I think the "Bullet" behaviour should work pretty well for an asteroid if you also animate the asteroid so it appears to tumble.
A combinaton of rotation and a tumbling animation is probably the best.
However if you use the "Rotate" behaviour oc Construct2, the facing angle of the asteroid will change all the time and combined with bullet this will make the asteroid go in an arc (or a circle depending on speed) which is probably not what you want.
Perhaps you could move the asteroids by updating their coordinates in the "each tick" section but I don't know if that will slow things down a lot. I'm pretty new at Construct 2 myself. I'll experiment some more and get back to you!