Starship Asterisk*

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.

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?

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

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°

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.

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.

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

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

http://en.wikipedia.org/wiki/Rings_of_Rhea wrote:
<<Voyager 1 observed a broad depletion of energetic electrons trapped in Saturn's magnetic field downstream from Rhea in 1980. These measurements, which were never explained, were made at a greater distance than the Cassini data. In August 2007, Cassini passed through Rhea's plasma shadow again, but further downstream. Its readings were similar to those of Voyager 1.

On November 26, 2005, Cassini made the one targeted Rhea flyby of its primary mission. It passed within 500 km of Rhea's surface, downstream of Saturn's magnetic field, and observed the resulting plasma wake as it had with other moons, such as Dione and Tethys. In those cases, there was an abrupt cutoff of energetic electrons as Cassini crossed into the moons' plasma shadows (the regions where the moons themselves blocked the magnetospheric plasma from reaching Cassini). However, in the case of Rhea, the electron plasma started to drop off slightly at eight times that distance, and decreased gradually until the expected sharp drop off as Cassini entered Rhea's plasma shadow. The extended distance corresponds to Rhea's Hill sphere, the distance of 7.7 times Rhea's radius inside of which orbits are dominated by Rhea's rather than Saturn's gravity. When Cassini emerged from Rhea's plasma shadow, the reverse pattern occurred: A sharp surge in energetic electrons, then a gradual increase out to Rhea's Hill-sphere radius. These readings are similar to those of Enceladus, where water venting from its south pole absorbs the electron plasma. However, in the case of Rhea, the absorption pattern is symmetrical. The simplest explanation for the symmetrical punctuations in plasma flow are "extended arcs or rings of material" orbiting Rhea in its equatorial plane. These symmetric dips bear some similarity to the method by which the Rings of Uranus were discovered in 1977. The slight deviations from absolute symmetry may be due to "a modest tilt to the local magnetic field" or "common plasma flow deviations" rather than to asymmetry of the rings themselves, which may be circular.

In addition, the Magnetospheric Imaging Instrument (MIMI) observed that this gentle gradient was punctuated by three sharp drops in plasma flow on each side of the moon, a pattern that was also nearly symmetrical.>>

Possible Rhean rings Ring 	orbital radius (km)
disk 	< 5900
1 	≈ 1615
2 	≈ 1800
3 	≈ 2020

http://en.wikipedia.org/wiki/Rings_of_Rhea wrote:
<<The Saturnian moon Rhea may have a tenuous ring system consisting of three narrow, relatively dense bands within a particulate disk. This would be the first discovery of rings around a moon. The discovery was announced in the journal Science on March 6, 2008.

In October 2009, it was announced that a set of small ultraviolet-bright spots distributed in a line that extends three quarters of the way around Rhea's circumference, within 2 degrees of the equator, may represent further evidence for a ring. The spots presumably represent the impact points of deorbiting ring material. There are no images or direct observations of the material thought to be absorbing the plasma, but the likely candidates would be difficult to detect directly. Further observations are planned for Cassini's first mission extension, with a targeted flyby scheduled for March 2, 2010.

Cassini's flyby trajectory makes interpretation of the magnetic readings difficult. The obvious candidates for magnetospheric plasma-absorbing matter are neutral gas and dust, but the quantities required to explain the observed depletion are far greater than Cassini's measurements allow. Therefore the discoverers, led by Geraint Jones of the Cassini MIMI team, argue that the depletions must be caused by solid particles orbiting Rhea: An analysis of the electron data indicates that this obstacle is most likely in the form of a low optical depth disk of material near Rhea’s equatorial plane and that the disk contains solid bodies up to ~1 m in size.

Simulations suggest that solid bodies can stably orbit Rhea near its equatorial plane over astronomical timescales. They may not be stable around Dione and Tethys because those moons are so much closer to Saturn, and therefore have much smaller Hill spheres, or around Titan because of drag from its dense atmosphere. Several suggestions have been made for the possible origin of rings. An impact could have ejected material into orbit; this could have happened as recently as 70 million years ago. A small body could have been disrupted when caught in orbit about Rhea. In either case, the debris would eventually have settled into circular equatorial orbits. Given the possibility of long-term orbital stability, however, it is possible that they survive from the formation of Rhea itself. For discrete rings to persist, something must confine them. Suggestions include moonlets or clumps of material within the disk, similar to those observed within Saturn's A ring.>>

DavidLeodis wrote:I think I've worked it out! I thought the main object was Saturn but I now think it is Rhea and that Saturn is off screen to the right. Apologies for my mental confusion regarding the image. but still :)ing.

owlice wrote:The non-obvious moon is Janus.

moons.jpg

primordial_punt wrote:Sorry folks. But the info accompanying this photo is wrong. The photo is actually being taken from BELOW the ring plane. Prometheus is in front of the rings while Rhea is behind. I wish APOD would be more careful.

Click to view full size image

Saturn's rings occupy the foreground of this image. The small moon Janus appears to hover above, while the far larger moon Rhea is partially obscured by the rings.

Janus appears to be located directly over the rings, but the moon is actually further away, at a range of about 1.1 million kilometers (684,000 miles) from the Cassini spacecraft. Rhea is 1.6 million kilometers (994,000 miles) from the spacecraft. This view looks toward the trailing hemisphere of Janus (179 kilometers, or 111 miles across) and the Saturn-facing side of Rhea (1,528 kilometers, or 949 miles across).

This view looks toward the northern, sunlit side of the rings from just above the ringplane.

Credit: NASA/JPL/SSI

primordial_punt wrote:Sorry folks. But the info accompanying this photo is wrong. The photo is actually being taken from BELOW the ring plane. Prometheus is in front of the rings while Rhea is behind. I wish APOD would be more careful.