Some satellites are powered by small nuclear reactors. Unlike earth-bound reactors, those in space are not shielded, because their radiation is thought to be harmlessly emitted in space. When one of these reactors passes near a satellite which is searching for naturally occurring gamma rays, however, its radiation is not harmless. Satellites record the gamma rays emitted by other satellites, and often miss entirely the naturally occurring gamma ray bursts which they are sent to track down.
Between 1961 and 1977 the USA launched 23 satellites with nuclear reactors in them. Four of them (17%) in into problems, including one that disintegrated on launch, tripling the amount of plutonium 238 in the Earth's environment. The former Soviet Union has launched 39 nuclear reactors on orbiting satellites since 1965, of which 6 (15%) have malfunctioned.