APOD: Saturn from Above (2013 Oct 21)

[alter-ego]

Anthony Barreiro wrote:
I'm guessing that little speck of light might be a star. Magnitude 4.6 Iota Antlia could be the culprit.

Iota Antlia is one of those dime-a-dozen K0III stars. For all intents and purposes it is a distant Pollux, pale yellow and not too bright. It's the kind of star that wouldn't stand out in any way, certainly not by visual inspection. It could certainly mimic one of Saturn's moons.

My only objection is - Antlia? That is definitely not where you would expect to see any of the major Solar system objects. Is the Saturnian system simply so huge that its outer parts, including its moons, spill into constellations far from the ecliptic?

Ann

Keep in mind Cassini's view angle is significantly out of the ecliptic plane by ~54°. However, I don't think any Antlia stars are in the field. Around the 10th, Saturn is fully immersed in Vela - not Antlia (~15°away).

Solar System Simulator (I added the constellation)

[Nitpicker]

Around the 10th, Saturn is fully immersed in Vela

Wow, your image led me to the Solar System Simulator:
http://space.jpl.nasa.gov/

Never knew that existed before. Thank you! And based on that, I'd have to concur that Saturn was within Vela from Cassini, during 10 Oct 2013. Based on shadow orientation, I'd estimate the time of the image to be closer to 18:00 UTC.

(Not so sure about the wee moons anymore though. Might have to ask Mr Ugarkovic.)

[Anthony Barreiro]

Iota Antlia is one of those dime-a-dozen K0III stars. For all intents and purposes it is a distant Pollux, pale yellow and not too bright. It's the kind of star that wouldn't stand out in any way, certainly not by visual inspection. It could certainly mimic one of Saturn's moons.

My only objection is - Antlia? That is definitely not where you would expect to see any of the major Solar system objects. Is the Saturnian system simply so huge that its outer parts, including its moons, spill into constellations far from the ecliptic?

Ann

Oops, I've been mis-spelling the constellation name. It is Antlia and the star is Iota Antliae. But this view from the Cassini spacecraft (i.e. the view in this APOD), is not at all aligned with the Ecliptic plane.

Yes, we're looking south onto Saturn's north pole, with the southern constellations behind Saturn, comprable to the view from Earth's southern hemisphere.

Thanks Nitpicker for the correct spelling of the genitive Iota Antliae. I haven't used that one before this discussion.

By the way, slightly changing the viewing distance and angle toward Saturn significantly changes the background constellations, so I don't really have a clue what that mystery object might be.

[kevincwolfe]

Uh, WOW! We've come a long way during the past couple of decades.
Saturn somehow appears to be solid. Still brushing up on fluid dynamics to try and understand that wacky hexagonal flow going on near the pole. Perhaps the axis of rotation in the upper atmosphere does not coincide with the rotation of the core? Gotta be something gravitational, fluids don't behave that way...except on Saturn.

There have been some experiments which were able to reproduce the hexagon in a lab here on Earth.

Click to play embedded YouTube video.

Where are the eddys on Saturn that exist during the bench-scale tests? They would seem to be a glaring component missing from the real-life example.

[geckzilla]

Yes, it's quite possible that Saturn has a lot more going on in it than a dish containing some water and dye on a spinning table in a lab.

[Anthony Barreiro]

Where are the eddys on Saturn that exist during the bench-scale tests? They would seem to be a glaring component missing from the real-life example.

Water is transparent. Saturn's clouds are opaque. There could be eddies at deeper layers that are hidden at visible wavelengths. I think the point of the tabletop experiment was simply to show that fluid dynamics can create a stable hexagonal pattern without needing any exotic conditions.

[Nitpicker]

By the way, slightly changing the viewing distance and angle toward Saturn significantly changes the background constellations, so I don't really have a clue what that mystery object might be.

Anthony, I haven't checked out Sky Safari yet, as I don't have any i-thingies or smart-thingies to run it on (not to mention that I'm cheap enough to baulk at shelling out cash for it). But thanks anyway.

Ignoring background stars for a moment, where does Sky Safari place Janus, Epimetheus, Pandora and Prometheus at 2013-10-10 18:00 UTC, plus or minus a couple of hours? Are they anywhere near the dots we can see in this APOD? Other factors which may need to be considered are that at the time, Earth and Saturn were separated by 90 light-minutes, and perhaps more importantly, the orbital elements of these tiny moons might not be current in the software. I don't know of an alternative source for up-to-date positions of these tiny moons, although I haven't looked too hard. Obviously not of major importance, but certainly interesting to me.

[Anthony Barreiro]

By the way, slightly changing the viewing distance and angle toward Saturn significantly changes the background constellations, so I don't really have a clue what that mystery object might be.

Anthony, I haven't checked out Sky Safari yet, as I don't have any i-thingies or smart-thingies to run it on (not to mention that I'm cheap enough to baulk at shelling out cash for it). But thanks anyway.

Ignoring background stars for a moment, where does Sky Safari place Janus, Epimetheus, Pandora and Prometheus at 2013-10-10 18:00 UTC, plus or minus a couple of hours? Are they anywhere near the dots we can see in this APOD? Other factors which may need to be considered are that at the time, Earth and Saturn were separated by 90 light-minutes, and perhaps more importantly, the orbital elements of these tiny moons might not be current in the software. I don't know of an alternative source for up-to-date positions of these tiny moons, although I haven't looked too hard. Obviously not of major importance, but certainly interesting to me.

At 1600 UT Prometheus and Pandora are in about the right places, but Epimetheus is at 4 o'clock and Janus is at 5:00. Being at similar distances from Saturn, their orbital periods are all between 15 and 17 hours, so they stay in roughly the same orientation over the course of 24 hours. When Prometheus and Pandora are in the right places, Epimetheus and Janus are on the opposite side of Saturn from that little mystery spot. Dust on the objective lens?

(Sky safari is available for desktop and laptop computers as well. I don't earn a commission on sales.)

[Nitpicker]

At 1600 UT Prometheus and Pandora are in about the right places, but Epimetheus is at 4 o'clock and Janus is at 5:00. Being at similar distances from Saturn, their orbital periods are all between 15 and 17 hours, so they stay in roughly the same orientation over the course of 24 hours. When Prometheus and Pandora are in the right places, Epimetheus and Janus are on the opposite side of Saturn from that little mystery spot. Dust on the objective lens?

(Sky safari is available for desktop and laptop computers as well. I don't earn a commission on sales.)

I did look for a Windows version, but not very hard (I'm cheap remember?)

I'd suspect a star before a dust mote. Dust on an objective lens or primary mirror is too far away from the focal plane to be detectable, it just reduces the amount of light captured from all incoming directions, by a miniscule amount.

I agree that all these moons have roughly the same orbital period. But I've been trying to find alternative sources for their orbital elements, or some information on the computer models used and their precision within a certain time period. Possibly foolishly, I'm more inclined to think that the software is using stale orbital elements, or something like that, as these little moon orbits can get pretty chaotic. (It wouldn't be the first time a hardware or software company released something that wasn't fully tested.) There don't appear to be many other commercial software packages which simulate these very small moons.

[Nitpicker]

Possibly foolishly, I'm more inclined to think that the software is using stale orbital elements, or something like that, as these little moon orbits can get pretty chaotic.

I just found perhaps the best source of ephemerides (as opposed to the less accurate mean orbital elements) from the JPL's HORIZONS system, viz:
http://ssd.jpl.nasa.gov/horizons.cgi

Using it, I generated the following table for five of Saturn's moons, for the day this APOD was taken:

I included Dione in the table as a reference, as its orbit is more widely known than the other four small moons under discussion in this topic.

The Ang values (a.k.a. the "SatPANG" values generated by the HORIZONS system) are angles in degrees from the North Celestial Pole (of Earth or Saturn?), measured anti-clockwise from Saturn-centre to moon-centre, with zero calibrated to about midnight and 180 calibrated to about midday on Saturn (as best I can tell).

What it shows, I believe, is that the four small moons are orbiting in the same order for the whole day, with Janus leading, followed by Epimetheus, then Pandora and Prometheus trailing. I believe this is enough to question the accuracy of Sky Safari's small moon simulations, as described by Anthony.

But I'm still struggling to positively identify the left-most dot in this APOD. It seems to be ahead of where Janus should be. I'm beginning to think there are still are few important unknowns in this puzzle, such as the time between exposures in this mosaic, the level of image processing done, and whether the Ang is measured from the Earth's pole or Saturn's pole. Regardless, I'm still having fun and learning a lot!

[Anthony Barreiro]

Possibly foolishly, I'm more inclined to think that the software is using stale orbital elements, or something like that, as these little moon orbits can get pretty chaotic.

I just found perhaps the best source of ephemerides (as opposed to the less accurate mean orbital elements) from the JPL's HORIZONS system, viz:
http://ssd.jpl.nasa.gov/horizons.cgi

Using it, I generated the following table for five of Saturn's moons, for the day this APOD was taken:

Saturn_Moon_Ephemerides.PNG

I included Dione in the table as a reference, as its orbit is more widely known than the other four small moons under discussion in this topic.

The Ang values (a.k.a. the "SatPANG" values generated by the HORIZONS system) are angles in degrees from the North Celestial Pole (of Earth or Saturn?), measured anti-clockwise from Saturn-centre to moon-centre, with zero calibrated to about midnight and 180 calibrated to about midday on Saturn (as best I can tell).

What it shows, I believe, is that the four small moons are orbiting in the same order for the whole day, with Janus leading, followed by Epimetheus, then Pandora and Prometheus trailing. I believe this is enough to question the accuracy of Sky Safari's small moon simulations, as described by Anthony.

But I'm still struggling to positively identify the left-most dot in this APOD. It seems to be ahead of where Janus should be. I'm beginning to think there are still are few important unknowns in this puzzle, such as the time between exposures in this mosaic, the level of image processing done, and whether the Ang is measured from the Earth's pole or Saturn's pole. Regardless, I'm still having fun and learning a lot!

I hadn't thought about the time between the individual exposures used to make this mosaic -- that may account for the discrepancy. The positions of Pandora followed by Prometheus are consistent between the apod image, sky safari, and the jpl ephemeris. So I would guess that Pandora and Prometheus are in one image, and the other little bit (which looks more and more like a moon and less and less like a star every time I look at it) is in another image taken at another time. And given that these moons all complete full orbits in 15 to 17 hours, they could all be anywhere over the course of "10 October 2013." The intention of the apod image is to show Saturn and the rings as a whole from above Saturn's orbit, not to show the locations of tiny moons at one moment in time.

Here's another Cassini image, a single exposure of Prometheus, Pandora, and Epimetheus, taken in 2004, just for comparison.

By the way, Sky Safari updates solar system object positions from NASA ephemerides, not just by calculation based on orbital elements. I haven't found any glitches previously, so I think the mosaic time lag is a more likely explanation for that little mystery spot.

[Anthony Barreiro]

Around the 10th, Saturn is fully immersed in Vela

Wow, your image led me to the Solar System Simulator:
http://space.jpl.nasa.gov/

Never knew that existed before. Thank you! And based on that, I'd have to concur that Saturn was within Vela from Cassini, during 10 Oct 2013. Based on shadow orientation, I'd estimate the time of the image to be closer to 18:00 UTC.

(Not so sure about the wee moons anymore though. Might have to ask Mr Ugarkovic.)

Aha. Somehow I missed this subthread until just now. Unfortunately none of our little moons are listed in this simulator, but when you run it for Saturn as seen from Cassini at 1800 UT 10 October 2013, Janus and Epimetheus appear to be right where sky safari also says they are.

[Anthony Barreiro]

I think I see three little moons near the outer edge of the rings along image top.

Can anyone confirm this and identify them?

To answer my own question without confirming anything, I'd guess that from left to right, we can see either Janus or Epimetheus (not sure which), then Pandora, then Prometheus.

You were right at the beginning, and we're still not sure which!

As T.S. Elliot said,

“We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time.”
― T.S. Eliot, Four Quartets

Or, as Emily Litella said,

Click to play embedded YouTube video.

[Psnarf]

Thank you for helping purge my data banks of false information. I wasn't aware of how ubiquitous these sorts of patterns are in fluid dynamics. With that huge ball called Saturn spinning around once every ten hours, I'd posit that it is fast enough to generate that pattern. I'd expect it to appear at the south pole, also, but I wasn't expecting the Spanish Inquisition. I learned that hydrogen gas gets so compressed a ways below the surface that it turns into metallic hydrogen. Is that another state of matter (as, liquid, solid, plasma, metallic), or is that what they call solid hydrogen?

[Chris Peterson]

Thank you for helping purge my data banks of false information. I wasn't aware of how ubiquitous these sorts of patterns are in fluid dynamics. With that huge ball called Saturn spinning around once every ten hours, I'd posit that it is fast enough to generate that pattern. I'd expect it to appear at the south pole, also, but I wasn't expecting the Spanish Inquisition. I learned that hydrogen gas gets so compressed a ways below the surface that it turns into metallic hydrogen. Is that another state of matter (as, liquid, solid, plasma, metallic), or is that what they call solid hydrogen?

Metallic just means it is an electrical conductor. Metallic hydrogen might exist in liquid, solid, or gaseous states. It isn't generally seen an an alternate exotic state of matter, although there may be some exotic quantum fluid possibilities.

[Nitpicker]

Around the 10th, Saturn is fully immersed in Vela

Wow, your image led me to the Solar System Simulator:
http://space.jpl.nasa.gov/

Never knew that existed before. Thank you! And based on that, I'd have to concur that Saturn was within Vela from Cassini, during 10 Oct 2013. Based on shadow orientation, I'd estimate the time of the image to be closer to 18:00 UTC.

(Not so sure about the wee moons anymore though. Might have to ask Mr Ugarkovic.)

Aha. Somehow I missed this subthread until just now. Unfortunately none of our little moons are listed in this simulator, but when you run it for Saturn as seen from Cassini at 1800 UT 10 October 2013, Janus and Epimetheus appear to be right where sky safari also says they are.

Hi Anthony, not sure how you can identify (or even see) Janus and Epimetheus at 4 or 5 o'clock in the little image given by the JPL's Solar System Simulator. Regardless, the JPL ephemerides at 18:00 put these two moons much closer to the mystery APOD dot at 11 o'clock (and critically, both ahead of Pandora and Prometheus for the whole day), so I still suspect there is something funny going on in Sky Safari. Or am I missing something?

I'd also be surprised if the individual exposures comprising this mosaic were taken more than an hour or so apart. At the speed Cassini was moving relative to Saturn, it would make it very hard to assemble the pieces if they were separated by more than a couple of hours.

[Nitpicker]

Ah ha! From the developers of SkySafari:
http://www.southernstars.com/support/re ... _data.html

•Planetary Satellites [ http://ssd.jpl.nasa.gov/?satellites ] - from JPL Solar System Dynamics. The authoritative source of orbital elements and physical parameters for the moons of the planets. SkySafari 4's planetary moon data comes from this site.

On the JPL page linked above, it says quite clearly:

Tables showing mean orbital elements are available. These should not be used for computing precise positions (please use HORIZONS instead), but instead might be used to gain a rough sense of a satellite's orbital shape.

The page for the JPL's HORIZONS system is:
http://ssd.jpl.nasa.gov/horizons.cgi

It looks like there is a also the "NAIF SPICE toolkit" available, to help third-party software developers produce accurate ephemerides (amongst other things):
http://naif.jpl.nasa.gov/naif/toolkit.html

But you'd have to be a certain kind of nerd to be interested in this stuff. [ Guilty.] I'd bet almost no amateur on Earth would be able to observe or detect these very small moons so close to the brightness of the rings, so the fact that SkySafari may not simulate all of their orbits as precisely as possible, is hardly the worst crime imaginable.

[geckzilla]

You could make a video game out of trying to operate Cassini and have it know exactly when to take a photo so a certain little moon appears in it... but I think it might drive people crazy.

[Nitpicker]

You could make a video game out of trying to operate Cassini and have it know exactly when to take a photo so a certain little moon appears in it... but I think it might drive people crazy.

Isn't that what the good people who operate Cassini actually do? Sure it may not be available on a Playstation, and the GUI might need some work, but I bet it still drives them crazy.

[geckzilla]

I don't know. There are so many moons of Saturn I'm amazed that some don't get counted twice by accident.

[Nitpicker]

You could make a video game out of trying to operate Cassini and have it know exactly when to take a photo so a certain little moon appears in it... but I think it might drive people crazy.

Isn't that what the good people who operate Cassini actually do? Sure it may not be available on a Playstation, and the GUI might need some work, but I bet it still drives them crazy.

Imagine playing the game:

Press button ... wait 3 hours ... press another button ... wait three hours ... oops pressed wrong button, press another ... wait three hours ... oops I've lost my bearings ...

I expect the Cassini operators live almost entirely for pleasure.

[geckzilla]

I imagined them sending a whole set of instructions to be carried out for a day at a time... actually, the fact that I have absolutely no idea how they operate the thing bothers me. I guess I have something to do tomorrow.

[Nitpicker]

I don't know. There are so many moons of Saturn I'm amazed that some don't get counted twice by accident.

In the case of Janus and Epimetheus, they were originally thought to be the same moon until 1978, and not confirmed until Voyager 1 in 1980.

[Nitpicker]

I imagined them sending a whole set of instructions to be carried out for a day at a time... actually, the fact that I have absolutely no idea how they operate the thing bothers me. I guess I have something to do tomorrow.

I hope your guess proves closer than mine.

[Anthony Barreiro]

Ah ha! From the developers of SkySafari:
http://www.southernstars.com/support/re ... _data.html

•Planetary Satellites [ http://ssd.jpl.nasa.gov/?satellites ] - from JPL Solar System Dynamics. The authoritative source of orbital elements and physical parameters for the moons of the planets. SkySafari 4's planetary moon data comes from this site.

On the JPL page linked above, it says quite clearly:

Tables showing mean orbital elements are available. These should not be used for computing precise positions (please use HORIZONS instead), but instead might be used to gain a rough sense of a satellite's orbital shape.

The page for the JPL's HORIZONS system is:
http://ssd.jpl.nasa.gov/horizons.cgi

It looks like there is a also the "NAIF SPICE toolkit" available, to help third-party software developers produce accurate ephemerides (amongst other things):
http://naif.jpl.nasa.gov/naif/toolkit.html

But you'd have to be a certain kind of nerd to be interested in this stuff. [ Guilty.] I'd bet almost no amateur on Earth would be able to observe or detect these very small moons so close to the brightness of the rings, so the fact that SkySafari may not simulate all of their orbits as precisely as possible, is hardly the worst crime imaginable.

It's good to know the limits of the app. Thanks for that. And for doubting your original inference, I can only say ...

Click to play embedded YouTube video.