I had never heard of Fermi having any particular expertise in astronomy, so always wondered why he should have thought of this. But it has just occurred to me: he's there in Chicago in 1942 or whenever, he's about to make the world's very first atomic pile, they've worked out with pencil and paper that it's not going to blow up Illinois and set the atmosphere on fire, but nevertheless he's wondering to himself... "I wonder if this is why advanced civilizations don't last long enough to travel in space."