Viewpoint: Yes, using plutonium as an energy source for Cassini carried only minimal safety risks, and it was the only currently feasible way to reach the outer planets in the solar system.

Viewpoint: No, using plutonium as an energy source for Cassini was not safe, given the known dangers of plutonium and the legacy of rocket launch failures.

Spacecraft must do a variety of things during a mission. They must be able to send information to mission scientists on Earth, and they must be able to receive and respond to instructions sent from Earth. The instruments they carry must function, and the computers that record images and data must be kept operational. Mechanical parts must be...