Celestia Forums

Hello, I'm new here, this is my first attempt at doing anything. You have to start somewhere. The SSC file is pasted below.
I basically pulled the data from JPL's browser and the rotation period from http://earn.dlr.de/nea/099942.htm. My first question is do people usually source their data, is it legal not to? I wasn't sure so I played on the safe side and put it in. Second off this data works much better than the plugin at the Motherlode (mine gets within the moon's orbit, 269,590 km to be exact), but it still doesn't put it into the error range that is the stated error range for the 2029 pass as stated on wikipedia at http://en.wikipedia.org/wiki/99942_Apophis Is there any other location that gives more precise data such that I can get it within this error range? Also is there any more advanced features for more advanced orbits than just a single elipse. Can we put in separate orbits for different Julian dates such that it slowly changes between them? For example, I'm guessing this error comes in from the fact that this data is for the current date and not for the date for the close approach and it's close passes with other orbital bodies shifts its orbit slightly such that it moves closer, and NASA (and others) takes these into effect. So for example if we gives 20 or so orbits over several years at any time the date will be much closer. Is there a feature to do this other than straight single "EllipticalOrbit" settings in .ssc files (and yes I know about defining xyz for them but that would be quite a large amount of data for one asteroid.) It could also come from how exactly Earth's orbit is being calculated in 2029, although I'm guessing you guys have that defined pretty exactly.

Thanks in advance,
Ergzay

# This file contains the orbital elements and some other data for the 99942 Apophis (2004 MN4) Near-Earth Asteroid (NEA)
# Author Matthew Lindner on October 14, 2008.

"2004 MN4" "Sol"
{
Class "asteroid"
Mesh "asteroid.cms"
Texture "asteroid.jpg"
Radius 0.27

EllipticalOrbit
{
Period 0.885920179879516
SemiMajorAxis 0.9224221637574083
Eccentricity 0.1912119299890948
Inclination 3.331425002325445
AscendingNode 204.4451349657969
ArgOfPericenter 126.4064496795719
MeanAnomaly 254.9635275775066
Epoch 2454800.5
}
Albedo 0.33
RotationPeriod 30.5376
}

# Data retrieved from
# JPL Small-Body Database Browser (http://ssd.jpl.nasa.gov/sbdb.cgi) and
# Database of Near-Earth Asteroids (http://earn.dlr.de/nea/099942.htm)

I forgot to add. For some reason for their calculations they used Gaussian Years in the JPL database for this object. You can check this by seeing the values for Period given by doing 'a^1.5' and dividing the period by it to give the year length in days and it turns out to match Gaussian Day closer than any other type of day. I had no clue what the year value was till I compared it to several off of wikipedia. http://en.wikipedia.org/wiki/Gaussian_year This is what I divided their period length by to get the term into years which is what Celestia goes by.

Celestia does not provide an automatic way to smoothly move from one disjoint trajectory to another. The Addon creator has to provide the intermediate positions.

Celestia's default planetary ephemeris isn't accurate enough for what you're trying to do. (It's a tradeoff, since more accuracy requires more computation or larger datasets.) You have to obtain accurate xyz ephemerides for all of the objects involved in a flyby. Celestia can use JPL DE405 or DE406 ephemerides, SPICE kernels, or xyz or xyzv trajectory files created by another program.

Probably the easiest way to have Celestia display accurate positions is the last: use JPL's Horizons ephemeris server. Its Web site is at http://ssd.jpl.nasa.gov/?horizons

You can send Horizons an email request for a file of heliocentric xyz or xyzv(1) positions for any solar system object. Some very simple editing is required in order to transform the format of that file into what Celestia wants. A sample email message is available at http://www.lepp.cornell.edu/~seb/celest ... mplate.txt

Detailed descriptions of how to use Spice, DE405 or xyz trajectories are in the Celestia WikiBook at http://en.wikibooks.org/wiki/Celestia#C ... g_Celestia

____
Note:
1. The use of xyzv trajectories requires Celestia v1.6.

[edit]
If you do want to use only elliptical orbits to display the flyby, it would be best to use parameters defined for the time of the flyby itself. They can be requested from Horizons. They'll only be accurate for that time, though. Gravitational influences will make the object travel away from that ellipse at other times.
[/edit]

selden wrote:Celestia does not provide an automatic way to smoothly move from one disjoint trajectory to another. The Addon creator has to provide the intermediate positions.

Celestia's default planetary ephemeris isn't accurate enough for what you're trying to do. (It's a tradeoff, since more accuracy requires more computation or larger datasets.) You have to obtain accurate xyz ephemerides for all of the objects involved in a flyby. Celestia can use JPL DE405 or DE406 ephemerides, SPICE kernels, or xyz or xyzv trajectory files created by another program.

Probably the easiest way to have Celestia display accurate positions is the last: use JPL's Horizons ephemeris server. Its Web site is at http://ssd.jpl.nasa.gov/?horizons

You can send Horizons an email request for a file of heliocentric xyz or xyzv(1) positions for any solar system object. Some very simple editing is required in order to transform the format of that file into what Celestia wants. A sample email message is available at http://www.lepp.cornell.edu/~seb/celest ... mplate.txt

Detailed descriptions of how to use Spice, DE405 or xyz trajectories are in the Celestia WikiBook at http://en.wikibooks.org/wiki/Celestia#C ... g_Celestia

____
Note:
1. The use of xyzv trajectories requires Celestia v1.6.

[edit]
If you do want to use only elliptical orbits to display the flyby, it would be best to use parameters defined for the time of the flyby itself. They can be requested from Horizons. They'll only be accurate for that time, though. Gravitational influences will make the object travel away from that ellipse at other times.
[/edit]

Where exactly do I get Celestia v1.6? Everyone talks about using it and the SVN, but unlike most open source program's websites download links are no where to be found. What's more is you cant use a search on the forums for 1.6 because it doesn't like to search by numbers. Also if I use the sample for my template, what format does that return the numbers in? xyz or xyzv?

Ergzay wrote:Where exactly do I get Celestia v1.6? Everyone talks about using it and the SVN, but unlike most open source program's websites download links are no where to be found. What's more is you cant use a search on the forums for 1.6 because it doesn't like to search by numbers. Also if I use the sample for my template, what format does that return the numbers in? xyz or xyzv?

Hi there.

Assuming that you use Windows, the way to get a beta version of 1.6.0 is to forst get Celestia 1.5.1 and then take a look at this thread: viewtopic.php?f=4&t=12221
and follow the instructions.

Hope this helps you.

Building Celestia from the source code of its svn archive on SourceForge is described in the Celestia WikiBook at http://en.wikibooks.org/wiki/Celestia#Development

The email template mentioned above causes xyz values to be returned.

Guckytos wrote:

Ergzay wrote:Where exactly do I get Celestia v1.6? Everyone talks about using it and the SVN, but unlike most open source program's websites download links are no where to be found. What's more is you cant use a search on the forums for 1.6 because it doesn't like to search by numbers. Also if I use the sample for my template, what format does that return the numbers in? xyz or xyzv?

Hi there.

Assuming that you use Windows, the way to get a beta version of 1.6.0 is to forst get Celestia 1.5.1 and then take a look at this thread: http://celestiaproject.net/forum/viewtopic.php?f=4&t=12221
and follow the instructions.

Hope this helps you.

Ah, no, using Mac OS X. But I'm an experienced linux user as well, as in I compile SVNs and open source stuff all the time for Mac OS X and Linux, just prefer Mac OS X for personal use. So either linux or Mac OS X instructions will suffice. I can figure it out from there.