New models of Adrastea, Metis & Thebe

[JackHiggins]

Hi all,

Since I couldn't get them anywhere else, I decided to make my own models of Jupiter's minor inner moons Adrastea, Metis & Thebe. These models are based on fairly blurred & fuzzy images from galileo.jpl.nasa.gov but they're still better than asteroid.cms...

You can download from my site http://homepage.eircom.net/~jackhiggins/celestia/

Here are some screenshots:
Thebe:



Adrastea



and Metis




(And yes, I know Thebe and Adrastea look very similar...) Anyway- Enjoy!

[billybob884]

Jack, where the binary asteroids and tiny moons on your site ever fixed? I know there was some sort of problem with them a while ago and I never really found out if they were or not. BTW, great job on the moons!

[JackHiggins]

Thanx!

Jack, where the binary asteroids and tiny moons on your site ever fixed?

The problem in that was only with the tiny moons part, and yes, I fixed that ages ago!

[billybob884]

The link for hte tiny moons file doesn't work

[JackHiggins]

Ok sorry bout that, i forgot to type the _ in the filename. It works grand now.

[ogg]

Very nice addon! I've got a question though... are the orientations of the moons now accurate? I would've thought that tidally locked moons with elongated shapes like these ones would have their long axes pretty much pointing toward the centre of Jupiter. I don't have any evidence to base this on except some assumptions about tidal forces... am I assuming too much?

[JackHiggins]

Yes, these 3 moons are all tidally locked to Jupiter, but the only one that I could find any orientation info for was Thebe, which is correctly oriented. The other two are just positioned in the way that I happened to make the models.

Also, today there's a slight change to the Thebe model, as one of the craters was a bit smaller than it should have been.

[granthutchison]

Jack, where did you get the orientation information for Thebe?
The Thebe model behaves rather oddly - the texture wrap seam is about 15 degrees offset from the model's long axis, and it doesn't follow the convention so far used in Celestia, in which the texture wrap seam is aligning with the X-axis. I'm guessing you've reorientated the model to force a particular orientation in Celestia, but this does mean that now the zero meridian of any applied texture map doesn't align correctly.
Your other two models follow standard Celestia practice, and it's easy enough to align their zero meridians and long axes with Jupiter, which seems like a good default position until we know otherwise:

Metis

Code: Select all

RotationOffset 6.2

Adrastea

Code: Select all

RotationOffset 54.8

Grant

[JackHiggins]

Grant,

According to the description of one of the images on photojournal.jpl.nasa.gov, the large crater on Thebe is on the side that always faces away from Jupiter, so that's how I got that orientation. Yes, I did reorient the model in Anim8or to face that way in Celestia.

I've changed it back 15 degrees again now, (I didn't actually realise it was standard practise to have the long axis centred along the x axis!). So, what would it's rotationOffset be now, if the cratered side was made to always face away from Jupiter? (Thanx for the other two btw!)

[granthutchison]

Not so much that the long axis has to align with the x-axis, but that the x-axis should be the axis of the model that's going to point at the parent planet (because that's how Celestia sets up its initial alignment) - and that axis usually turns out to be the long axis. You also want the seam in the applied texture to be aligned with the x-axis, so that the prime meridian of any applied map comes out with the correct orientation.
Thebe is now well-behaved, as far as I'm concerned, and the calculated offset of

Code: Select all

   RotationOffset   29.5


gets the prime meridian of its applied texture pointing at Jupiter. However, I have a suspicion that the orientation of the model is now 90 degrees away from the way you intended! If that's so, you need to rotate the model through 90 degrees, but then rotate the applied texture backwards through 90 degrees so that it maintains its current alignment.
(Sorry, I seem to be turning into the Alignment Police ... You should of course feel free to produce models and textures aligned any way you want! I'm just keen to keep the Celestia core package "pure", and sometimes that enthusiasm rubs off inappropriately on other people's work.)

Grant

[JackHiggins]

Oh, ok I get what you mean now! Yes, the orientation was out by 90 degrees, (according to what JPL say anyway...) It looks fine at

Code: Select all

RotationOffset   119.5

The texture for this one is only the asteroid texture though, and I doubt galileo will return many more hires images of thebe, so there's really no need to rotate this texture by 90 degrees...

I'll keep it in mind though!