The forum software doesn't always seem to link internally correctly. Try downloading it directly
from here. Word of warning: it's still a very early build and has a few bugs, but if you look at the LRS in the initial system, then jump to another system (the singularity-looking things are jump points), do an LRS, then jump back to Sol, you'll see that the original Nebulas were removed on the first jump, and replaced on the second.
Asteroids, Mines, and Nebulas get destroyed differently from named objects-- on exiting the system, I do this:
<destroy_near type ="nebulas" centerX="50000" centerY="0" centerZ="50000" radius ="75000"/>
<destroy_near type ="asteroids" centerX="50000" centerY="0" centerZ="50000" radius ="75000"/>
<destroy_near type ="mines" centerX="50000" centerY="0" centerZ="50000" radius ="75000"/>
This builds a sphere centered in the middle of the sector and destroys all the unnamed items, and then on re-entering the system, you create them again as usual. Note that I haven't factored in whales or space monsters yet, but it's a start.