the mass of which nearly sums to the mass of the original nucleus. The fission of uranium or plutonium therefore produces a spectrum of different pairs of nuclei, generally in a bimodal mass distribution. The fission products will include isotopes of many elements and this includes the inert gases such as xenon and krypton. Some of the radioactive isotopes of these inert elements have half-lives or the order of days, and these isotopes have therefore decayed away and are not found in our solar system. However, a recent fission reaction such as in a reactor or in a nuclear explosion will produce small quantities of these radioactive isotopes. If they are detected in the atmosphere, their presence is evidence that a reactor has been operating, or that a recent nuclear detonation has taken place. This is the basis of the air sampling effort as a means for the detection and characterization of such activities. As noted in the following table by Chien C. Lin, the isotopes of particular interest have short half-lives, so detection depends on sampling the atmosphere promptly following such operations. Note that the distribution and ratios of xenon isotopes can also be used to determine whether a particular detonation was of a uranium or plutonium device. The explosion that was sampled by this first Grab Bag flight is known as Joe 4 and took place on August 12, 1953 at Semipalatinsk in Kazakhstan. It was a 40-kiloton uranium-235 bomb that ultimately produced a total yield of 400 kilotons. It was estimated that 15–20% of the energy was released by fusion (60–80 kilotons) and the rest was from fast fission. The Grab Bag air sampling program was quickly expanded following the successful flight in August 1953, and there subsequently were more than two hundred Grab Bag flights launched during the subsequent two years. It was one of the largest programs that took place in the Balloon and Meteorological Systems organization at General Mills Inc. On February 19, 1963, U. S. Patent 3,077,779, for this high-altitude air sampling system was assigned to H. E. Froehlich, Roger A. Kizzek, Donald F. Melton, and Richard L. Schwoebel. References Nuclear weapons testing Soviet Union–United States relations Ballooning