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APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 4:04 am
by APOD Robot
Moons Beyond the Rings of Saturn
Explanation: What's happened to that moon of Saturn? Nothing -- Saturn's moon Rhea is just partly hidden behind Saturn's rings. In April, the robotic Cassini spacecraft now orbiting Saturn took this
narrow-angle view looking across the
Solar System's most
famous rings. Rings visible in the foreground include the thin
F ring on the outside and the much wider
A and B rings just interior to it. Although it seems to be hovering
over the rings, Saturn's moon
Janus is actually far behind them.
Janus is one of Saturn's smaller
moons and measures only about 180 kilometers across. Farther out from the camera is the heavily cratered
Rhea, a much larger moon measuring 1,500 kilometers across. The top of
Rhea is visible only through
gaps in the rings. The Cassini mission around Saturn has
been extended to
2017 to better study the complex planetary system as its season changes from
equinox to
solstice.
[/b]
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 4:18 am
by Mr America
ZZZZZZZZZZZz
Had a total eclipse today-far more astronomical an event than this
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 4:32 am
by owlice
Eclipse images are here:
http://asterisk.apod.com/vie ... 51#p126951
The image under discussion appeared on the Observation Deck here:
http://asterisk.apod.com/vie ... 2&p=125790
(And a couple of other threads, too.)
This is a fabulous and amazing image! Gorgeous, too. What a fascinating planetary system!
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 5:10 am
by moonstruck
Well Mr. ZZZZZZZZZZZz..Sorry you are so bored. Some of us actually enjoy the pictures of the universe that our technology has given us. Maybe you could Post some pics of the total eclispe today of the more astronomical events.
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 6:59 am
by bystander
If you do post pictures of the 11 July 2010 total solar eclipse,
please
post them to
2010's total eclipse of the Sun on the
Observation Deck.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 11:28 am
by orin stepanek
hmph! Some moons just don't follow the in group!
![Mr. Green :mrgreen:](./images/smilies/icon_mrgreen.gif)
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 1:12 pm
by León
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
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 5:23 pm
by Tina
I think this is the most wicked picture I've ever seen! Saturn and its rings are so facinating. Thank you so much for posting:)
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 5:31 pm
by owlice
Hi, Tina, and welcome! There are more images of Saturn, its rings, and moons on
this thread on the
Observation Deck. Enjoy!
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 6:56 pm
by Foozinator
Janus looks like it might be on the same plane as the rings (and may or may not be orbiting on the ring plane or just crossing at the time), but Rhea appears to be orbiting several degrees off the plane of the rings. How many of Saturn's moons have orbits at angles to the plane of the rings?
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 7:07 pm
by Chris Peterson
Foozinator wrote:Janus looks like it might be on the same plane as the rings (and may or may not be orbiting on the ring plane or just crossing at the time), but Rhea appears to be orbiting several degrees off the plane of the rings. How many of Saturn's moons have orbits at angles to the plane of the rings?
All of them. Most of the moons are inclined by a few tenths of a degree; there are also a bunch of tiny ones with extreme inclinations. Janus has an inclination of 0.163°, and Rhea has an inclination of 0.345°.
It is impossible to assess inclinations in a single image (except for determining a non-zero value) since regardless of inclination, any satellite could be captured crossing the equatorial plane.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 7:17 pm
by Foozinator
Chris Peterson wrote:... Most of the moons are inclined by a few tenths of a degree; there are also a bunch of tiny ones with extreme inclinations. Janus has an inclination of 0.163°, and Rhea has an inclination of 0.345°. ...
Very interesting. Are the rings themselves on Saturn's equatorial plane, or are they inclined as well?
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 8:20 pm
by biddie67
I always have trouble with the perspectives of which moons are closer in toward Saturn and which ones are further out.
Is little Janus as irregularly shaped as it appears to be?
Do these moons, small or large, cause some kind of splatter patterns in the rings as they "bust" through the rings from one side to the other?
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 8:27 pm
by primordial_punt
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.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 9:02 pm
by Chris Peterson
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.
I'm not sure what image or APOD you are viewing. This one shows two moons imaged from slightly above the ring plane (that is, we are seeing the northern face of the rings). The moons are tiny Janus, which is just outside the F-ring, beyond the rings as seen in the image, and much larger Rhea, which is much farther outside the rings, also beyond the rings from this view. Prometheus is not in the image at all, and neither moon is between the camera and the rings.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 9:11 pm
by bystander
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.
Not sure where you got your information, but JPL-Caltech disagrees:
PIA12643: Rings, Rhea and Janus
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
The rings are in the foreground. Janus is further away and Rhea further yet. The image was taken from just above the ringplane.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 9:12 pm
by León
Radius of the orbit of Rhea is 527 000 km and 151,000 km of Janus, in the image rhea is closer to Saturn, so you must be behind Janus although the image seems to say otherwise. The last ring extends to 480 000 km.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Mon Jul 12, 2010 9:29 pm
by Guest
Would be better in 3D, I wonder if there was a second photo taken shortly after this one to form a stereoscopic pair?
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Tue Jul 13, 2010 11:04 am
by DavidLeodis
I'm very confused (easily done I admit.
![Embarassed :oops:](./images/smilies/icon_redface.gif)
) but I can only discern 1 moon yet the explanation (and the caption to the image in the JPL Phototojournal website) implies I should be able to at least see Janus and Rhea. It may be my eyesight or I need a larger monitor, but could someone please give me an indication of where the non-obvious moon (presumably Rhea) is in the image. Thanks.
![Smile :)](./images/smilies/icon_smile.gif)
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Tue Jul 13, 2010 11:39 am
by owlice
The non-obvious moon is Janus.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Tue Jul 13, 2010 11:51 am
by DavidLeodis
owlice wrote:The non-obvious moon is Janus.
moons.jpg
Thanks owlice. So what I thought was Saturn is Rhea. I totally failed to work out what was what in the image.
![Embarassed :oops:](./images/smilies/icon_redface.gif)
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Tue Jul 13, 2010 11:55 am
by DavidLeodis
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.
![Embarassed :oops:](./images/smilies/icon_redface.gif)
but still :)ing.
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Tue Jul 13, 2010 12:01 pm
by owlice
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.
![Embarassed :oops:](./images/smilies/icon_redface.gif)
but still :)ing.
Yes; the two moons are pictured, as are the rings, and Saturn is off to the right. What I "circled" (or whatever that shape is!) are the moons.
Rhea Rings Beyond the Rings of Saturn?
Posted: Tue Jul 13, 2010 2:07 pm
by neufer
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.>>
Code: Select all
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.>>
Re: APOD: Moons Beyond the Rings of Saturn (2010 Jul 12)
Posted: Tue Jul 13, 2010 3:23 pm
by bystander