The June 1970 print edition of the Bulletin of the Atomic Scientists provided "Some Recollections of July 16, 1945" by Gen. Leslie Groves, the military head of the Manhatten Project. July 17 was the date of the first successful atomic bomb test.
One of the things he recalls is that the danger of radioactive "fallout" from the bomb had only been understood as a potential problem for a few months before the test:
The hazard that I feared the most was that of radioactive fallout on the areas over which the radioactive cloud would pass. This had not been considered for too many months as it was only at the turn of the year that Joseph Hirschfelder had brought up the possibility that this might be a serious problem. I learned later that the possibility of this danger had been indicated in the British Maud report, but I had been unaware of the existence of this report. It was this fallout hazard that caused us to be fearful of exploding the bomb when rain was likely, since we thought rain would be certain to bring down an excessive fallout over nearby areas. We also could not ignore the old tales that heavy battle cannonading had sometimes brought on rain, although we knew of no scientific basis for any such phenomenon. I have never believed in ignoring such tales no matter how unreasonable they seemed, for sometimes they are unexpectedly I justified by events.
In planning for the test, one consideration was that until the thing actually blew up, they didn't know how strong the blast might be. The possibility was even consdered that it might ignite the earth's atmosphere.
As a side issue, I would point out that in my planning, I had provided for what I considered to be impossible: that the destructive force of the bomb would be 'many times our maximum estimates. I had left with Mrs. Jean O'Leary, my administrative assistant in Washington, a number of messages, any one of which would become effective upon the use of its code word from me. These provided for varying provisions for the declaration of martial law over extended areas. Fortunately, as I expected, none of these eventualities took place, but it was like too many things in the Manhattan Project - we were dealing with unknowns outside the rwealm of man's experience and we simply had to try to imagine everthing that migh happen.
Groves was very aware of the political climate in which the test took place and of the credibility problems that delays can bring to any large project:
I was most disinclined to postpone the test because of its effect on the issuance of the Potsdam Ultimatum. The President was expecting to hear from me through Secretary Stimson as to whether the bomb was a success or a failure. Both of them had been told that I expected the test to take place by the middle of July and that we would be ready to drop the bomb as soon after July 31 as flying conditions permitted. I knew that President Truman wanted to issue the ultimatum long enough ahead of this date so that the Japanese would have time to reply. As it turned out, he was not only able to issue it on time, but also to make it much more vigorous than would have been the case if he had not known of our success.
My third point of concern was the effect of a test delay on our schedule of bombing Japan. Our first combat bomb was to be a U-235 one, and while a successful test of the plutonium bomb without the complications of an air drop would not be a guarantee, it would be most reassuring. Moreover, it would give credence to our assurances to the President as to the probable effectiveness. A misfire might well have weighed heavily on the argument by some, particularly Admiral Leahy, that we were too optimistic and that we should wait for a successful test. After all, this was the first time in history since the Trojan Horse that a new weapon was to be used without prior testing.
And Groves recalled the hopes of those in the Project that the new weapon would help bring the war to a speedy conclusion. After the successful test, he writes:
As for me and the other members of the Project, we were now able to devote all our efforts to what had always been our goal, the prompt ending of the war by the use of nuclear weapons.
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