You have to pick the one effected by the laser in the conditions or you'll effect all of the asteroids.
This only subtracts HP on the asteroid that is overlapping the laserbeam.
And this only reduces HP on the asteroid that is overlapping laser and the mouse cursor is over.
(Remember that the events above is reducing the HP with 1 per tick so if your game runs at 60fps it is -60HP/sec. If you add every 1sec or something like that in the conditions it will only drain 1hp/sec)
But you can do all kinds of picking so it only picks the object you want. This is just some simple examples.