To power the Cassini spacecraft by solar energy alone would require huge, unwieldy solar arrays about a quarter the size of a football field. Plutonium decays over time, emitting radiation or helium nuclei and electrons. The RTG uses only decay heat from the radioactive material, which means there are no nuclear reactions involved. It also means the radioactive material can be encapsulated to prevent release into the atomosphere. Plutonium-238 gives off primarily alpha rays.
Alpha rays are less energetic than beta or gamma rays, which are associated with nuclear weapons.
Clothing or even a sheet of paper is sufficient to shield against alpha rays, while gamma rays can be blocked only with dense material, such as lead. The Cassini spacecraft is powered by three RTGs, each filled with 72 marshmallow-sized, ceramic pellets of plutonium oxide. For safety, each pellet is encased in a capsule made of iridium, a dense metal resembling platinum. It also is corrosive-resistant. Each capsule is further protected by a graphite shell that can withstand extreme high temperatures.