Asteroid on collision course

In a story I'm working up, an asteroid is launched(long story made short) from the asteroid belt in a collision course with earth. 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. The planetoid itself is rather large, with an H of 10 or so.  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) you folks and other amateurs out there, like comet hunters. Visual magnitude, I mean. 8, 10? Naked eye visibility? Your best guess from what you know about observing, and the professional astronomers' capabilities with their big scopes would be much appreciated.

I would think it would have very little lateral motion if its on a collision course(its headed to arrive one month after "launch", and where the earth will be then), 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.

Thanks for your help.