I have updated the model (same link as above, but I'm giving it again below). The "secret" is now MUCH lower polygonal, to help getting a better frame rate. Now, it's only a small gem inside some room.
The corridors have been updated as well. I think it's a bit more interesting to navigate inside. The asteroid is also better looking from the outside (less dullish).
The goal here is to enter the asteroid, find the jewel, and go out. Without passing by a wall. There's no collision detection in Celestia, so if you suddenly see the star field, while you were inside the asteroid, then it's because you have "hit" a wall. As I said, you'll need to turn up the ambient light, and use the navigation keys ("a" to accelerate, "s" to stop, keypad to move around). I suggest using a velocity of about 300 m/s (the asteroid has a 10 km radius). Beware, there are two dead ends inside.
I'm not satisfied with the texture mapping, which is distorded and blurry inside, and I don't see how to map it in a better way.
http://nho.ohn.free.fr/celestia/Cham/Astrocave.zip
"Well! I've often seen a cat without a grin", thought Alice; "but a grin without a cat! It's the most curious thing I ever saw in all my life!"