No plutonium dispersal or impact.
Dr. Lynn Cominsky 4, Chair of Astronomy and Physics at Sonoma State University, Dr. Phil Plait, PhD, Professor of Astronomy at Sonoma State University, and Dr. David Walls, Professor Emeritus of Sociology at Sonoma State University, “Cassini: Well Worth the Small Risk”, Santa Rosa Press Democrat, 10/1/2004, https://web.sonoma.edu/users/w/wallsd/cassini.shtml
In numerous articles and his book The Wrong Stuff, Grossman argued that an accident during Cassini’s launch or an inadvertent reentry to the Earth’s atmosphere during its "gravity assist" flyby could result in disintegration of the RTGs and a catastrophic release of plutonium into the atmosphere, threatening thousands if not millions of human lives.
Few reputable scientists supported Grossman’s analysis. RTGs had been used safely before, most notably on the Voyager 1 and 2 missions to the outer planets, launched in 1977, and the Galileo mission to Jupiter, launched in 1989. The late astronomer Carl Sagan, a strong opponent of nuclear weapons, endorsed the Cassini mission and urged the peace movement to focus on more urgent tasks.
Cassini’s RTGs, scientists pointed out, are not nuclear reactors. The natural decay of the RTGs’ plutonium generates heat that is converted into electricity. No chain reaction takes place, and there is no possibility of a nuclear explosion. Further, the plutonium is packaged in small encased pellets and surrounded by a graphite shell. Disintegration of the plutonium is highly unlikely in the event of an accident. In fact, in the two instances when NASA-designed RTGs accidentally reentered the Earth’s atmosphere, no radiation was released, and the RTGs remained intact.
Despite the minimal risk involved, Grossman’s argument was taken up by some activists from the peace, environmental, and anti-nuclear movements. In Censored 1997, Project Censored designated Grossman’s articles as its No. 1 under-reported story of 1996, writing that NASA could expose "billions" of people to radiation. The height of rhetorical excess was reached by peace activist and physician Helen Caldicott, who wrote in opposition to Cassini in The Nation, "Plutonium is so toxic that one pound distributed around the Earth in particles small enough to be inhaled could induce lung cancer in every person on the planet." The likelihood of this hypothetical scenario is infinitesimal.