Ok, this sounds funny and interesting, but manhole covers are made of cast iron, and as such are brittle. A nuclear explosion quite possibly would turn that cast iron disk into dust. It could have, perhaps, been made of cast ductile iron, but it would have then at least broken apart into chunks. The energy dissipation of destroying a large chunk of iron would have cancelled most of its potential velocity. Very likely, the reason it wasn't seen in the second frame is because it was dust, not because it was in orbit.