18 May 2007

I have been reading Peter Grego's Firefly book Solar System Observer's Guide. I am currently reading about Saturn and learned that the rings extend far beyond what we normally see and that all but five of the planet's 19 moons and numerous other orbiting bodies are within the ring structure. That got me to thinking about how I might graphically display the rings and moons. I found that my concepts would not work in Excel. The best I could do was to create one chart depicting the ring structure and another for the location and size of the rings (Vertical location of the moons in the graphic does not indicate where they are relative to the plane of Saturn's rings. I had to move them up and down on the chart so they didn't overlap as badly.

There is mention that a “sizeable solid body” will be torn apart if it gets closer to the planet than its Roche Limit. But it never says how “sizeable” sizeable is. So I did a search for the Roche Limit of Saturn figuring that I could compare that to my distance chart to see when a planet becomes “sizeable.” I combined that information with information from Grego’s book and other web sources and came up with my own version of a chart listing where things are relative to the center of Saturn.

Roche Limit

Planet	Roche Limit
	Kilometers	Miles
Mercury
Venus
Earth	18,470	11,470
Mars
Jupiter	175,000	108,000
Saturn	147,000	92,000
Uranus	62,000	39,000
Neptune	59,000	37,000

If a planet and a moon have identical densities, then the Roche limit is 2.446 times the radius of the planet.

Saturn's Rings, Planets, and the Roche Limit

Description	Distance from Saturn's center (km)	Width or Diameter (km)
Saturn - cloud tops	60,300	&ngsp;
D Ring - inner edge	66,900	7,150
D Ring - outer edge	73,150
C Ring - inner edge	74,510	17,500
Maxwell Division	87,500	270
C Ring - outer edge	92,000
B Ring - inner edge	92,000	25,500
B Ring - Outer edge	117,500
Cassini Division - inner edge	117,500	4700
Huygens Gap	117,680	285-440
Cassini Division - outer edge	122,200
A Ring - inner edge	122,200
Encke Minima	128,200	3,500
Pan	133,583	20
Encke Division	135,700	325
Daphnis	136,500	7
Keeler Gap	136,530	35
A Ring - outer edge	136,800
Prometheus	137,350	94
Atlas	137,640	28
F Ring	140,210	30-500
Pandora	141,700	92
Roche Limit	147,000
Epimetheus	151,422	114
Janus	151,472	178
Epimetheus	151,500	119
G Ring - inner edge	164,000	8,000
G Ring - outer edge	172,000
E Ring - inner edge	180,000	300,000
Mimas	185,520	392
Methone	194,000	3
Pallene	211,000	4
Enceladus	238,100	499
Tethys	294,660	1,060
Telesto	294,660	24
Calypso	294,660	19
Dione	377,400	1,118
Helene	377,400	32
Polydeuces	377,4000	13
E Ring - outer edge	480,000
Rhea	527,100	1,528
Titan	1,222,830	5,150
Hyperion	1,481,100	286
Iapetus	3,560,800	1,436
Kiviuq	11,365,000	16
Ijiraq	11,440,000	12
Phoebe	12,952,000	220
Paaliaq	15,199,000	22
Skathi	15,647,000	8
Albiorix	16,404,000	32
S/2004 S11	16,950,000	6
Erriapo	17,616,000	10
Siarnaq	18,160,000	40
Tarvos	18,247,000	15
Narvi	18,719,000	7
S/2004 S15	18,750,000	6
S/2004 S09	19,800,000	5
S/2004 S18	19,650,000	7
S/2006 S1
S/2006 S2
S/2006 S3
S/2006 S8

So it looks like a body is too big to exist inside of the Roche Limit at around 100 km.

After it all I found this Planetary Society image that shows the ring structure and the major moons.

Grego also says that inexperienced observers, which is definitely me, often mistake a gray area on the inside of Ring B in the ansae with Ring C. This leads me to wonder whether or not I have seen Ring C. But reading farther he talks about Ring C crossing the planet and I am reasonably certain that I have seen the ring there.