I think your code always re-position the asteroids when they are out of the layout , am I correct ? Make a variable that will change to 1 when the asteroids enter the layout and that will block the asteroids respawn ...
On start of Layout
If AsteroidsEnterred = 0
-- Spawn asteroids and position them
If Asteroids | Is on screen
-- Set AsteroidsEnterred = 1
Here you go , Cheers !