//asteroid collision with shot script

with(other)
{
    //this destroys other (the shot that hit this asteroid)
    instance_destroy();
}

score += 10;

spawn_asteroid(size-1, scale/2, random(360));
spawn_asteroid(size-1, scale/2, random(360));

//if you need to destroy the base object, do it outside the } like this
instance_destroy(); // destroys the asteroid itself.