I'm writing an SF story in which a main belt asteroid is diverted into a collision course trajectory with earth (long story), and unknown to us here on earth.
Its H magnitude is 12. As it nears earth, I would assume it gets brighter in V--apparent magnitude. Considering all the eyes on the sky these days, how bright, apparent mag V, would it get before being discovered here on earth? 9 or 10 seem reasonable?
And if you could give its distance in AU, at discovery, that would be super (trying to find out how much time we have left ;) )!