is, they have non-zero inclination). That serves to weed out bodies that get perturbed out of zero inclination.

Capt. Obvious wrote:Did anyone notice the date is July 7th again? There are two July 7, 2015 on APOD. Something changed in the Matrix.

Sorry. Fixed it. Time warp smoothed. - RJN

Just a technical observation, it appears that since Otto posted the page so early (Tue Jul 07, 2015 8:07 pm) that July 7, 2015 became the default date for http://apod.nasa.gov/apod/ap150708.html. Giving July two days with the same date. A Cosmologist might need to research this.

8D <;))',><

Medik 1-7 wrote:Just a technical observation, it appears that since Otto posted the page so early (Tue Jul 07, 2015 8:07 pm) that July 7, 2015 became the default date for http://apod.nasa.gov/apod/ap150708.html. Giving July two days with the same date. A Cosmologist might need to research this.

8D <;))',><

Otto gets his cue from APOD (apod.nasa.gov). APOD is on US Eastern Time and Otto is on US Central Time. Shortly after midnight ET, after APOD posts, when someone selects the Discuss link, Otto goes to work. Usually it is still before midnight CT for Otto. The wrong date on APOD is probably just a finger error.

Nice image...

Hello, Dione!!!

:---[===] *

Chris Peterson wrote:

CURRAHEE CHRIS wrote:Now, why would the ring be so thin? What force is essentially keeping it in check? Is it gravity? It seems odd because it is such a flat, smooth looking plane.

Any insight is appreciated.

Because Saturn is so oblate, the only stable orbits around it have near-zero inclination, particularly for bodies close to the planet. Also, a ring system isn't really a disc, because everything has to orbit around a common center. So for a system with non-zero thickness, bodies that are not on the disc plane have to cross it twice each orbit (that is, they have non-zero inclination). That serves to weed out bodies that get perturbed out of zero inclination.

We understand that the moons shepherd and control the shape of the rings. But does the ring system and the various moons all orbit at the same velocity? I seems to me that there would have to be differences in order to shepherd the rings. if so, why isn't the ring material and the smaller moons ejected from what seems to be a stable system? Or is the ring system just apparently stable for our short lifetimes and will disintegrate in more geological/astronomical time frames.

Dad is watching wrote:We understand that the moons shepherd and control the shape of the rings. But does the ring system and the various moons all orbit at the same velocity?

No. The orbital speed of any moon or ring particle is dictated by ordinary Newtonian dynamics. The closer the particle or moon to Saturn, the faster its orbital speed.

I seems to me that there would have to be differences in order to shepherd the rings. if so, why isn't the ring material and the smaller moons ejected from what seems to be a stable system?

The primary effect of shepherding is to create resonant zones which are unstable- basically, gaps in the overall structure. Certainly, material is occasionally ejected.

Or is the ring system just apparently stable for our short lifetimes and will disintegrate in more geological/astronomical time frames.

Like any system of more than two bodies, the Saturnian system is unstable. But it has a high degree of metastability, and the odds are that the system can be treated as stable over a long time- probably billions of years.