Revised orbital/rotation parameters for Gaspra, Ida & Dactyl

[granthutchison]

By setting the orbits of Gaspra and Ida to match the dates of their flypasts by Galileo, and by giving them correct orientations for their rotation axes and prime meridians, I've come up with Celestia encounters that appear to be pretty close to the real thing.

Code: Select all

"Gaspra" "Sol"
{
   Class "asteroid"
   Mesh "gaspra.3ds"
   Texture "asteroid.jpg"
   #Texture "gaspramosaic.jpg" #Phil Stooke
   Color   [ 0.52 0.47 0.42 ]
   BlendTexture true

   Radius 9.5

   EllipticalOrbit
   {
   Epoch           2448559    #1991 Oct 29 - Galileo encounter
   Period          3.2837     #average
   SemiMajorAxis   2.2096348  #at epoch
   Eccentricity    0.1738752  #at epoch
   Inclination     4.0975771  #at epoch
   AscendingNode   253.445592 #at epoch
   ArgOfPericenter 129.045896 #at epoch
   MeanAnomaly     280.769206 #at epoch
   }

   RotationPeriod       7.042073
   EquatorAscendingNode 109.59
   Obliquity            69.28

   RotationOffset       238.865

   Albedo 0.1
}

"Ida" "Sol"
{
   Class "asteroid"
   Mesh   "ida.3ds"
   #Mesh   "ida2.3ds"
   Texture "asteroid.jpg"
   #Texture "idamosaic.jpg" #Phil Stooke
   Radius 45

   EllipticalOrbit
   {
   Epoch      2449228      #1993 Aug 28: Galileo encounter
   Period          4.8417       #average
   SemiMajorAxis   2.863731     #at epoch
   Eccentricity    0.043109     #at epoch
   Inclination     1.137110     #at epoch
   AscendingNode   324.586055   #at epoch
   ArgOfPericenter 113.017101   #at epoch
   MeanAnomaly     131.594945   #at epoch
   }

   RotationPeriod       -4.633632
   EquatorAscendingNode  172.77
   Obliquity         23.04
   RotationOffset        0.54

   Albedo 0.24
}

"Dactyl" "Sol/Ida"
{
   #Data for Dactyl taken from:
   #Petit et al.: The Long-Term Dynamics of Dactyl's Orbit
   #(Icarus 1997: 130; 177-197)
   InfoURL "http://www.lpl.arizona.edu/~hurfordt/research/papers/Icarus130.pdf"

   Class "asteroid"
   Texture "asteroid.jpg"
   Mesh "roughsphere.cms"
   Radius   0.7

   EllipticalOrbit
   {
   Epoch            2449228.2028 #1993 Aug 28 16:52:05
   Period           -0.96534 #stable 5:1 resonant orbit (Petit et al.)
   SemiMajorAxis    83.5     #stable 5:1 resonant orbit (Petit et al.)
   Eccentricity     0.13     #stable 5:1 resonant orbit (Petit et al.)
   LongOfPericenter 230      #stable pericenter at 90?E Ida longitude (Petit et al.)
   AscendingNode    90       #VERY approximate - chosen to place Dactyl north
                                  #of Ida's equator, as seen at time of Galileo encounter
   Inclination      8        #estimated at 7-9 degrees (Petit et al.)
   }

   RotationPeriod  -23.16816 #enforce synchronous rotation
   RotationOffset  57        #place prime meridian facing Ida

   Albedo 0.2
}


The Gaspra encounter took place on 29 Oct 1991, Ida on 28 Aug 1993.

The orbital data for Dactyl represent just one of many options compatible with the brief observation period - various resonant orbits are possible, and since they are non-chaotic (and so can be approximated with a simple ellipse) one of them seemed like a good choice for Celestia. I chose a 5:1 resonance with a pericentre over Ida's convex side (pericentres elsewhere are unstable); but some day I'll also model the other most likely resonance, at 9:2, with the pericentre alternating between the convex and the concave sides!

To see these at their best, you really need an Ida shape model that is correct - the Celestia version is mirror-reversed. I've made one called ida2.3ds available at http://www.lns.cornell.edu/~seb/celestia/hutchison/index.html#3.0, but it's a poor thing at present - I've been unable to export my original VRML model to Celestia with any smoothing, so Ida is horribly faceted. If anyone out there can load it into some modelling software and turn on automatic normals, please do so - and let me know so that we can replace the existing specimen!
Also zipped in with the shape model are a couple of public domain photomosaic maps by Phil Stooke - gaspramosaic.jpg and idamosaic.jpg - these look distinctly odd when laid on to the asteroids, since they are composed of a jigsaw of hires, lores and absent data. But if you then fly past with Galileo, suddenly the layout of the mapped areas makes sense.

Feedback gratefully received, as ever, and thanks as usual to Selden for providing the web space and his own time to put these files up.

Grant

[praesepe]

Hi Grant!

First of all, congratulations for the new orbital parameters for Gaspra, Ida and Dactyl, now the encounters are amazing!

I've been playing around with your Ida mesh and edited the normals in order to smooth the model; Now i think is pretty correct, take a look:



I'm sending the model to you via e-mail

[granthutchison]

That's great, praesepe - the smoothed model looks very good.
Your revised version should be replacing my clunky original at Selden's website very soon now.
Next, to get Eros sorted out ...

Grant

[chris]

Nice work on both the orbital parameters and the new model. I'd like to make these changes part of the Celestia 1.2.6. Praesepe, the texture on Ida looks great; where did you get it? Is it derived from the Phobos texture, or is it actually from Galileo images?

An improved Eros will be neat to see . . . With some of my adjustments to the sampled orbit code (double precision, cubic interpolation), we should be able to recreate the NEAR mission.

--Chris

[granthutchison]

Praesepe, the texture on Ida looks great; where did you get it? Is it derived from the Phobos texture, or is it actually from Galileo images?

Chris, I got it from Phil Stooke's Small World Atlas - the original texture is at
http://www.ssc.uwo.ca/geography/spacemap/icylmos2.jpg
I just resized and flipped it to allow for Celestia's treatment of textures (same with the Gaspra texture).
It's not much good for general use, because it's a mixture of high-res, low-res and no-res, so Ida looks very peculiar when you're free to fly around it. But it is very interesting to watch the Galileo approach using this texture on Ida, because you can see Ida's rotation bring progressively higher resolution texture into view as Galileo approaches and sweeps past, while the no-data areas are all in shadow during the encounter.

Phil Stooke has a number of interesting public-domain textures available at the above site, and at his Cartography of Non-Spherical Worlds. I've resized, flipped and longitude-adjusted quite a number by now, and was planning to ask Selden to post a collection of them in a zip file on his website, RSN.

Grant

[praesepe]

Although the shadow and low ressolution areas of Phil Stooke's maps, I think that they could be really interesting for modeling purposes in Celestia. In conjunction with Grant's revised orbital parameters, Celestia recreates almost perfectly the view of Ida from Galileo, so new revisions accompained with nice textures could increase the realism and accuracy of this great program

[granthutchison]

Praesepe's smoothed (and therefore hugely improved ) version of my Ida model is now available, together with the Ida and Gaspra textures, from Selden's site at:

http://www.lns.cornell.edu/~seb/celestia/hutchison/#3.0

Grant