The terrestrial flyby was the final instance when the probe posed any conceivable danger to human beings. The maneuver was successful, with
Cassini passing by 1,171 km (728 mi) above the Earth on August 18, 1999.
[47] Had there been any malfunction causing the probe to collide with the Earth, NASA's complete environmental impact study estimated that, in the worst case (with an acute angle of entry in which
Cassini would gradually burn up), a significant fraction of the 33 kg
[34] of plutonium-238 inside the RTGs would have been dispersed into the Earth's atmosphere so that up to five billion people (i.e. almost the entire terrestrial population) could have been exposed, causing up to an estimated 5,000 additional cancer deaths over the subsequent decades
[48] (0.0005 per cent, i.e. a fraction 0.000005, of a billion cancer deaths expected anyway from other causes; the product is incorrectly calculated elsewhere
[49] as 500,000 deaths). However, the chance of this happening were estimated to be less than one in one million.
