In a SF story I'm working up, an asteroid is launched(long story made short) from its orbit in the asteroid belt in a collision course with earth. It is 30km wide or so, one of 10,000 such planetoids in the belt, IIRC. From the viewpoint here on earth, it is in the milky way of Gemini and Orion. Visual magnitude about 15, 16, at its distance. Dim. This is an alternate history in our equivalent of 1990 or so; before the interest in NEOs and the umpity dozens of surveys conducted to track them down. As it approaches, it'll get brighter and brighter, natch'. My question is, how bright would it have to be before its noticed by astronomers; professional, or (most likely) the many amateurs out there? Visual magnitude, I mean. 8 or 10? Before naked eye visibility surely. Your best guess from what you know about the pros and their observatories and the many aficionados would be much appreciated. I would think it would have very little lateral motion if its on a collision course, and if photographed by a survey, would appear to be just another star in the star clouds. So it would most likely be noticed by a lucky amateur and his 17" dobsonian.