I have often seen the ISS in the evening sky. Its elevation is about 200 miles up. It is a fairly large object measuring more than 310 feet wide and 225 feet long. It is about 150 feet in height. It is too big to fit into a football field at 310' x 225'. It is quite the sight. A couple of weeks ago after I worked Richard I saw the ISS by stepping outside. It was a great dark morning and the ISS probably had the brightness on the solar scale of 1. The sun is -27 and the brightest object for us on earth. The -27 being the bright side ; as you go to a more positive number the object is dimmer.
AO 51 is about 25 cm on a side. That is about the size of a sack of groceries. I do not know the exact height of AO 51 but I know it is higher than the ISS (200 miles). I think AO 51 is about 250 to 300 miles up.
I believe it is impossible to get a visual sighting of something that small at that height. There are thousands of objects in orbit around our earth. A bag of groceries at 300 miles up would take more seeing ability than a human set of eyes can provide.
Jim W9VNE