APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)

[Ann]

Hmmmm, I like the enhanced color image of Rhea. Normally I hate false color, but when it comes to enhanced color I can't help that I like it.

And hey, I just discovered that I have become the color commentator here at Starship Asterisk! I like that, too!

(But I miss the Inadvertant Bot Sire. I loved that title.)

Ann

[bystander]

(But I miss the Inadvertant Bot Sire. I loved that title.)

There is an amusing ( ) story behind that, and I guess it was better than Sanitation Guru or Sanitation Tech (see here). But, if I was an Inadvertent Bot Sire, then geckzilla is a Deliberate Bot Dam. After Art's comment (here), I think my new title is well earned.

[neufer]

Is little Janus as irregularly shaped as it appears to be?

The Planetary Society Blog By Emily Lakdawalla
Cassini eyes Janus
Jul. 13, 2010 | 11:56 PDT | 18:56 UTC

Click to view full size image

Cassini gazed at Janus (193 by 173 by 137 kilometers in diameter) from a distance of 96,000 kilometers on July 26, 2009 to capture this natural-color, global view; even the night side is faintly illuminated by light reflected from Saturn or the rings. Janus is the larger of a pair of moons that share an orbit around Saturn; its partner is Epimetheus. Credit: NASA / JPL / SSI / color composite by Gordan Ugarkovic

[neufer]

Cracks visible through Rhea, of monumental image to appreciate the cleaning between discs, back janus, with both sides gives the name to January, looks at the bullet-shaped image, Epimetheus is not observed to have the same orbit Nor is it http://photojournal.jpl.nasa.gov/catalog/PIA12638 can be seen in what he calls attention

<<Epimetheus and Janus are co-orbital: Janus's mean orbital radius from Saturn is currently only 50 km less than that of Epimetheus, a distance smaller than either moon's diameter. In accordance with Kepler's laws of planetary motion, the closer orbit is completed more quickly, but only by about 30 seconds. Each day the inner moon is an additional ¼° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction boosts the inner moon's momentum and saps the outer moon's momentum. With this added momentum, the inner moon's distance from Saturn and orbital period are increased, and the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons "trade" orbits, never approaching closer than about 10,000 km. The exchange takes place about once every four years; the last close approach occurred on January 21, 2006, the next will be in 2010. At that time, Janus's orbital radius will increase by ~20 km, while Epimetheus's decreases by ~80 km; Janus's orbit is less affected because it is 4 times more massive than Epimetheus.

As far as it is currently known, this arrangement is unique in the solar system.>>

• The horseshoe orbits of Janus and Epimetheus can be easily seen in this rotating frame depiction.

  

  Click to view full size image

  
  Epimetheus (lower left) and Janus seen on March 20, 2006, two months after swapping orbits.
  The close proximity between the two moons is an illusion;
  they are actually being seen on opposite sides of their common orbit.

[Bill in Tennessee]

Why do both satellites of Saturn (Rhea and Janus) have orbits that are off the plane of the rings? I would think that because the rings are made up of debris that have found homeostasis in their own orbit as determined by the gravity of Saturn, that the moons would have the same orbit. Is it that the moons have a slightly different orbit and actually cross through the rings on a periodic basis? As the photo is shown, it appears that the rings have one orbit, and that the two satellites shown have orbits, one above the plane of the rings, and the other below the plane of the rings. Is this an optical illusion? I am just curious as to why the different bodies would seem to have different orbital planes.

[Bill in Tennessee]

Follow up...I did not mean those are Saturn's ONLY moons, but rather the two moons that are visible in that photo.

[Chris Peterson]

Why do both satellites of Saturn (Rhea and Janus) have orbits that are off the plane of the rings? I would think that because the rings are made up of debris that have found homeostasis in their own orbit as determined by the gravity of Saturn, that the moons would have the same orbit. Is it that the moons have a slightly different orbit and actually cross through the rings on a periodic basis? As the photo is shown, it appears that the rings have one orbit, and that the two satellites shown have orbits, one above the plane of the rings, and the other below the plane of the rings. Is this an optical illusion? I am just curious as to why the different bodies would seem to have different orbital planes.

There is no mechanism to force the moons into orbits with zero inclination. The rings, however, behave like a fluid. Individual particles collide with one another, transferring angular momentum. This is quite unlike the moons, where only gravity is significant in determining orbital characteristics.

Most of the moons are outside the ring system (not considering the extremely nebulous distant ring components), so they don't strictly pass through the rings, but simply through the ring plane. There are some tiny interior moons that bob up and down through the rings, though. But these moons have cleared a zone around their orbit, so they only interact weakly with actual ring material.

[neufer]

Why do both satellites of Saturn (Rhea and Janus) have orbits that are off the plane of the rings? I would think that because the rings are made up of debris that have found homeostasis in their own orbit as determined by the gravity of Saturn, that the moons would have the same orbit. Is it that the moons have a slightly different orbit and actually cross through the rings on a periodic basis? As the photo is shown, it appears that the rings have one orbit, and that the two satellites shown have orbits, one above the plane of the rings, and the other below the plane of the rings. Is this an optical illusion? I am just curious as to why the different bodies would seem to have different orbital planes.

There is no mechanism to force the moons into orbits with zero inclination. The rings, however, behave like a fluid. Individual particles collide with one another, transferring angular momentum. This is quite unlike the moons, where only gravity is significant in determining orbital characteristics.

Tidal forces from Saturn, the rings, and other moons keep most
lunar inclinations within one degree for the first 2 million kilometers.

(Such small inclinations are simply exaggerated in this APOD.)

Code: Select all

Moon		semi-major axis  Period (d)  inclination
-----------------------------------------------------------
Pan	          133,584 	 0.57505 	0.001°
 
Daphnis         136,505 	 0.59408 	≈ 0°
 
Atlas 	       137,670 	 0.60169 	0.003°

Prometheus      139,380 	 0.61299 	0.008°

Pandora         141,720 	 0.62850 	0.050°

Epimetheus      151,422 	 0.69433 	0.335°
Janus	        151,472 	 0.69466 	0.165°

Aegaeon         167,500 	 0.80812 	0.001°

Mimas	        185,404 	 0.942422   1.566°

Methone         194,440 	 1.00957	 0.007°

Anthe           197,700 	 1.03650	 0.1°

Pallene         212,280 	 1.15375 	0.181°

Enceladus       237,950 	 1.370218   0.010°

Tethys	       294,619 	 1.887802   0.168°
Telesto         294,619 	 1.887802   1.158°
Calypso         294,619 	 1.887802   1.473°

Dione	        377,396 	 2.736915   0.002°
Helene	       377,396 	 2.736915   0.212°
Polydeuces      377,396 	 2.736915   0.177°

Rhea  	       527,108 	 4.518212   0.327°

Titan  	    1,221,930 	15.94542 	0.3485°
Hyperion      1,481,010 	21.27661 	0.568°

Iapetus       3,560,820 	79.3215 	 7.570°

[Chris Peterson]

Tidal forces from Saturn, the rings, and other moons keep most lunar inclinations within one degree for the first 2 million kilometers.

Yes, like most planetary satellites, there are forces that tend to produce low inclinations. But not zero inclinations. This is different from the rings, where fluid dynamic effects truly produce an inclination so near zero that there's no effective difference.

[Bill in Tennessee]

So then it is possible (assuming two perfectly spherical bodies with both having evenly distributed masses, and also assuming perfectly circular orbits) for a satellite to have an inclination that would have it permanently orbit at some point above (or below) the plane of the larger body's equator? I accept the statements of those here on this discussion board, as I'm sure you are more learned than I am on this, but I hope you can understand that to me that it seems counter-intuitive.

[neufer]

So then it is possible (assuming two perfectly spherical bodies with both having evenly distributed masses, and also assuming perfectly circular orbits) for a satellite to have an inclination that would have it permanently orbit at some point above (or below) the plane of the larger body's equator?

Assuming either two spherical bodies or a distant orbit (or a tiny moon)
it is probable that a satellite would have a FIXED inclination so as
to permanently orbit both above & below the plane of the larger body's equator.

Tidal forces from the sun would still cause the satellite's orbit to precess vis-a-vis the fixed stars.

[Chris Peterson]

So then it is possible (assuming two perfectly spherical bodies with both having evenly distributed masses, and also assuming perfectly circular orbits) for a satellite to have an inclination that would have it permanently orbit at some point above (or below) the plane of the larger body's equator? I accept the statements of those here on this discussion board, as I'm sure you are more learned than I am on this, but I hope you can understand that to me that it seems counter-intuitive.

Sure. The Solar System is full of moons in stable orbits with inclinations significantly different from zero. Of course, I use "stable" a little loosely, since these are all multiple body systems and nothing is perfectly stable. And there are lots of artificial satellites orbiting the Earth that are in stable, high inclination orbits. In general there is no connection between the orbits of bodies and the rotation of those bodies.