Celestia Forums

I've split this question from the: Lagrange Point addon thread, as that thread is getting a bit long.
Please use the voting buttons (and or add to this thread) to give your feedback.

Background:
In the last version of this addon, I implemented the Lagrange-Points as ReferencePoints, which means that they are never visible and cannot be labelled. This is fine for positioning spacecraft, but not so good for visualization purposes.
(the old DEMO version which did have coloured spheres for visualization, is now deprecated).

1. With the improved facilities for marking objects in CELX, I'm thinking of writing a script to mark and label the lagrange points. This will be far better than the old DEMO version.
Would you find this script useful or not?

2. A question has also been raised regarding whether to include unstable trojan points in the addon ie. Pluto-Charon system. (I'll repeat that question and my response in the second post of this thread)

If you are currently using the addon and would find the script useful, please give feedback.
I would also welcome any additional comments on how you are specifically making use of the addon.
(It was originally designed primarily for the purpose of placing spacecraft in the vicinity of lagrange points.)

If there's little or no feedback, I will assume everyone's pretty happy with it as is (or no-one's using it at all), so I probably won't waste my time on any enhancements.

Regards
CC

ajtribick wrote:The fact that L4 and L5 are unstable at mass ratios greater than 24.96 doesn't mean they do not exist as stationary points. In fact since the L1, L2 and L3 points are always unstable and yet are included, it seems inconsistent to exclude the L4 and L5 points when they too are unstable.

You do have point... it does seem inconsistent, however it's not. I'll explain my reasoning...
It's known that the collinear Lagrange points are unstable, and that the triangular/trojan points are generally stable [edit: except when the mass ratio of the primary/secondary bodies is less ]than 24.96, hence by excluding the unstable trojans, a user of this addon will always know that all included trojans are stable ones. (Hence, it's consistent )
If I was to include the unstable trojans, then a user would never know (without doing the maths themselves) which ones were stable, and which one's weren't. (although, FYI the only known unstable trojan points in the solar system are those in the Pluto-Charon system)

I've no objection to including the unstable points, but IMO we would need a way of distinguishing the unstable trojans from the others. I would be happy to do a bit of work on this if you really want them included. They could perhaps be labelled as "unstable".

Let me know if you really want them, however it does seem to me that unstable trojans will be of little practical interest for parking spacecraft etc, as they offer no advantage over the more conveniently located collinear points.

Then again, not all Trojan points that meet the mass ratio criterion are stable, since the solar system is NOT a set of isolated 2-body pairs. An object in one of the Trojan points is thus subject to perturbations from the other objects in the solar system. For example, the Earth-Moon L4 and L5 points are unstable thanks to the perturbations of the Sun, the other planets, etc.

If you are going to eliminate Trojan points based on stability, then all such unstable Trojan points should be eliminated or you are misleading people about the stability of the vicinity of the points.

Alternatively keep all the Lagrangian points in, and leave the stability assertions for anyone who wants to do dynamical integrations...

Here and in the following post(s) are some previews of popular locations in the Solar System with Lagrange Points marked and labeled by the planned scripts.
Please let me know any thoughts.

In the vicinity of Saturn...

Cool addon!
Displaying Lagrange points could be a nice feature if the script could be run at contextual level... (perhaps a feature for the interactivity we need with the next UI?)

ElChristou wrote:Cool addon!
Displaying Lagrange points could be a nice feature if the script could be run at contextual level... (perhaps a feature for the interactivity we need with the next UI?)

I will be including the ability to label only the lagrange points of the currently selected object. (if that's what you're after)

Chuft-Captain wrote:

ElChristou wrote:Cool addon!
Displaying Lagrange points could be a nice feature if the script could be run at contextual level... (perhaps a feature for the interactivity we need with the next UI?)

I will be including the ability to label only the lagrange points of the currently selected object. (if that's what you're after)

This would be a useful feature. The main problem with the Lagrange point add-on right now is that there are too many objects. Being able to show just the Lagrange points for the selected object solves that problem completely.

--Chris

chris wrote:

Chuft-Captain wrote:

ElChristou wrote:Cool addon!
Displaying Lagrange points could be a nice feature if the script could be run at contextual level... (perhaps a feature for the interactivity we need with the next UI?)

I will be including the ability to label only the lagrange points of the currently selected object. (if that's what you're after)

This would be a useful feature. The main problem with the Lagrange point add-on right now is that there are too many objects. Being able to show just the Lagrange points for the selected object solves that problem completely.

--Chris

I think it's still useful to be able to see them all at once, and therefore when and where they make close approaches to other lagrange-points or celestial bodies. With your involvement with the ESA, you're no doubt aware of spacecraft mission planning involving the use of L-points to minimize fuel use (Interplanetary Super-Highway).

I think it would be a safe bet that the Cassini mission planning team factors in a judicious amount of L-point hitch-hiking in order to save fuel.
EDIT: (on further thought, I suspect they rely on gravity assists)

Although I'm making no claims that my addon is useful for those purposes.

I really think it would be useful to only display the Lagrange points of selected objects.
And with that I mean, the points of one or more objects (if possible with the scripting).
Would it perhaps be possible to show lets say the "normal" moons only and on the other hand the "minor" moons only. And then showing them all together, and then selecting only one moon and getting its points?

Anyway, is this addon getting into the offical release? An addon like this is IMHO definitely a candidate for that.

Best regards,

Guckytos

chris wrote:

Chuft-Captain wrote:

ElChristou wrote:Cool addon!
Displaying Lagrange points could be a nice feature if the script could be run at contextual level... (perhaps a feature for the interactivity we need with the next UI?)

I will be including the ability to label only the lagrange points of the currently selected object. (if that's what you're after)

This would be a useful feature. The main problem with the Lagrange point add-on right now is that there are too many objects. Being able to show just the Lagrange points for the selected object solves that problem completely.

--Chris

Chris,
This would also be a useful feature for moon-orbits, as they suffer from the same problem as the lagrange points...the sheer number of moons at Jupiter and Saturn can leave the orbit lines as difficult to sort out as a tangled mess of spaghetti (even if only minor or major moon orbits are enabled). This situation will only worsen as new moons are continually discovered at an increasing rate.

Perhaps an option could be added to show only the orbit of the selected object (regardless of which options are specified in the Render menu.
As far as I know, the "O" key is currently case-insensitive, so this function could be assigned to either the upper or lower case of the "O" key. (eg. lowercase "o" could display orbits as specified in the render menu, and uppercase "O" would display only the orbit of the selected object regardless of the Render menu settings).
In addition, although this is inspired by the situation with the numbers of moons, I can see no reason why this functionality should not be a generic function common to all objects.

I think there are many times a user might like to quickly see the orbit of a single selected object without having to go to the render menu and enable all other orbits of the same class, so a toggle like this would be useful.

Guckytos wrote:Anyway, is this addon getting into the offical release? An addon like this is IMHO definitely a candidate for that.

It's unlikely this addon will ever become part of an official release for the following reason...

Although by design the addon inherits the accuracy of the associated orbits within Celestia, the current source of mass information (Wikipedia) used to calculate the lagrange point locations does not live up to Chris' standards of reliability and scientific credibility.
It's possible that more reliable or reputable sources might be found for future upgrades of the addon, but this is unlikely to happen in the short term.

Regards
CC

Chuft-Captain,

I think the other posters' consensus is about right. To summarise:

- Unlabelled L points are too anonymous and cluttered.
- Showing L points for just one orbit is far better than for all orbits, especially because the L points should always be labelled.
- There's no point discriminating between stable/unstable. In any case:

- Even bodies orbiting unstable L points have some timescale of persistence, even if calculating the timescale is way beyond most people.
- Getting masses for all relevant bodies could be a pain, Celestia's kinematic model never really needs them.
- The solar system has no such example of the mass ratio being exceeded anywhere[edit], except Pluto and Charon, as you rightly point out below - tsk![/edit].

I'm not sure spacecraft actually use L points to travel through, but they are still interesting. Plus several new missions are using L points as 'home' (SOHO has done L1 for some time), there's STEREO (L4 and L5), and soon Herschel and Planck at L2, followed by GAIA and the James Webb Space Telescope.

Spiff.

Spiff,

I recently split off this thread from the original thread: http://celestiaproject.net/forum/viewtopic.php ... &sk=t&sd=a
That thread contains a lot more discussion about the dynamics, so some of your q's may be answered there.
Also see here: http://www.physics.montana.edu/faculty/ ... range.html

The idea was that this thread would be strictly for discussion of the new scripts for labeling of lagrange points, rather than discussion of the dynamics. Perhaps it was a mistake to split the thread, as those discussions are tending to be repeated here anyway.

To address specifically a couple of your q's:

Spaceman Spiff wrote:First, let's be sure what the mass ratio limit appies to. If the Sun is our primary body of mass M0, Earth is our secondary body of mass M1 and we have a trojan at L5 of mass M2, then the relevant mass ratio to stability is M1/M2, right?

The relevant ratio is M0/M1. The mass of the 3rd "captured" object (whether it's a trojan asteroid or a spacecraft) is irrelevant to this calculation as long as it is insignificant in mass when compared to the primary and secondary bodies. eg. The mass of a trojan asteroid is tiny compared to the mass of the Sun and Jupiter; The mass of a spacecraft is tiny compared to that of the Earth or the Moon.
This is also why a number of tiny moons of Saturn are able to occupy stable orbits in the triangular lagrange regions of a couple of the much larger Moons -- Tethys (Telesto and Calypso), and Dione (Helene and Polydeuces) respectively.


For a stationary observer at Tethys-L4 for example, Telesto would appear to orbit the lagrange point...ditto for the other 3.

If you have the Lagrange Points addon installed, then you can see this effect demonstrated in Celestia by CLICKING HERE

Spaceman Spiff wrote:Secondly, I thought the trojan's orbit would be stable if M1/M2 is greater than 24.96, not in exception to it.

The Sun's mass divided by Jupiter's mass (M0/M1) IS greater than 24.96.

Spaceman Spiff wrote: - The solar system has no such example of the mass ratio being exceeded anywhere.

Pluto-Charon is the example.

Spaceman Spiff wrote: Plus several new missions are using L points as 'home' (SOHO has done L1 for some time), there's STEREO (L4 and L5), and soon Herschel and Planck at L2, followed by GAIA and the James Webb Space Telescope.

Yes, I'm aware of them. If and when XYZ trajectories become available for these missions (I'm not sure if SOHO's trajectory was ever published), I will be very interested to see the relationship between their trajectories and the respective lagrange points in my addon.

Hope this helps,
CC

At risk of re-opening an old thread....

I was originally planning to release this visualization tool around Christmas 2010, and then for various reasons all Celestia related stuff was put on hold.

Well, now in the last couple of days I've been able to spend some serious time scripting for Celestia, which has allowed some good progress on this tool.

So here is a presentation demonstrating the planned features and user interface, submitted for your valued and considered feedback . . .



Download the Presentation in the format that suits you:

. . . . . OpenOffice Presentation (ODP)

. . . . . Adobe Reader (PDF)

Please comment on any issues you might anticipate, such as choice of key assignments, other suggestions, etc.
I think I've allowed for the simple contextual approach suggested by the 2 Chris's, while still retaining the bells and whistles that I like (being able to see it all).
Anyway, any further comments are welcome, so that any potential issues can be addressed before release.

CC

PS. I guess I'll have to upload this as a Powerpoint version as well for those that don't have OpenOffice. ( it's free, you know! :hint: :hint: ).

Wow. I was teaching Lagrange points to the Classical Mechanics class last week, I could have shown this. Still, I showed them the cloud of Trojan asteroids and I think they liked them better than the demonstration of the equilibria in the rotating frame and their stability. I will check and let you know about your script, Chuft.

G

Thanks Guillermo,

I made some changes last night to the menu and user interface.

New menu design looks something like this: which I think looks a bit better than before,
and HELP screen now will stay up as long as required, which I think is better.

Also, thinking of removing the ( - ) Hide option, or at least assigning to a different key, mainly because I tend to use "-" 's in the names of many objects in my addons and I suspect other people do so as well. Having the key mapped makes it un-availble when searching for objects in the ENTER menu, so I think unfortunately it's not a good choice (although it was perfect in term of the implied meaning - subtraction).

What's the current status of your extraction script and servers? Last post I saw about it, there were some issues with either your server or the database server.

CC

Chuft-Captain wrote:What's the current status of your extraction script and servers? Last post I saw about it, there were some issues with either your server or the database server.

Oh, I have made no changes to the asteroid extraction script. It does what it does reasonably well, so there is no real need to improve it.

Regarding my Lab server, i believe it's working most of the time.

Hi,

I lost this Lagrange Point add-on after a recent replacement of my old PC. Tried to download it from this thread:
http://shatters.net/forum/viewtopic.php?f=6&t=11670&st=0&sk=t&sd=a
but it is a broken link. Anybody can help by pointing me to the right link?

Hi Joe,

Sorry but the sitesled.com site where I hosted the original addon is down much of the time these days. It used to be more reliable. You can keep trying it (occasionally their servers are working - If you can visit sitesled.com, then the download link should work as well).
I'm planning to re-release the original addon together with the viewer tool when I finish that, so I'll get a more reliable host next time ... (but not before I get over the rather nasty flu that I've had for the last week).

I was hoping for some feedback from people on the viewer tool, before I finalize it for release, but nothing forthcoming so far.

CC (...short for *cough* *cough* )