From The Concise Encyclopedia of Economics, by Henderson (p.447):

"One of the smallest risks that statisticians have measured – dying as a result of cancer caused by the release of plutonium from a deep space probe that loses control during its swing around the Earth to gain velocity and burns up in the atmosphere – measures in at three-millionths of one percent."

Found this email message archive that has some calculations in this regard. In case you were worried about dying this way.