J. Robert Oppenheimer: Memorandum on the Radiological Dangers of a Nuclear Detonation

Explanation and Analysis of the Document

Thomas Farrell, chief of staff to General Leslie Groves, was the second-ranking military officer in the command chain of the Manhattan Project (the project's name reflecting the fact that its headquarters had once been in New York City). This memo from Oppenheimer seeks to inform Farrell about possible dangers to military personnel from direct radiation and from radioactive fallout in the vicinity of a detonation of the “special bomb.” Oppenheimer also notes the importance of “meteorological effects,” such as temperature, wind, and especially rain in determining where and how soon the radioactive cloud rising from the explosion would return to the ground.

Likened at the time to gas warfare, which had recently been internationally outlawed because its use was considered immoral, the dropping of bombs on Japan created fallout that killed more than one hundred and fifty thousand people in the immediate aftermath and, in the ensuing years, was responsible for deaths from cancer and for birth defects among the descendants of the people exposed to it. The radioactivity of the heavy metals used in the bombs also took a toll on the scientists working on nuclear weapons at Los Alamos and elsewhere. Although Oppenheimer's own throat cancer might well have been associated with smoking, Enrico Fermi and many other scientists would eventually die of cancers that were thought to be directly related to their exposure to radiation.

In discussions of the moral issues of the atomic bombings of Japan, this memo from Oppenheimer to Farrell is often cited as evidence that the scientists working on the bombs did not understand the nature, extent, and effects of radioactive fallout. For example, Oppenheimer concludes his memo by stating that the “probable results of monitoring will be that it will be quite safe to enter” the test area. However, he applied that statement only to entry “some weeks” after the detonation. The document also makes it clear that Oppenheimer and his team had a quite accurate understanding of how deadly the weapon would be. The memo states that the bomb itself contained “about 109 times as much toxic material … as is needed for a single lethal dose”—in other words, it contained enough radioactive material to kill a billion people. Moreover, apart from this lingering fallout, direct radiation from the nuclear blast would be “lethal within a radius of about six-tenths of a mile.” Oppenheimer does observe that little was known about how radioactive fallout would move through the atmosphere, but considerable knowledge in this area was soon gained from the Trinity Test.

Oppenheimer's memo is a purely technical document that was not directed to a lay audience. It simply states the radiation effects predicted for a nuclear detonation and the dangers those effects might pose to personnel in the vicinity. Yet even this brief factual presentation makes it abundantly clear that he and his team had created a new kind of weapon, one that could destroy an entire city and poison its hinterland. Moreover, precisely because of this new kind and scale of destructive power, the bomb obviously would be most effective when used against cities, not against traditional military targets that might be spread over many square miles in the field. It is fair to ask, then, why men like Oppenheimer and Fermi and many others, dedicated to pacifism and internationalism, worked to create such a terrible weapon. On the one hand, Nazi Germany seemed such a threat to the most basic institutions of civilization that it was easy to justify going to almost any lengths to defeat it. Indeed, although the bombs were actually used against Germany's ally Japan (because Germany had, in the meantime, been overcome by conventional means), the rationale for the development of the atomic bomb was the defeat of Germany and especially the fear that, since most of the basic research supporting nuclear fission had been done by German scientists before the war, Germany might develop its own bomb.