The atomic bomb on Hiroshima was 15 kilotons.
That should make a very visible crater from space.
I wonder if it struck glacial ice instead of land?
Can you imagine the tsunami wave that would make if it struck the ocean off a major city?
I wonder if it exploded in the atmosphere like the fairly recent Russia meteor?
And I'll guess at least few American military officers thought the Early Warning Station was under nuclear attack!
You’re assuming “they” had sufficient warning to “do something” about the impact. Assuming also that “they” were able to detect, and track the detect, and predict the crater location of the inbound object.