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Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Tue Jul 13, 2010 9:03 pm
by 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
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Tue Jul 13, 2010 10:44 pm
by bystander
Ann wrote:(But I miss the Inadvertant Bot Sire. I loved that title.)
There is an
amusing (
![Embarassed :oops:](./images/smilies/icon_redface.gif)
) 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.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Wed Jul 14, 2010 1:13 am
by neufer
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
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
Epimetheus and Janus
Posted: Wed Jul 14, 2010 1:37 am
by neufer
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.
![Image](http://upload.wikimedia.org/wikipedia/commons/2/27/PIA08170_Epimetheus_and_Janus.jpg)
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.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Thu Jul 15, 2010 2:28 pm
by 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.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Thu Jul 15, 2010 2:32 pm
by 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.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Thu Jul 15, 2010 2:48 pm
by Chris Peterson
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.
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.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Thu Jul 15, 2010 4:00 pm
by neufer
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.
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.)
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°
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Thu Jul 15, 2010 4:14 pm
by Chris Peterson
neufer wrote: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.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Fri Jul 16, 2010 1:59 pm
by 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.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Fri Jul 16, 2010 2:37 pm
by neufer
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?
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.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Fri Jul 16, 2010 3:30 pm
by Chris Peterson
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.
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.