Reacting to nuclear power systems in space: American public protests over outer planetary probes since the 1980s

• Focuses on nuclear power enabling US deep space probes; without them could not explore region.
• In 1970s/1980s nuclear accidents created US wariness of nuclear powered spacecraft.
• Discusses protests over nuclear power sources on Galileo (1989) and Cassini (1997).
• Resolution of protests through legal means; more stringent mission assurance.
• Since, New Horizons (2006) and Curiosity (2011) launched with little protest.

The United States has pioneered the use of nuclear power systems for outer planetary space probes since the 1970s. These systems have enabled the Viking landings to reach the surface of Mars and both Pioneers 10 and 11 and Voyagers 1 and 2 to travel to the limits of the solar system. Although the American public has long been concerned about safety of these systems, in the 1980s a reaction to nuclear accidents – especially the Soviet Cosmos 954 spacecraft destruction and the Three Mile Island nuclear power plant accidents – heightened awareness about the hazards of nuclear power and every spacecraft launch since that time has been contested by opponents of nuclear energy. This has led to a debate over the appropriateness of the use of nuclear power systems for spacecraft. It has also refocused attention on the need for strict systems of control and rigorous checks and balances to assure safety. This essay describes the history of space radioisotope power systems, the struggles to ensure safe operations, and the political confrontation over whether or not to allow the launch the Galileo and Cassini space probes to the outer planets. Effectively, these efforts have led to the successful flights of 12 deep space planetary probes, two-thirds of them operated since the accidents of Cosmos 954, Three Mile Island, and Chernobyl.