Cometary orbits

[godsowndevil]

Hey all.
I'm from india and im a research assistant at the Indian Planetary Society. I'm currently working on a project which deals with cometary orbits. Is there any simulation software available which deals with orbits? Or some other open source softwares which can be modified to suit my purposes? Can celestia work in that case?

[selden]

I'm a little confused: Celestia can draw cometary orbits, including hyperbolic orbits, although Ellipticity exactly equal to 1 doesn't work.

Is is just that you don't know how to translate the orbital parameters into terms that Celestia understands? A brief description of how to translate the parameters used in IAU notices is in http://www.lepp.cornell.edu/~seb/celestia/transforming_ephemeris.html

Does this help?

p.s. Celestia is a 3D visualization program. It does not simulate gravity, so it can't calculate how planetary encounters change cometary paths. Orbital trajectories have to be precalculated for Celestia to be able to draw them. You can use JPL's Horizons ephemeris service to calculate heliocentric trajectories that Celestia can display.

See http://ssd.jpl.nasa.gov/horizons.html for more information about Horizons. A template that I often use for getting trajectory information about known objects is at http://www.lepp.cornell.edu/~seb/celestia/horizons-template.txt. Supposedly you can give Horizons an initial Keplerian orbit and it'll calculate the object's trajectory for you. I've never tried to do that, though.

[godsowndevil]

what i am looking for is something which can simulate the orbit of any body which is put withing the gravitational interaction range of a planet or the sun. Like suppose if an object is placed at some distance from earth, would the body orbit the earth like a satellite, or go around like a comet. Also if a third body like Jupiter for example, has its closest approach to earth, will the gravity of Jupiter perturb the oribit of the body. i know this sounds complicated but thats just cuz of my communication skills.. lol.. i hope i have been able to convey my problem.

[selden]

Celestia can't do that, unfortunately. I think you need a program to simulate gravity. Tony Dunn's GravitySimulator may be more appropriate.

See http://www.orbitsimulator.com/

[godsowndevil]

thanks a lot.. i'll check it out and see if it suits my purpose..

[godsowndevil]

The gravity simulator software is quite good... only problem is that i cant modify it as per my requirements.. i can only add one main reference body and see the simulations of the other bodies around it... i need to see the orbit if some other body is brought in to perturb this config. but im sure it'll be helpfull for other projects. is there any other gravity simulator available which i can modify as per my requirements...

[tony873004]

...i need to see the orbit if some other body is brought in to perturb this config...

You can do that. Cosmic "bowling" is something Gravity Simulator is pretty good at doing. If you give a more detailed description of what you're trying to accomplish I can tell you how.

On the Graphics Options window is a button labeled F for floating mode. It unlocks the center object and allows you to see its orbit around the system barycenter and how it is affected by the additional body. (I think that's what you mean?)

[tony873004]

...Supposedly you can give Horizons an initial Keplerian orbit and it'll calculate the object's trajectory for you. I've never tried to do that, though...

By trajectory do you mean state vectors (position x,y,z & velocity x,y,z)?

[selden]

It'll at least calculate a sequence of heliocentric positions for a specified interval. I don't think it'll calculate changing orientations, though.

[godsowndevil]

what i want to do exactly is. take a body, jupiter for example. put a large number of smaller bodies at a specific distance around jupiter and then study the motion of these smaller bodies. as these bodies go around jupiter, they will orbit with jupiter around the sun. suppose jupiter comes to its point of closest approach to earth. though earth is small, its gravity may give small perturbations to the smaller bodies. i want to see whether such interactions with earth or sun or other planets will affect the orbit of the smaller bodies around jupiter.

[godsowndevil]

and then this simulation can be generalised to involve bodies around many planets at the same instant... so a more complicated simulation. atleast i hope that this can be done. as i'm not very good with programming, i'm looking for simulators which can do this.. or else i'll have to write my own program which will take loads of time.

[selden]

Oh.
Ouch.

I trust you realize that the effect of the gravitational field close to a highly elliptical body like Jupiter is not easy to calculate. It can't be simplified to a point mass located at Jupiter's center of mass.

I know Horizons takes into account the gravitational effects of all of the major bodies in the solar system, including the 4 largest asteroids. I don't know if it takes into account the ellipticity of the gas giants. It might.

[godsowndevil]

yeah... i know its quite difficult to do that... that is why i was looking for some simulators on the net.. though now looks like i'll havta write my own code... hopefully at some time in the future i'll be successfull...

[tony873004]

You can do that in Gravity Simulator.
Like Seldon said, Jupiter is not spherical, but flattened due to its high rotation speed. But approximating it as a sphere will still give you pretty good results. And yes, Horizons does take Jupiter's shape into account. Horizons takes EVERYTHING it can think of into account including General Relativity.

Withouth even making your own simulation in Gravity Simulator, you can run a simulation that comes with the download called "fullsystem.gsim". It contains the entire Jovian system: every moon of Jupiter known to man as of a few months ago. It also contains Earth, so you can watch Earth's closest approach. You can even run the simulation over again after deleting Earth to see what affect it caused.

Although the close approach by Earth will indeed have an effect on Jupiter's moons, the effect will be so small that it will be hidden many places to the right of the decimal point. (i.e. insignificant). The gravitational solar tide is by far the most dominant force perturbing the moons of Jupiter, or any planet for that matter. The Sun induces an effect called the Kozai Mechanism on the Jovian system which prevents moons from wandering into polar orbits. The Sun also sets a maximum distance that objects can orbit Jupiter (or any planet). You can compute this distance without simulating it by using the formula for the Hill Sphere (Wikipedia has a good article).

You can also make up your own planetary systems to put together "what-if" situations. If Eath orbited closer to Jupiter it could have an effect, until Jupiter gave Earth the heave-ho from the solar system. Jupiter had done a remarkable job of clearing its vicinity of the solar system of stray objects. The exception is the Jupiter trojans.

There's a simulation that comes with the Gravity Simulator download called "sl9.gsim". It begins a few years before the comet Shoemaker Levy 9 slams into Jupiter in 1994. It shows you the close approach that broke the comet apart as well as the collision. Even though Gravity Simulator treats Jupiter as a sphere, this is still accurate enough to take Horizons-generated numbers from 4 years prior to the collision and propogate them forward, simulating both the close encounter and the collision.

I hope this helps.

[godsowndevil]

i know this sounds quite silly... i created about 3-4 objects around jupiter... but im not able to get them to perform the simulation.. i tried lots of things like enter the time and the period of simulation... but nothing worked... i dunno wots wrong... could u suggest anything.. ??

[tony873004]

i know this sounds quite silly... i created about 3-4 objects around jupiter... but im not able to get them to perform the simulation.. i tried lots of things like enter the time and the period of simulation... but nothing worked... i dunno wots wrong... could u suggest anything.. ??

Pay attention to the Semi-major axis. If you leave it at its default of 0 then you are creating your objects inside of Jupiter where they instantly merge with Jupiter. Do you objects at least exist after you create them? They should be the last objects in the Focus Object combo box.

Also, the semi-major axis defaults to AU as its units. So its possible that your objects are too far from Jupiter. Check the kilometers option instead, and make sure that they're somewhere in the range of 1 million to 10 million kilometers and this should be fine. You can leave all the other values at their defaults.

[godsowndevil]

thanks... i'll try that

[godsowndevil]

yeah... now i got the orbits... i realised that i was putting distances either too far away or too near..
i wanted to know this... when i used auto save... the data txt file that was created... was too complicated to comprehend... is there any way to make it more readable???

[tony873004]

i wanted to know this... when i used auto save... the data txt file that was created... was too complicated to comprehend... is there any way to make it more readable???

I agree it looks a bit cryptic, but it's meant to be imported into a spreadsheet program where you can add titles, etc.

If you use Excel, make sure to open it as a comma delimated file.

Then, column by column the data means:

Object name
Semi-major axis
eccentricity
inclination
argument of periapsis
longitude of ascending node
mean anomoly
object's x position
object's y position
object's z position
object's x velocity
object's y velocity
object's z velocity
blank column
Next object... etc...

The last 6 values are useless by themselves. You need to subtract one object's vectors from another object's vectors or these numbers don't represent anything real.

After you've got this stuff imported into Excel, (Microsoft Works Spreadsheet that comes free with most computers works well too. And it figures out for you that it's comma delimated data), its really easy to make all sorts of cool graphs with the numbers.