Ephemeris Accuracy

[revent]

CAP-Team,

What are you trying to say?
That that's a good value or bad value?

Which version of Celestia did you use?
Celestia v1.5.0 has had many improvements in the way it calculates trajectories.

The most accurate trajectories will be seen if you configure Celestia to use JPL's DE405 or DE406 ephemerides. See the Celestia WikiBook for details.

FYI, JPL released a new planetary ephemeris, DE-421, in Feb.
ftp://ssd.jpl.nasa.gov/pub/eph/planets/bsp/de421.bsp

[Nastytang]

Celestia/data/asteroids.ssc contains the definition for Toutatis.

Rather than modifying that file with a text editor, you should create toutatis.ssc in your Extras folder containing this text:

I`m !!! I wrote a reply and I don`t see it ??

Any ways Thanks Selden for the INFO !!

I agree it better to add to the extra files the to mod the main file to Celestia

[revent]

I think the orbital elements of most asteroids and comets can be updated with much more accurate numbers from NASA's Horizons system, but question is at what epoch do you generate them, since they're more accurate at a time you know it passes at a known distance from another body.

I.e. you know that a spacecraft or whatever passes asteroid x at a certain time at a certain distance. If you generate the ephemeris for that asteroid at that time, the ephemerides will be accurate.
But Celestia doesn't take into account any pertubations the asteroid might undergo into the future or past.

You can't use euclidean elements to model close approaches, because the 'roid is being perturbed during the whole period of the close approach. The orbit won't be the same before or afterward. Elements for the exact moment of C/A would get that point right, but would be wrong for any time before or afterward.

What you /can/ do is use the telnet or email interface to horizons to get a spice kernel for the asteroids orbit.

[CAP-Team]

I have experience in using Spice kernels in the program Xplanet (which works great there), and I've tried to use spice kernels in Celestia with less success.

For instance I tried to use the SPICE kernels for the Cassini mission, but I ended up replacing moons (or double moons) at different locations with wrong rotation speeds.

I hope the SPICE implementation in Celestia will improve much in the future.

For instance for a given object (planet, moon, spacecraft) have an option

Code: Select all

spiceorbit {
  beginning blabla
  ending blabla
  baricenter "sol"
  kernels {
    kernel1.bsp
    kernel2.bsp
  }
}


This would then replace any other orbit for that object during the given time.

I saw that some changes have been implemented in Celestia so that you can specify certain time intervals for objects, so that you can use a sampled orbit for a specific period without the need of creating multiple objects.

[chris]

I have experience in using Spice kernels in the program Xplanet (which works great there), and I've tried to use spice kernels in Celestia with less success.

For instance I tried to use the SPICE kernels for the Cassini mission, but I ended up replacing moons (or double moons) at different locations with wrong rotation speeds.

I hope the SPICE implementation in Celestia will improve much in the future.

You can use SPICE in Celestia already. The user volcanopele has created a complete recreation of the Cassini mission. It's used here: http://ciclops.org/view.php?id=3213&js=1

For instance for a given object (planet, moon, spacecraft) have an option

Code: Select all

spiceorbit {
  beginning blabla
  ending blabla
  baricenter "sol"
  kernels {
    kernel1.bsp
    kernel2.bsp
  }
}


This would then replace any other orbit for that object during the given time.

You can do that now--you need to use the Modify disposition with your object definitions so that you don't end up with duplicate objects. For instance, to replace the orbit of Tethys with an ephemeris in a SPICE kernel:

Code: Select all

Modify "Tethys" "Sol/Saturn"
{
    SpiceOrbit
    {
         Kernel "sat2000-2020.bsp"
         Target "Tethys"
         Origin "Saturn"
         Beginning "2000 1 1"
         Ending "2020 1 1"
         BoundingRadius 300000
         Period 1.8872
    }
}


I saw that some changes have been implemented in Celestia so that you can specify certain time intervals for objects, so that you can use a sampled orbit for a specific period without the need of creating multiple objects.

Right. You could set up a Timeline so that SPICE trajectory is used for an object over some interval of interest--say the duration of the Cassini mission--and the default orbit is used at other times. There's also new code in the SVN version of Celestia that lets you specify multiple kernels, for example:

Code: Select all

Kernel [ "hayabusa.bsp" "itokawa_1989-2010.bsp" "hayabusa_hp.tf" ]


The Beginning and Ending for the SPICE trajectory may now also be omitted, though you still need to specify them for the object itself.

--Chris

[CAP-Team]

If I understand correctly, this means you don't have to declare any object in the spice file with the spice kernel?

If I want to use just the Cassini ephemerides in the spice kernel, I only have to put the kernel data in the ssc file for Cassini, and not AND Cassini AND saturn AND moons etc?

[chris]

If I understand correctly, this means you don't have to declare any object in the spice file with the spice kernel?

If I want to use just the Cassini ephemerides in the spice kernel, I only have to put the kernel data in the ssc file for Cassini, and not AND Cassini AND saturn AND moons etc?

It depends on how the SPICE kernel is structured. If the trajectory of Cassini is defined with respect to Saturn, then you'll either need to have a kernel that contains to orbit of Saturn, *or* you'll have to redefine Cassini to be in a Saturn-centered frame instead of the current heliocentric frame. The problem is that SPICE can't compute the heliocentric position of Cassini without knowing the position of the intermediate object, in this case Saturn.

Typically, SPICE kernels have a mission trajectory divided into segments, each of which may have a different origin. For example, the kernel for the NEAR mission to Eros has segments that are geocentric, heliocentric, and Eros-centric. If you wanted to replay the whole mission, you need a SPICE kernel with the orbits for Eros, Earth, and the Sun. (Or, you could get clever with Celestia's Timeline feature, but it's a lot simpler just to get the right SPICE kernels.)

--Chris

[CAP-Team]

OK that's working alright when I try a spice kernel for the voyager 2 spacecraft.

Code: Select all

"Voyager 2" "Sol"
{
  Class           "spacecraft"
  Mesh            "voyager.cmod"
  MeshCenter      [ -39.39 -0.05 -30 ]
  Orientation     [ 90 1 0 0 ]
  Radius                   0.019
  Albedo                   0.1

  Beginning     "1977 08 20 15:33:00"
  Ending        "2049 12 31 00:00:00"

  SpiceOrbit {
    Kernel        "vgr2_st.bsp"
    Target        "-32"
    Origin        "10"
    Beginning     "1977 08 20 15:33"
    Ending        "2049 12 31 00:00"
    BoundingRadius    300000
    Period                 1.8872
  }
}


I can't draw the orbit though.

[Derek]

OK that's working alright when I try a spice kernel for the voyager 2 spacecraft.

Code: Select all

"Voyager 2" "Sol"
{
  Class           "spacecraft"
  Mesh            "voyager.cmod"
  MeshCenter      [ -39.39 -0.05 -30 ]
  Orientation     [ 90 1 0 0 ]
  Radius                   0.019
  Albedo                   0.1

  Beginning     "1977 08 20 15:33:00"
  Ending        "2049 12 31 00:00:00"

  SpiceOrbit {
    Kernel        "vgr2_st.bsp"
    Target        "-32"
    Origin        "10"
    Beginning     "1977 08 20 15:33"
    Ending        "2049 12 31 00:00"
    BoundingRadius    300000
    Period                 1.8872
  }
}


I can't draw the orbit though.

The "period "needs to be extended this shows the orbit from 01-01-2007 until 2049. I still don't understand why the orbit isn't drawn prior to 2007, although by running time back Voyager2 orbit can be tracked so it would appear that all the orbit info is in the file.?????

[revent]

The "period "needs to be extended this shows the orbit from 01-01-2007 until 2049. I still don't understand why the orbit isn't drawn prior to 2007, although by running time back Voyager2 orbit can be tracked so it would appear that all the orbit info is in the file.?????

Actually, the trajectory for Voyager 2 isn't really an 'orbit', as it's moving faster than solar escape velocity. That means that declaring a 'Period' for Voyager 2 is meaningless, as the quantity is actually undefined, but Celestia uses the value for 'Period' to determine how much of the trajectory to draw as an 'orbit'. For an object that doesn't have an orbit, Period should be either set to zero or not defined.