Celestia Forums

Hello All.
I am new to Celestia & this forum but I was wondering if anyone has made a version of Celestia with all the Jovian moons ? The moons in particular that I am interested in are Epimetheus & Janus ( moons #5 & 6 ,respectively, of Saturn. ) These two moons are co-orbital & I would LOVE to see the mechanics of this visualised in Celestia !!! :D

BTW : Great Site & excellent product !!!

Hi, I'm new here myself ,,,
Go to the main Celestia site, then to the link Add-ons.
In there you will find
"Minor Moons of the Giant Planets
This file contains data for 37 small moons of Jupiter, Saturn, Uranus, and Neptune. To install this file, just download it and copy it into the extras directory of your Celestia installation. "

I just did it a short time ago! and both wee moons are in there
Good program init!

That is odd, since I made the reply (above) to 'Anonymous' the Forum Index has made me the Author of the thread ! Strange!

Cheers Malcolm :)
I had a look at the Add-ons section but when I saw that it said ...

These add-ons will only work with Celestia 1.2.0! If you don't already have this version of Celestia, get it now from the download page.

I foolishly assumed that they would only work for that specific version of Celestia. I have installed the ssc file now & it works nice. Very nice indeed ! I still don't understand the physics of co-orbital moons but it is very interesting !!! :D

BTW : I started this thread while not logged in ... I think thats why it was originally "Guest" then changed to Anonymous. :/

Hi Demon,

>I had a look at the Add-ons section but when I saw that it said ...
>These add-ons will only work with Celestia 1.2.0!
>I foolishly assumed
snip

Yeh! It made me pause as well, but being a carefree foolish impetu, err
I carried on regardless

>I still don't understand the physics of co-orbital moons

Nope, me neither,,,,except,, if they are Lagrange (sp) (trojans) with some other (more massive?) object in the same orbit but,
gosh its more than 30y since I tried to understand orbital dynamics !
but you say there are only these two in this orbit ?

It is 4am local, not the right time for deep understandings,
c y'all later

Well, actually even though it is now 4:15am ! Do I distantly recollect,,whilst sipping my glass of Laphroaig before going to bed,,,
something about wee rocks in close adjacent orbits slingshoting each other as they pass ? Not absolutly a singular orbit.
I suppose,though,that they dont slingshot each other I suppose the energy needed would be supplied by the central massive primary ?
Dunno, vague memories is all,,
g'night

Here is an except from the page that I read about Janus & Epimetheus ... & have been intrigued ever since !

Janus and Epimetheus are "co-orbital". The orbital radii of Janus and Epimetheus differ by only 50 km, less than the diameter of either. Their orbital velocities are thus very nearly equal and the lower, faster one slowly overtakes the other. As they approach each other they exchange a bit of momentum the end result of which is to boost the lower one into a higher orbit and to drop the higher one to a lower orbit. They thus exchange places. The exchange takes place about once every four years. The orbital data given here is as of the time of the Voyager encounters.

What I don't understand is ... do the moons actually come very close to each other when the orbit exchange takes place ? If they do ... what mechanism stops them from gravitating together & coliding !?
These are the mysteries that make astronomy so interesting !! :D

I think the Pro-Epi co-orbital system is a nice example for a physics class. It's a strange result of their orbital dynamics... they both orbit their parent planet but in a sense they also orbit each other. An orbit is what happens when you are falling toward something but 'miss' it. That's what happens to these two moons when they approach; the co-orbital system survives because they 'miss' each other by exchanging inner and outer positions... It can be explained in terms of angular momentum, but it's a somewhat alien way of thinking and a bit counter-intuitive.
The angular momentum of each moon is about the same because they have similar masses and similar orbital velocities at a similar radius from their primary. Their masses don't change of course, but when they interact their orbital velocities and orbital radii can change. The point to remember is that if the velocity goes up, the radii must go down and vise versa (because momentum is a conserved quantity).
As the inner one approaches the outer from behind (because it's velocity is greater), it is drawn outwards by gravitation. However it has more or less the same angualar momentum as before and so, as the radius of it's orbit increases, it's orbital velocity decreases. Ie, by being drawn toward the outer one it is slowed down. The outer one experiences the same thing but in reverse: by being drawn inward it's radius decreases and so the ratio of radius to velocity shifts toward velocity. It shifts 'down' in radius and 'up' in velocity, and so starts powering away from the one that was previously the inner, faster moon, which is now of course the outer, slower one. The difference in velocity means that this newly inner moon will itself catch up with the newly outer one in 4 years when the process will repeat and the two swap orbits again.
It only works because they are more-or-less the same, small mass, and the velocity difference small as well. If one was significantly bigger then the other then the smaller one would be hurled around too much by the other and/or they'd crash.

The point is, Celestia can't simulate this because it uses Keplerian orbital elements to model a stable, eliptical orbit, which would be what the moons do if moons were only gravitationally attracted to the primary planet, and not to other moons as well. Because the Pro-Epi(methian) phenomena is a 3-body gravitational interaction (ie not just 1 planet and 1 moon), it's difficult to model without using numerical methods (ie there's no analytic solution that will give you a nice, computer-friendly formula to describe how they move).

Which is a shame, but there it is.

PS Prometheus means 'foresight' and Epimetheus means 'hindsight'. So half the time the names are very appropriate and half the time they're very wrong indeed...

Cheers ogg ! Very well explained. I feel I have a better understanding of the co-orbital system now :) & I will stop sitting in front of Celestia, centered on Janus spinning round & round ... waiting for Epimetheus to come hurtling in. :D

BTW : Do people who frequent this forum have an IRC channel for Celestia or astronomy in general ? Because often when I am at work ... I just want to chat about astronomy ... sad but true ! :]

'mornig all.

Thanks ogg, very nicely described, I think I even understood that
Better than my 'slingshot' !

So now I am wondering :-
With them being so close to such a massive primary is tidal drag (or do I mean tidal friction?) going to disturb them and cause them to coele,,err coales,, err, crash into eash other ?
Or are they too small for tidals to be important in the 'near' future ?

Woops! Spot the embarrissing error. The co-orbital moons are *Janus* and Epimetheus, not *Prometheus* and Epimetheus, as I wrote above. Just substitute 'Janus' for 'Prometheus' in everything I wrote. That's what you get for rambling from memory and not reading the question properly I guess. I should have guessed that the names (given the lagging-leading thing) were too ironic to be true. Perhaps they should be re-named? I think my way is better.

Prometheus is the inner, 'shepherd' moon for Saturn's thin, outer 'F' ring. The outer one is Pandora and together they keep the ring in line, so I was confusing one kind of interacting moon-pairs with another.

BTW, tidal forces *do* seem to play a role in the Saturn system wrt Prometheus and might account in a large part for the rings etc. It seems to be an open question though about what's going to happen to any of the moons because of this. See the following URLs for example...

http://www.jpl.nasa.gov/saturn/hst19.html
http://www.aas.org/publications/baas/v3 ... 99/148.htm

it's a really messy, dynamic system from the sound of it.

Nicely done! Thank you all for a great orbital physics lesson!

BTW - do Aussies refer to the rest of the world as "up overs"?
Cheers!
Ron A

Well, I'm actually from New Zealand; new to Canberra and all things Australian are a mystery to me. My mum always complained that NZ is far more 'down under' then Australia, but I'm not sure that we should be trying to claim that title...
Besides, our entire defense policy is based on trying to ensure that other nations remain ignorant of our existence, so perhaps I'd better shut the hell up.