Celestia Forums

A bit of a noob, but i've got an idea

So i'm wondering how nasa plots out the trajectory of probes and etc. It would be fun to create an object, give it an intial velocity and have some control over it. Say let it have constant thrust like the ion probe. Or let it burst thrust. Thus, it would react to the forces of the Sun and planets, maybe even model the whiplash thing around planets for speed.


Also, does the Sol system orbit around the galaxy?

I like your idea, though first we would need to incorperate gravity effects into Celestia. I think being able to mess around with objects and gravity would be pretty fun... I wrote about it in a earlier post somewhere. In the meantime, try Mostly Harmless. It only affects the camera, but it's halfway there...

As for your question, in real life Sol, the planets, and every single star in the galaxy slowly revolves around the centre of the Milky Way. However, this isn't featured in Celestia, since calculating movement for all the stars in Celestia would probably end up melting your processor.

I was looking at voiger 1 and 2 the orbits are way off!!!
Is there a way to make the orbit turn, so you can sling shot around bodies.
ben experimenting with the start end dates make the orbit long and the stop the probe on that orbit and start it on the next planit. It workes ok but it could be better.
hope this is a beature in Celestia.

Auscreely

Auscreely,

Chris has provided updated trajectory files for the Voyager (and other) probes which should be fairly accurate. Have you looked at them? They're in http://www.shatters.net/~claurel/trajectories/

To first approximation, no object in Celestia affects any other object. They all just go their own merry ways as they've been told, no matter what other objects do. There's no such thing as gravity, for example. A planet close to a star can have a long period while a planet far away can have a short period.

In other words, for an object to "slingshot" around a planet, you have to describe the trajectory in detail. At the moment, Celestia only lets you do that using either "elliptical orbits" or "sampled orbits".

Using JPL's Horizons ephemeris system it's relatively easy to generate sampled orbits for any of the 90,000 objects in the solar system that it knows about. It solves the equations of motion numerically, even including the gravitational effects of some of the minor planets. Some of the documenttion hints that it can be persuaded to generate sampled trajectories for imaginary objects if they're properly described.

Generating sampled orbits for other planetary systems is not so easy right now. Someone would have to write an appropriate program, I suspect.

auscreely wrote:I was looking at voiger 1 and 2 the orbits are way off!!!
Is there a way to make the orbit turn, so you can sling shot around bodies.
ben experimenting with the start end dates make the orbit long and the stop the probe on that orbit and start it on the next planit. It workes ok but it could be better.
hope this is a beature in Celestia.

Auscreely

The next release of Celestia (which will be ready in time for the Galileo/Amalthea encounter) has more accurate planet orbits, plus I have a new Voyager package with more trajectory samples. Together, these two things mean an accurate recreation of the Voyager mission is finally possible in Celestia. Just hold on for a couple more days . . .

--Chris

Chris will the new version of Celestia enable one to position a spacecraft at a particular set of coordinates prior to rolling and engaging the spacecraft's engines? What I'd like to do is to specify a longitude and latitude of a planet together with an altitude above its surface. Once the spacecraft is re-positioned to these coordinates, I'd like to orient the spacecraft toward the center of the planet, pitch the ship's longitudinal axis up by a few degrees, accelerate and execute a grazing hyperbolic flyby with periapsis near the planet's atmosphere.

Peter,

This got long, so here's a quick summary:

Celestia 1.2.5pre6 can do what you want, but it's not trivial.

Orbits and rotations have to be predefined.
Orbits can be xyz or elliptical: translate from long/lat.
Rotations don't have xyz.
Use Beginning and Ending to make different objcets visible.

Long-winded response:

Bear in mind that Celestia only uses predefined movements. You can't change an object's trajectory or orientation in realtime as you can with a program like Orbiter. The way you phrased your question didn't make it clear if you realized this.

With an xyz trajectory (SampledOrbit), in principle you can cause an object to go anywhere you want, although creating that file may be a bit difficult. Alternatively, you can use an "EllipticalOrbit". You will have to convert the coordinate system from the longitude and lattitude that you mentioned into orbital parameters, of course.

The xyz capability hasn't been coded for rotation elements, unfortunately. If it becomes available, such an xyz script would make it possible to specify arbitrary orientations at particular times. Maybe Chris (or some other skilled C++ programmer) could be persuaded to add that feature.

Until that happens, an object's orientation is controlled by the rotation parameters that are defined in its .SSC file.

The available rotation parameters are

RotationPeriod
RotationOffset
RotationEpoch
Obliquity
LongOfRotationAxis
PrecessionRate

However, the new Beginning and Ending parameters are available in v1.2.5pre6 to control when objects are visible. You can define several different objects, perhaps in the same .SSC file. They can have the same object models and trajectories but different rotation parameters. With appropriate Beginning and Ending times, the different objects would seem to be the same object changing its orientation appropriately. Getting the objects to align correctly at the replacement times sounds like it could be a bit tricky, though.

I made a few simple tests replacing moons at specified times. They worked fine. Of course, that's easier than aligning a complicated model at the right orientation, but the principle's the same.

Does any of this help?

Thanks for the help Selden. I just started using 1.2.5pre6. For some reason I can't get the gravity to work by pressing F9. I did understand the point about predefined movements.

As soon as I get gravity to work, I'll calculate the orbital paramets for a hypothetical hyperbolic flyby and see how the close I can come to the planet's atmosphere.

I forgot to put my name onto the last message. It was from Peter3210.

Peter,

There's no gravity in Celestia, only predefined orbits. That's why xyz trajectories have to be defined when multi-body gravitational effects are important.

It sounds like you're confusing Celestia with Mostly Harmless, which Mark Griffith based on Celestia v1.2.4. I suspect Mark will merge the updates for 1.2.5 into MH when they're finalized. Mostly Harmless uses F9 to enable gravitational effects on the observer. In Celestia F9 does nothing.

Sorry.