the efforts necessary to gather the fissile material
That's what the plutonium path is all about. It doesn't require a lot, and between the uranium ore from Africa they got when the captured Belgium, and the not as rich ore from Czechoslovakia, it was judged it might be enough.
You say it's "inconceivable", but it was indeed "conceived", considered to be possible, if perhaps unlikely, by lots of scientists and engineers working on the Manhattan Project. Who as it progressed had a better and better idea of exactly what was required. And who knew how long we dicked around with the concept before committing to it, and then putting Groves in charge.
There's also the danger of 20/20 hindsight. For example, the critical March 1940 Frisch–Peierls memorandum (https://en.wikipedia.org/wiki/Frisch%E2%80%93Peierls_memoran...) laid out the scientific path to success (fast fission) and estimated about 1 kg (2.2 lb) of U-235 was required. Later it was learned it was 52 kg, and plutonium-239 a much more modest 10 kg. Suppose the universe instead allowed 1 pound of Pu-239? Is that still "inconceivable"?
Was the V-2 effort "inconceivable"? It required a similar order of magnitude of effort, we knew more about the project because they had to test fly it, the threat was obvious and it did cause a lot of harm, and yet the "risk by air" was not enough to take it out prior to that harm.
The final detail is that even if low priority (you're not going to convince many it was truly impossible), the consequences of their getting it before it was too late for the Third Reich were ... well, it would have been existential for them.
The final detail is that even if low probability (you're not going to convince many it was truly impossible), the consequences of their getting it before it was too late for the Third Reich were ... well, it would have been existential for them.
That's what the plutonium path is all about. It doesn't require a lot, and between the uranium ore from Africa they got when the captured Belgium, and the not as rich ore from Czechoslovakia, it was judged it might be enough.
You say it's "inconceivable", but it was indeed "conceived", considered to be possible, if perhaps unlikely, by lots of scientists and engineers working on the Manhattan Project. Who as it progressed had a better and better idea of exactly what was required. And who knew how long we dicked around with the concept before committing to it, and then putting Groves in charge.
There's also the danger of 20/20 hindsight. For example, the critical March 1940 Frisch–Peierls memorandum (https://en.wikipedia.org/wiki/Frisch%E2%80%93Peierls_memoran...) laid out the scientific path to success (fast fission) and estimated about 1 kg (2.2 lb) of U-235 was required. Later it was learned it was 52 kg, and plutonium-239 a much more modest 10 kg. Suppose the universe instead allowed 1 pound of Pu-239? Is that still "inconceivable"?
Was the V-2 effort "inconceivable"? It required a similar order of magnitude of effort, we knew more about the project because they had to test fly it, the threat was obvious and it did cause a lot of harm, and yet the "risk by air" was not enough to take it out prior to that harm.
The final detail is that even if low priority (you're not going to convince many it was truly impossible), the consequences of their getting it before it was too late for the Third Reich were ... well, it would have been existential for them.