Protective measures...
Here is how Cassini's (probe enroute to Saturn) plutonium is protected (in case of an accidental re-entry):
"The plutonium is packed into 216 insoluable heat resistant ceramic pellets. The pellets are then encased in a module protected by a heat shield made of iridium, a graphite impact shell and insulation made of carbon fibers."
"Misinformation and exaggerated claims of risk should not be allowed to slam the door on deep-space discovery." says Charles Walker and Pat Dasch, the president and excecutive director of the National Space Society.