No nuclear reactors are known to be aboard any currently operational satellites or spacecraft. Late in the twentieth century, the Soviet Union launched a number of military satellites that contained compact nuclear reactors. A few of them, however, nearly ended in disaster. Cosmos 954, which was launched in September 1977, spiraled into the atmosphere and crashed into the Canadian arctic on January 24, 1978, scattering radioactivity across a large stretch of land. Some of the debris was emitting lethal doses of radiation when it was recovered. Fortunately, nobody was killed or injured, but cleanup of the affected area took months. Another Soviet spacecraft, Cosmos 1402, launched in August 1982, suffered the same fate, falling to Earth on January 23, 1983. Fortunately, this re-entry occurred far out in the Indian Ocean, and no known debris was ever found.

Today, for safety reasons, no spacecraft are launched with nuclear reactors on board. It remains a very attractive idea, however, to have nuclear-powered spacecraft for deep space journeys that carry them far from Earth. The challenge is to make sure that, even with a catastrophic failure, our planet and its people would not be put at risk. In the 1960s, the United States studied a nuclear-powered rocket engine idea called NERVA (Nuclear Engine for Rocket Vehicle Application), but the project was cancelled in 1972. In 2003 NASA started a new nuclear space program called Project Prometheus; after a few years, however, its funding was deeply cut, and then it too was cancelled.