Cobalt⁶⁰ is a radioactive isotope of the fairly common metal cobalt. It can be easily produced by absorbing slow neutrons in the natural and stable cobalt⁵⁹. It has a half-life of five years and it emits penetrating gamma rays. These properties make it useful in cancer therapy. Many cancerous growths are more sensitive to radiation than healthy tissue. Therefore radiation can be used to reduce—sometimes even to destroy—dangerous tumors. The penetrating rays of cobalt⁶⁰ can reach the cancer even deep inside the human body. The lifetime of cobalt⁶⁰ is long enough so that this substance is easily installed in hospitals. But the same properties which make cobalt⁶⁰ useful also make it potentially dangerous. A nuclear explosion produces many neutrons and these could be absorbed in ordinary cobalt. The radioactivity produced in this way lives long enough to become widely distributed. Its ray can easily penetrate a foot of masonry and several hundred feet of air. A cobalt bomb would indeed be a most unpleasant object. (See pictures 7 and 8.) One widely discussed possibility is that future nuclear tests will be used to develop a cobalt bomb or other bombs for radiological warfare. Actually tests have little to do with the cobalt bomb. Once one has a powerful nuclear weapon, such as a hydrogen bomb, it is relatively easy to make a radiological bomb. Further tests are not necessarily required. To the extent that any testing need be carried out, it is only necessary to activate a moderate amount of substance to find out in what way a certain bomb would function as a tool of radiological warfare. Tests of this kind would add only a negligible amount of radioactivity to the atmosphere. Therefore, in connection with the test program we need not worry about the cobalt bomb or any related experiment. The question of the cobalt bomb or radiological warfare in general is not whether it is feasible—it is—but rather whether it serves a useful military purpose.