All thermonuclear bombs have been exploded only for test purposes. Even testing seems to be dangerous, however, at least if it is carried on in the open atmosphere. The radioactivity liberated spreads over the world and may do slow but cumulative damage. Controlled Fusion However effective a fusion bomb may be in liberating vast quantities of energy, it is not what one has in mind when speaking of a fusion power station. The energy of a fusion bomb is released all at once and its only function is that of utter destruction. What is wanted is the production of fusion energy at a low and steady rate—a rate that is under the control of human operators. The sun, for instance, is a vast fusion furnace 866,000 miles across, but it is a controlled one—even though that control is exerted by the impersonal laws of nature. It releases energy at a very steady and very slow rate. (The rate is not slow in human terms, of course, but stars sometimes do release their energy in a much more cataclysmic fashion. The result is a “supernova” in which for a short time a single star will increase its radiation to as much as a trillion times its normal level.) The sun (or any star) going at its normal rate is controlled and steady in its output because of the advantage of huge mass. An enormous mass, composed mainly of hydrogen, compresses itself, through its equally enormous gravitational field, into huge densities and temperatures at its center, thus igniting the fusion reaction—while the same gravitational field keeps the sun together against its tendency to expand. There is, as far as scientists know, no conceivable way of concentrating a high gravitational field in the absence of the required mass, and the creation of controlled fusion on earth must therefore be done without the aid of gravity. Without a huge gravitational force we cannot simultaneously bring about sun-center densities and sun-center temperatures; one or the other must go. On the whole, it would take much less energy to aim at the temperatures than at the densities and would be much more feasible. For this reason, physicists have been attempting, all through the nuclear age, to heat thin wisps of hydrogen to enormous temperature. Since the gas is thin, the nuclei are farther apart and collide with each other far fewer times per second. To achieve fusion ignition, therefore, temperatures must be considerably higher than those at the center of the sun. In 1944 Fermi calculated that it might take a temperature of 50,000,000° to ignite a hydrogen-3 fusion with hydrogen-2 under earthly conditions, and 400,000,000° to ignite hydrogen-2 fusion alone. To ignite hydrogen-1 fusion, which is what goes on in the sun (at a mere 15,000,000°), physicists would have to raise their sights to beyond the billion-degree mark. [Illustration: _A supernova photographed on March 10, 1935._] [Illustration: _The same star on May 6._]