generator (RTG) with approximately 1 kilogram of Plutonium-238 on board when on April 21, 1964 it burned up and re-entered the atmosphere. Dr. John Gofman claimed it increased the rate of lung cancer worldwide. He said "Although it is impossible to estimate the number of lung cancers induced by the accident, there is no question that the dispersal of so much Plutonium-238 would add to the number of lung cancer diagnosed over many subsequent decades." Other satellite failures include Kosmos 954 and Kosmos 1402. Mechanism Cancer is a stochastic effect of radiation, meaning it is an unpredictable event. The probability of occurrence increases with effective radiation dose, but the severity of the cancer is independent of dose. The speed at which cancer advances, the prognosis, the degree of pain, and every other feature of the disease are not functions of the radiation dose to which the person is exposed. This contrasts with the deterministic effects of acute radiation syndrome which increase in severity with dose above a threshold. Cancer starts with a single cell whose operation is disrupted. Normal cell operation is controlled by the chemical structure of DNA molecules, also called chromosomes. When radiation deposits enough energy in organic tissue to cause ionization, this tends to break molecular bonds, and thus alter the molecular structure of the irradiated molecules. Less energetic radiation, such as visible light, only causes excitation, not ionization, which is usually dissipated as heat with relatively little chemical damage. Ultraviolet light is usually categorized as non-ionizing, but it is actually in an intermediate range that produces some ionization and chemical damage. Hence the carcinogenic mechanism of ultraviolet radiation is similar to that of ionizing radiation. Unlike chemical or physical triggers for cancer, penetrating radiation hits molecules within cells randomly. Molecules broken by radiation can become highly reactive free radicals that cause further chemical damage. Some of this direct and indirect damage will eventually impact chromosomes and epigenetic factors that control the expression of genes. Cellular mechanisms will repair some of this damage, but some repairs will be incorrect and some chromosome abnormalities will turn out to be irreversible. DNA double-strand breaks (DSBs) are generally accepted to be the most biologically significant lesion by which ionizing radiation causes cancer. In vitro experiments show that ionizing radiation cause DSBs at a rate of 35 DSBs per cell per Gray, and removes a portion of the epigenetic markers of