How Bright Must an Asteroid Be to Catch an Amateur Astronomer's Eye?

[chasrob]

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.

 

[Drakkith]

I think it would depend mostly on luck. Someone would have to be imaging that area of the sky and then notice that they picked up an asteroid afterwards. These are easily missed if you aren't looking for them. So I don't think you could give a time frame of when it would be detected. Also, it would not remain stationary in the sky, as the asteroid isn't headed straight for us, but will have to have it's orbit meet ours.

 

[chasrob]

Yes, it will have some motion in the sky, as it's heading toward where the Earth will be(in a month after the launch. But, AIUI, asteroids were detected by their streaks across photo plates, at least before today's dedicated searches. Amateurs must number in the thousands around the world, many with superb equipment, taking CCD pics, that would make a 9 or 10 mag object stand out. Hell, the amateur variable star observers probably observe every square degree of the sky down to 10 or 12 every night.

Many comet hunters search "memorized" areas, in collusion with others, as I understand it. I would think even a fixed star of mag 9 or 10 would be noticed by them.