The two men were the first to calculate that an atomic bomb would require about 1 lb of the isotopeuranium-235. Before it had been assumed that the bomb itself would require many tons of uranium, implying that it was theoretically possible, but not a practical military device. An earlier letter to President Franklin D. Roosevelt, signed by Albert Einstein (but written by Leo Szilard), had suggested it may need to be delivered by ship but could not be small enough to drop from the air.
The memo was written in two parts. The second was an explanation of the science supporting their conclusions. The first was an elegant and comprehensive outline of the implications of their calculations. It included a proposal that the best defence against such a weapon would be to develop one before Germany did so. In a few short pages these two scientists had anticipated the policies of deterrence which would later shape Cold Wargeopolitics.
The memorandum opens with:
Strictly Confidential
Memorandum on the properties of a radioactive “super-bomb”
The attached detailed report concerns the possibility of constructing a “super-bomb” which utilizes the energy stored in atomic nuclei as a source of energy. The energy liberated in the explosion of such a super-bomb is about the same as that produced by the explosion of 1000 tons of dynamite. This energy is liberated in a small volume, in which it will, for an instant, produce a temperature comparable to that in the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the centre of a big city.
In addition, some part of the energy set free by the bomb goes to produce radioactive substances, and these will emit very powerful and dangerous radiations. The effect of these radiations is greatest immediately after the explosion, but it decays only gradually and even for days after the explosion any person entering the affected area will be killed.
Some of this radioactivity will be carried along with the wind and will spread the contamination; several miles downwind this may kill people.
