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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?
Robert L. Hutton of Fort Meade, MD asks: If Cassini does manage to get recaptured by Earth on its return flyby and burn up in the atmosphere, can we expect the plutonium to make it to the surface? How much radiation would reach the ground relative to the cosmic background radiation? How large an area would this radiation affect? What types of radiation would be emitted (alpha, beta, gamma, slow neutrons, etc.) and how much damage to life could we expect from it?
Steven Aftergood of the Federation of American Scientists responds:
Eventually-- over perhaps a few years-- any plutonium released into the atmosphere would be deposited on the Earth's surface. This is what happened following the disintegration of a plutonium power source in the upper atmosphere in 1964. Depending on the scenario, the plutonium 238-- an alpha emitter-- could be widely dispersed around the globe, adding a fraction to natural background radiation.
There is voluminous and contentious literature on the potential consequences of such a release. A lot depends on how one assesses the effects of very low incremental doses of radiation. I do not have an informed opinion on this question, except to say that release of Cassini's plutonium inventory into the environment would be unacceptable if there were a significant chance that it could happen. I do not believe that there is.
Dr. John Gofman of the University of California, Berkeley responds:
Part 1. Yes, we expect the plutonium will definitely make it to the surface in a period of two to five years. All of it. We know that from worldwide fallout of plutonium from weapons tests. Those tests put 440,000 curies of plutonium-239 equivalent in the atmosphere, and it all came down. We even know that as it spread over a large part of the Earth, people inhaled about 42 picocuries into their lungs from this. Today the 42 would be higher because there were more weapons tests since that measure. Cassini has about 400,000+ curies of alpha emitter on board, so a full vaporization to fine particles (less than 10 microns) would do the same thing as the weapons test plutonium did .Part 2. The emissions would essentially all be alpha particles.
Part 3. Cosmic rays have been killing people since people were on Earth, so there is little comfort in the 30 or so millirems of cosmic radiation per year we receive. Cancer is in proportion to dose and there are no safe doses from cosmic rays or other ionizing radiations. The key to your question resides in what form will the plutonium be in when it descends to Earth.
NASA tries to assure us that fine particles just won't happen because they say it is in a ceramic form.That is a questionable claim. There is reason for real concern that a year or two of bombardment of the "ceramic" from the monumental alpha particle intensity may lead the ceramic structure to be weakened or totally destroyed. In that event, on flyby, an accidental reentry could lead to a powdered plutonium oxide material inside the iridium.
I think this issue needs addressing: how well do the RTGs hold up after one to two years. NASA vacillates in its reports on the chance and amount of fine particles in a worst case accident. I will emphasize that if the particles are large when they come down, the lung cancers will not occur. But there will be some long-term entry of some deposited plutonium into the biosphere.
People ask, "If plutonium is so toxic, why aren't we all dead?" That one is easy. I have calculated that only one part in every 2,300,000 parts that descend to Earth gets into people's lungs. That is because there is not a person in every square foot of Earth. As to how large an area is covered depends on the condition of a burn-up, but it could spread the plutonium over the equivalent of a hemisphere.
Damage to life: We have only poor data (thanks to a less than good job by the Atomic Energy Commission) on the toxicity of plutonium. Using beagle dog data scaled up to humans, I reach the conclusions that for Pu-238, it takes about one tenth of a microgram to guarantee one fatal lung cancer in humans. You have to know how many people will inhale how much depending on the kind of accident in the flyby but for every 0.1 microgram deposited in human lungs you can conservatively figure on one fatal lung cancer.
NASA people often say we will not get the fine particles even in the worst case, and so all calculations are irrelevant. I do not say that NASA is wrong. I simply say I do not believe that they know what the real situation will be, in view of no real extent of experience, especially with my concern about the interior of the RTG modular units when they have been exposed in the capsule for the year or two in flyby times. I certainly hope that NASA is correct for humanity's sake, but I consider such assurances poorly supported. We really need to know how well the "ceramic" structure will hold up under bombardment, that can finally produce nearly one bond broken per atom of plutonium. It is a serious problem and deserves a hard look, not a lick.
Dr. Gary Bennett, former NASA scientist, responds:
There is some question as to whether any of the plutonium-238 fuel would be released in a postulated reentry during the flyby because the fuel pellets are encased in four layers of protection including two separate protective layers of a carbon-carbon composite material that is designed for reentry protection. Recent calculations indicate that the second layer of composite material (known as the graphite impact shell) may survive. However, using more pessimistic assumptions, the Final Safety Analysis Report states that "...the mean fuel release predicted in the event of an EGA [Earth Gravity Assist] reentry accident which produces a fuel release is 2600 grams" with the majority of this calculated to be released on the ground as non-respirable-sized material.In terms of radiation exposure the number is quite low-- about 1 millirem over a 50-year period. Cosmic background radiation gives us about Millirems per year or 2,500 millirems over the same 50-year period. From all sources (radon, cosmic rays, naturally occurring radiation on Earth, medical and dental x-rays, building materials, fallout, etc.) everyone will receive about 15,000 millirem over the same 50-year period. My luminous dial wristwatch will give me two millirems in just one year.
The Final Safety Analysis Report calculates for such a postulated accident that about 19 square kilometers (about 7 square miles) would receive enough plutonium-238 to exceed the Environmental Protection Agency's guidelines that require consideration of clean-up actions. (It should be noted that the most likely location of such a postulated entry would be over an ocean not land.) Plutonium-238 is primarily an emitter of alpha particles (helium nuclei) which are easily stopped by the materials encasing it. The plutonium-238 is in the form of a high-fired ceramic which means it is not easily dissolvable nor does it easily break into very small particles. Repeated tests have shown that on impact it tends to break into large pieces that cannot be ingested. There are also some gamma rays and neutrons emitted.
Using the pessimistic assumptions of a release of fuel in the event of a postulated reentry during the flyby of Earth, the Final Supplemental Environmental Impact Statement has estimated that the mean health effect consequence would be 120 over a 50-year period. (These are defined as latent cancers.) However, considering the very low doses (about one millirem), some experts have questioned this number, pointing out that the most likely result will be no "health effects." Unfortunately, some critics of the Cassini mission, using more emotion than science, have claimed much higher numbers of health effects. If these critics were right, life on this planet should have ended eons ago from the much higher background radiation. To date none of the critics have submitted their work for scientific peer review.
Next: Why was plutonium selected to power Cassini?
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