The greatest problem encountered to date is the extreme instability of the plasma and the corresponding difficulty of maintaining it at the proper temperature longer than a few millionths of a second. Many physicists now think that the successful exploitation of thermonuclear energy will not occur for many years. When and if it is achieved, however, the deuterium present in the oceans of the earth will represent an almost inexhaustible source of energy. Isotopes Have Many Uses The ability to produce and control nuclear reactions is affecting, and will doubtless continue to affect, human life in two outstanding ways. One way is by making tremendous amounts of energy available, either as explosions or as energy released from controlled reactions for peacetime use. The other way is by producing a vast variety of radioactive isotopes, first in the particle accelerators (“atom smashers”) mentioned earlier, and now in large quantities in nuclear reactors. The presence of a radioactive isotope can be detected by instruments like the familiar Geiger counter; for this reason isotopes make wonderful tracers. These telltale atoms, which, in effect, continually cry “Here I am,” can trace the course of a chemical element through any kind of chemical reaction. Chemists are taking advantage of this new way of tagging atoms to study reaction patterns that, heretofore, have been obscure. As a consequence, a scientist’s ability to synthesize scarce chemicals is being increased. The exact role of numerous essential trace elements in the growth and metabolism of living things, including people, is being studied by the use of tagged atoms. Radioisotopes at Work [Illustration: IN MEDICINE: _Iodine-131 reveals spread of thyroid cancer in patient’s body._] [Illustration: IN SPACE: _Plutonium-238 is the fuel for the atomic generator powering this TRANSIT satellite._ Courtesy The Martin Company] [Illustration: IN FOOD PRESERVATION: _Potatoes stored for 18 months at 47°F. Potato at right had been irradiated, that on left had not._] [Illustration: IN INDUSTRY: _Radioactive iridium was used to inspect the hull of the carrier_ Independence. Courtesy Technical Operations, Inc.] As sources of radiation, radioactive isotopes are frequently replacing more expensive and less convenient sources such as radium and X-ray machines. The medical treatment of diseased tissue has been greatly expedited by the new sources. In industry many applications of radiation sources have been made. They are used, for example, in thickness gauging and in making radiographs to check the quality of large castings. The sterilization and preservation of food is another promising use for inexpensive radioactive sources.