The risks of mishap are real. In 1964 a U.S. satellite crashed to earth with 2.1 pounds of plutonium on board. It disintegrated and the plutonium was dispersed through the atmosphere scattering onto every continent and every latitude of the planet. Increased lung cancer rates have been attributed to that crash. Dr. Ernest Sternglass, professor emeritus of radiological physics at the University of Pittsburgh School of Medicine, estimates that the number of deaths from a Cassini mishap could run as high as 20 - 40 million people.
Major accidents are inevitable if nuclear materials continue to be sent into space.
He points out that solar energy could supply the electricity in space instead of plutonium.