Risk of nuclear powered space probes

In recent years, three probes have been sent into outer space: Galileo, Ulysses and Cassini. Each has used for its electricity power source, a radioactive thermoelectric generator (RTG), heated by a source of plutonium 238. Cassini, launched in October 1997, will reach its scientific objective in late June or early July 2004, concurrent with PSAM7. The authors of this paper had the privilege of providing the final review of the Cassini safety assessments for the Office of Science and Technology of the President, before final approval by the President of the United States, as required by law and international treaty, before the launch of any nuclear device into space.