Starship Asterisk*

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

biddie67 wrote:Is little Janus as irregularly shaped as it appears to be?

http://www.planetary.org/blog/article/00002582/ wrote:
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

León wrote: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

http://en.wikipedia.org/wiki/Epimetheus_%28moon%29 wrote:
<<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 wrote: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.

Chris Peterson wrote:

Bill in Tennessee wrote:
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.

http://en.wikipedia.org/wiki/Moons_of_Saturn wrote:

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°

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

Bill in Tennessee wrote: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?

Bill in Tennessee wrote: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.