Ok, I found a way to do it. I added two variables and a sub event.
sub event does For Each Asteroid: add x and y to variables.
then there is another sub event with no condition that creates the particle at X/2 Y/2.
Is that the best way to do it, or is there a more compact way?
<img src="http://i.imgur.com/iQl8S6k.png" border="0" />