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| RISKS VS. RETURNS Is the Cassini Mission safe? October 21, 1997 |
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Return to this forum's introduction.
Questions answered in this forum:What is the Cassini mission all about? What could cause an accident when Cassini swings by Earth? What damage would an accident cause? Why was plutonium selected to power Cassini? What other energy sources could have been used to speed up the mission? How can NASA risk public safety for planetary exploration?
Cathy Middlecamp of Madison, WI asks: What is unique about plutonium-238 that it was selected to power the RTGs? Why couldn't another alpha emitter (with a reasonably long half-life) have been selected? What other isotopes were considered and why were they rejected?
Steven Aftergood of the Federation of American Scientists responds:
Plutonium-238 with its 87.8 year half life offers a relatively desirable specific power-- the amount of heat produced per unit of mass. Power supplies using isotopes with a longer half life would have to be a good deal heavier.
In the past, the Soviet Union developed power supplies using polonium-210. Years ago, studies were done of power supplies using isotopes of strontium or cesium, which have shorter half-lives than plutonium-238. I believe these isotopes were rejected, among other reasons, because of their relatively high radioactivity and related difficulties in fabrication and handling.
Dr. John Gofman of the University of California at Berkeley responds:
Dear Cathy:You have already said you want the same reasonable half-life and you want alpha emitters. So why go looking for something else when plutonium-238 is available, meets your requirements, and has the added feature that its oxide, PuO2 can be made into a poorly soluble material, which helps keep any released plutonium from getting into the biosphere?
But there has to be a reason you ask this question. Any alpha-emitter will do the same biological harm as Pu-238 if it reaches tissue, There are no alpha particles of the same energy class that will produce less biological harm per unit disintegration of the emitter. If you think there is some special chemical toxicity of plutonium that worries you, I do not think that would be correct. The toxicity of plutonium -238 is essentially its alpha particle emission. I realize that many people are frightened by the word plutonium . Other nuclides might be available, but unless you can show that they will keep away from biological tissue better than PuO2, there is no advantage I can imagine.
With respect to cancer induction, alpha particles are really potent, but if you get the same radiation dose from any alpha particle emitter, the induction of cancer will be the same.
Dr. Gary Bennett, former NASA scientist, responds:
In selecting a radioisotope fuel for a radioisotope thermoelectric generator (RTG), a number of tradeoffs must be made. If the radioisotope has too short a half life (the time it takes for half the material to decay) then it will not be useful for missions like Cassini which are scheduled to last 11 years (seven years to get to Saturn plus four years of scientific study at Saturn). If the half life is too long (generally more than 100 years) then the amount of heat produced in a given amount of radioisotope will be too low, making the RTG too large and too heavy. Also, to protect people and the spacecraft's electronic components, it is highly desirable to avoid radioisotopes that emit gamma rays. It turns out that alpha emitters (like plutonium-238) are much easier to shield (the alpha particles are essentially absorbed in the fuel and its containers) and have good power densities (heat per unit volume or per unit mass). They could also be produced in existing production reactors.In the early days of the RTG program other radioisotopes, such as polonium-210 and curium-244 were considered but they were rejected because they don't meet the criteria (primarily half life) mentioned above. Strotium-90 has been used in some terrestrial RTGs, but it leads to bulky RTG designs and it has a shorter half-life than plutonium-238.
It should be pointed out that contrary to the claims of the critics, plutonium is neither "the most toxic substance known to man" nor "the most toxic substance known in the universe." There arc literally dozens of radioisotopes that are more radioactive than plutonium-238. Moreover, in a world dealing with the AIDS virus, the Ebola virus, "mad cow" disease and botulism, plutonium-238 is clearly not "the most toxic substance known to man." As to the universe, black holes, neutron stars, supenovae, and the Van Allen radiation belts (to name just a few things) are clearly more "toxic."
Plutonium was named for the planet Pluto because plutonium was the second element discovered after uranium just as Pluto was the second planet discovered after Uranus (for which uranium had been named). This nomenclature followed that used to name neptunium, the first element found after uranium (just as Neptune is the first planet discovered beyond Uranus). (It has been fashionable in some nontechnical circles to make the silly statement that "plutonium was named for the god of the underworld or hell"-- a good sound bite, but that's all it is.)
While the plutonium-238 used in the Cassini RTGs was produced in special nuclear reactors, traces of plutonium (essentially plutonium-239) do occur naturally on Earth in some uranium ores-- and because of the short half-life of the plutonium isotopes relative to some uranium ores relative to the 4-5-billion-year age of Earth, that plutonium was produced naturally on Earth. Plutonium like uranium, is no doubt also produced in the explosions of stars. (All the uranium on Earth came from stellar explosions; however, since all the isotopes of plutonium have much shorter half lives than uranium-238 and uranium-235, none of that early plutonium is left).
Next: What other energy sources could have been used to speed up the mission?
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