Celestia Forums

Hi folks,

yep, I finished it this weekend. Four major impact events of the 20th century:
- Tunguska 1908
- Sikhote Alin 1947
- Jackson Lake 1972
- Shoemaker-Levy 1994
Of these, Tunguska and Sikhote Alin are true impacts of small bodies on earth. Jackson Lake misses but pierces through earth's atmosphere, of Shoemaker-Levy I set up a simple model of the last three orbits and the breakup at the last pericenter encounter (13 of 21 fragments included).

Download at http://www.geocities.com/diane_va/download

I also uploaded it to the Celestia Motherlode.

Have fun ridin' the cannonball,

~Medusa.

I tried all of them and could follow them. But I didn't see the cms models, only transparent black spheres with the atmospheres around them.
I have Celestia 1.3.2.

Brendan

Medusa,

In the Readme on the Web page, you mention that the gravitationally modified Jackson-Lake Bolide trajectory can't be shown by Celestia. Actually, it can!

If you provide an XYZ trajectory that describes the path of a body, Celestia can display a non-eliptical orbit. It will take some work to create that trajectory, though.

What you have to do is use JPL's Horizons ephemeris service and provide it with Keplerian orbital parameters for a portion of the orbit that's remote from the Earth. Horizons can be told to do the appropriate gravitational calculations and to send you the resulting list of heliocentric xyz trajectory coordinates. With minor modifications, that list can be used by Celestia to display a continuous trajectory.

Abbreviated information about how to use Horizons is available at http://ssd.jpl.nasa.gov/horizons.html. The full documentation can be downloaded using their telnet and e'mail interfaces.

A copy of the e'mail command file that I've used to get xyz trajectory information is at http://www.lepp.cornell.edu/~seb/celestia/horizons-template.txt.

I'm having no trouble seeing the textures or models.....but the main problem I'm having, is witnessing Celestia crash every time I'm about to get to the 'fireball' stage of each impactor.

I don't seem to be able to see an 'atmosphere' around any of them at all.

regards,
TERRIER

Nice work Diane,nice to see a little animation in Celestia,I especially like the Shoemaker-Levy impactor,cheers Jestr

Brendan wrote:I tried all of them and could follow them. But I didn't see the cms models, only transparent black spheres with the atmospheres around them.

Hi Brendan: try to place all the mesh files in a 'models' subfolder in the 'Impacts' folder. It will work fine.
Medusa, this is a very nice addon, thanks, Jeam

Hi all in here,

time for some replies...

@Brendan: about the textures and .cms problem: should be not present if all files are in the right Celestia directories. Check out the INSTALL file.

@Selden: thanks for that hint. I didn't mean that "it can't be done in Celestia" in my README but just I couldn't do it that fast. Maybe I'll put in xyz coordinates for Jackson-Lake in a later version (though there maybe problems cause the body is so close to earth. Depends on type and accuracy of the interpolation procedure the JPL horizon service uses -- at least if it does trajectory interpolation and not true gravitational force imfluence computation).
Possibly it was a plain translation problem, I'm not a native speaker.

@Terrier: I had the same problem when I made the "atmosphere" higher than now. On my system, if the atmosphere of a body exceeds 4 body radii, Celestia crashes without any error mesage. Couldn't find out what cases it, if I do a rudimentary backtrace provided by KDE, it seems to come from some OpenGL driver but there are also lots of unreferenced hex addresses.
Maybe you can do some experiments about the atmosphere height, possibly it works on your system for "smaller atmospheres" (even if I guess it doesn't look that good if the "shine" becomes very small).
After all, it's just cosmetics.
BTW, does your Celestia installation show the faint blue "comas" I made for the SL9 fragments? There should happen the same crash if it's really an atmosphere problem (at least it was this way on my system).

@Jestr: thanks for the compliment....

~Medusa.

Medusa wrote:BTW, does your Celestia installation show the faint blue "comas" I made for the SL9 fragments? There should happen the same crash if it's really an atmosphere problem (at least it was this way on my system).

Hello Medusa: yes, i can see the faint blue coma (very interesting improvement) ...sometimes, and not for all the fragments: pointing some different fragments at different times, Celestia crashes without any message. Earth Impactors work fine for me, even the brief illumination. Jeam

I put in the correct places,but when I tried to follow Tunguska until your impact, the following error happened:
The instruction in "0x0bff50ab" make reference to the memory in "0x1401a00c".The memory couldn?t be read.

Besides,is this a tutorial or a addon ready to go?I didn?t understand.I make no modifications

Selden,

Can JPL's Horizons ephemeris service account for atmospheric drag? From what I've read, the Jackson Lake bolide's mass was estimated based on its measured deceleration in the atmosphere which implies, to me, that the deceleration is enough to ruin any JPL Horizons calculation, unless it can account for such effects.


More info:
http://www.maa.agleia.de/Comet/Other/1972.html

Unfortunately, although Horizons does include corrections for atmospheric optical effects near the horizon (for alt-az ephemerides, for example) I can't find any mention of drag effects.

Sorry.

Medusa wrote:
Maybe you can do some experiments about the atmosphere height, possibly it works on your system for "smaller atmospheres" (even if I guess it doesn't look that good if the "shine" becomes very small).
After all, it's just cosmetics.
BTW, does your Celestia installation show the faint blue "comas" I made for the SL9 fragments? There should happen the same crash if it's really an atmosphere problem (at least it was this way on my system).

~Medusa.

Firstly the good news..... I am now able to see (close-up) all the impactors run their courses to the end, with much smaller atmosphere settings.......
.....unfortunately the bad news is that the only atmosphere setting I seem to be able to use is 0.0

I've never been able to see the blue "comas" on the SL9 fragments. Celestia crashes when I try to see these up close too.

BTW I thought I'd take a look at the error report which you can send to microsoft. It contained 43 modules and was the size of an encyclopaedia.

regards,
TERRIER

Hi all here,

I guess the "Atmosphere" problem is a bit larger than I thought.
I first performed a lot of experiments to check out if it generally works with Celestia. Well, it did not. As I mentioned, at my machine the limit of the atmosphere was 4 planet radii. From a physical point of view, a true atmosphere of a normal planet never can be that high (at least not at densities higher than in a vacuum tube). There could be kind of philosophical discussion if the coma of a comet can be viewed as a very extended atmosphere but I drop that here.
I used it as a trick to get some desired view on the screen, something that was well outside of the normal parameters of the program but worked after some checks. The normal case for a software product of Celestia's complexity would have been it simply doesn't work this way so the impactors addon would have come without any atmospheres.

Nevertheless, the atmosphere trick works well with my Celestia installation if I keep the height below 4 radii and the body isn't that irregular (that's the reason I created .cms files for all of them). From the KDE crash report, I suspect the error to come from the OpenGL driver. As far as I see, transparent objects are handled on a very low level close to the graphics hardware if possible. Celestia is a very sophisticated software which is intended to run on a lot of different platforms, so we shouldn't wonder if some of is ahve problems which never arise for others.

I will lookup the *exact* versions of my libraries needed by celestia and post it here next time, maybe it helps in some error diagnostics.

~Diane.

Diane,
I don?t undertand why you didn?t answer me.Do you have any idea of why I am receiveing this error message(above)?Can you tell me if the Celestia Impactors is a complete addon or a do it yourself addon?
I wonder if it?s possible that the error message have to be with lack of dsc or stc entries,for example.

Daniel,

Please work through all of the examples in http://www.lepp.cornell.edu/~seb/celestia/addon-intro.html.
Do not take any shortcuts.

Then you will able to recognize the different kinds of Addons and how to reorganize them so they work well.

The files in Diane's Impactors Addon need to be moved into the correct folders. The Addon will not work if you just restore it into your \extras\ folder.

I've managed to reproduce it with a simple example, so I've reported the Atmosphere crash in the Bugs forum.