the Asteriod is in progress...Im learning how to work with rocks in Bryce...sorry its taking so long...
If you are making floating rocks (like an asteroid field). One interesting method is to determine the number of frames to use (unfortunately this will take a little trial and error, although I almost never go beyond 15).
Next add one extra frame for smooth looping.
Keyframe the first frame and the last,
For a single (slow) orbit, go to the middle the scatter the rocks using the random scatter button on the top.
Keyframe that middle frame.
Go to next to last frame keyframe that one and then delete the last frame. Since it was only there to make the next to last loop smoothly with the first frame.
To do double loop (faster orbits and twice the movement).
Keyframe the 1st and the last frame
keyframe the middle (to the same value)
Randomize the rocks in the 1/4 and 3/4 points.
Keyframe them
keyframe the next to last
delete the last.
The extra frame (which is removed) is a simple way to get smooth animations for any movement in Bryce.