I wonder at what certain projected area onto Earth would an object be viewable from the naked eye?
Some random google result tells me the smallest objects visible to the naked eye are approximately 0.1mm long, let's assume this means 0.01mm^2 area. Presumably this is with the object very close to you, i.e. on Earth's surface. The radius of the Earth is roughly 6.4e6 meters, so this gives 2.4e-22 sr. In real life you'd obviously have other considerations like light given off by the object and whatnot, but this might work as a very crude estimate.