Editing Cassini–Huygens (section)

== Plutonium power source ==
[[File:Cassini's RTG.jpg|thumb|upright|A ''Cassini'' [[GPHS-RTG]] before installation]]
Because of Saturn's distance from the Sun, [[Solar cell|solar arrays]] were not feasible as power sources for this space probe.<ref name=Solar/> To generate enough power, such arrays would have been too large and too heavy.<ref name="Solar"/> Instead, the ''Cassini'' orbiter was powered by three [[GPHS-RTG]] [[radioisotope thermoelectric generator]]s, which use heat from the decay of about {{convert|33|kg|lb|abbr=on}} of [[plutonium-238]] (in the form of [[plutonium dioxide]]) to generate direct current electricity via [[thermoelectrics]].<ref name=Solar/>
The RTGs on the ''Cassini'' mission have the same design as those used on the ''[[New Horizons]]'', [[Galileo (spacecraft)|''Galileo'']], and [[Ulysses (spacecraft)|''Ulysses'']] space probes, and they were designed to have very long operational lifetimes.<ref name=Solar/>
At the end of the nominal 11-year ''Cassini'' mission, they were still able to produce 600 to 700 watts of electrical power.<ref name="Solar"/> (Leftover hardware from the ''Cassini'' RTG Program was modified and used to power the ''New Horizons'' mission to [[Pluto]] and the [[Kuiper belt]], which was designed and launched later.<ref name=IECEC/>)

[[Power distribution]] was accomplished by 192 [[Solid-state electronics|solid-state]] [[power switch]]es, which also functioned as [[circuit breakers]] in the event of an overload condition. The switches used [[MOSFET]]s that featured better efficiency and a longer lifetime as compared to conventional switches, while at the same time eliminating [[Transient (oscillation)|transients]]. However, these solid-state circuit breakers were prone to erroneous tripping (presumably from cosmic rays), requiring them to reset and causing losses in experimental data.{{sfn|Meltzer|2015|p=70}}

[[File:Radioisotope thermoelectric generator plutonium pellet.jpg|thumb|left|A glowing-hot plutonium pellet that is the power source of the probe's radioisotope thermoelectric generator]]
To gain [[momentum]] while already in flight, the trajectory of the ''Cassini'' mission included several [[gravitational slingshot]] maneuvers: two fly-by passes of [[Venus]], one more of the Earth, and then one of the planet [[Jupiter]]. 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 {{convert|1171|km|mi|abbr=on}} above the Earth on August 18, 1999.<ref name=cassini.quickfacts/>
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&nbsp;kg<ref name=Grandidier/> of nuclear fuel 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<ref name=eis/> (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<ref name=Friedensen_1999/> as 500,000 deaths).  However, the chance of this happening were estimated to be less than one in one million, i.e. a chance of one person dying (assuming 5,000 deaths) as less than 1 in 200.<ref name=eis/>

NASA's risk analysis to use plutonium was publicly criticized by [[Michio Kaku]] on the grounds that casualties, property damage, and lawsuits resulting from a possible accident, as well as the potential use of other energy sources, such as solar and fuel cells, were underestimated.<ref name=Kaku_1997/>