"[Plutonium] is so toxic that less than one-millionth of a gram, an invisible particle, is a carcinogenic dose. One pound, if uniformly distributed, could hypothetically induce lung cancer in every person on Earth." - Helen Caldicott
|Fig. 1: Artistic depiction of Cassini. (Courtesy of NASA)|
This quote, which is attributed to Dr. Helen Caldicott, founder of Physicians for Social Responsibility, exemplifies the the perception of using plutonium as a power source in robotic space missions. While the general field of nuclear power often evokes a passionate, negative response from the public, nuclear-powered spacecraft are of particular concern due to their ability to spread radioactive material on a global scale. This comes from the fact that if a launch vehicle carrying a nuclear or radioisotope powered craft were to fail during launch - a fairly common occurrence for launch vehicles - then the radioactive debris could be spread over the entire Earth by the strong winds of the upper atmosphere.  More startling is the fact that almost this exact scenario played out in 1964 when the Transit-5BN-3 satellite unexpectedly re-entered the atmosphere, burned up, and released enough radioactive material into the upper atmosphere to produce ~20,000 curies of radiation.  According to the prominent physicist/physician and Manhattan Project collaborator, John Gofman, this sole event is to blame for a global rise in lung cancer.  Furthermore, it is noted that the amount of radioactive material on the Transit-5BN-3 satellite is far from the largest quantity launched in a single spacecraft; that title belongs to NASA's Cassini orbiter.
If this is the case, if radioisotope-powered spacecraft pose such a stupendous risk to health and environment, why would NASA and other space agencies continue to launch mission that rely on plutonium? Is the government being irresponsibly cavalier about the true risks or are the opponents of nuclear-powered spacecraft guilty of fear mongering?
To address the question at hand let us examine the previously mentioned example of NASA's Cassini orbiter; an artistic representation of which is given in Fig. 1. Cassini was an unmanned spacecraft that was tasked with exploring the Saturnian system. Launched in 1997, Cassini was powered by 33kg of plutonium-238, the largest amount ever employed by a single craft, which acted as the heat source for the radioisotope thermal generators (RTGs) that convert thermal energy to electrical power. Of particular, yet unrelated note, Cassini was the first mission to land a man-made object on an outer Solar System body with the landing of the Hyugens probe on Saturn's moon Titan in December 2004. 
Cassini generated a relatively small yet impassioned group of opponents due to its risk of ejecting radioactive material on the Earth. Not only was this risk present during launch in 1997 but also during the Earth flyby maneuver that was scheduled - and successfully executed - in August of 1999. While the mission was a success by any metric and never exposed the public to any radioactive material, it is still a controversial issue due to the risks, perceived or real, that it posed to global health.
|Fig. 2: Comparative radiation dosage chart. (Courtesy of Randall Monroe)|
In an attempt to determine the validity of the claims that Cassini threatened the lives of 100,000+ people, let us perform a worst-case analysis of a failure. It is noted that some figures are taken from unreliable sources such as Wikipedia, however since this is just an exercise to illustrate potential risk and not exact calculations, these figures should suffice. Cassini contained 32.7 kg of Pu-238 dioxide; for the sake of argument we'll assume that this is pure plutonium.  Therefore at 634 GBq/g the total activity of that mass is 2.073 × 1016 decay/sec. At 8.96 × 10-13 J/decay for Pu-238 this total activity equates to 1.858 × 104 J/sec. In a worst-case failure of Cassini the plutonium would be distributed evenly across the world, affecting every person evenly. Using the rough estimate that 6 billion people were alive at the launch of Cassini this equates to 3.097 × 10-6 (J/s) of radioactive fallout per person. If we use the rough, conservative estimate that the average weight of a person is 60 kg, then the average absorbed dose for each person on the Earth would be 5.161 × 10-8 Gray/sec. Since Pu-238 is primarily an alpha emitter we will use the conservative, worst-case weighting factor of 20 to convert to Sievert, the effective dose comes out to 1.032 × 10-7 Sievert/sec for every person on Earth. Over a period of a year this would expose every person on Earth to 3.25 Sieverts of an effective dose of radiation. For perspective, this dosage is roughly 30 times greater than the lowest one-year dose clearly linked an increased risk of cancer.
Doesn't this validate the concerns expressed by the Cassini critics, specifically that the mission posed a serious health risk to everyone on Earth? Why the implication previously presented that these figures are just a result of paranoia?
Focusing a more critical eye on the above approximations, it is quickly evident that blatant numerical trickery has been applied to generate such a terrifyingly high dosage. The assumption that every person receives an equal dose that cumulatively sums to the total radioactive material originally contained in the craft is absurd. It completely ignores the fact that the overwhelming majority would never have any contact with humans. Furthermore it ignores the fact that plutonium must be ingested to produce significant health risks. Claiming all of the plutonium would be absorbed by humans is simlar to claiming that having Clostridium tetani (tetanus bacteria) occurring naturally in the environment is tantamount to having all such bacteria equally distributed and injected into each person on Earth.
So we are left without a conclusive answer as to the actual danger posed by the Cassini mission; all that we've accomplished is showing that the claim that plutonium contained within could "induce lung cancer in every person on Earth" is unfounded. This statement, in fact, could be labelled as fear-mongering due to its lack of detailed justification and intended effect. This isn't to say that the release of radioactive material posed no health risks, but we have to find realistic, researched assessments of hazards. To this end we are left with the figures produced by NASA on the environmental impact of a failure. NASA's 1997 Supplemental Environmental Impact Statement for the Cassini Mission concluded that a worst-case failure would be a failure during the Earth gravity-assist phase and would result in 120 mean health effects.  To put this number into perspective, the World Health Organizations report on the Health Effects of Transport-Related Air Pollution estimates tens of thousands of health effects per year are attributable to emissions from transportation vehicles, in Europe alone.  In turn, the Cassini critics, such as the celebrity scientist Dr. Michio Kaku, accuse NASA's report of being self-serving and completely incapable of accounting for human error in design. While there may be some truth to these accusations they remain pure supposition unless detailed, defensible research is performed by the accuser. Such supposition is not science and does not move the conversation forward, it only serves to muddle the question by attempting to excite peoples' fundamental reaction to new technology: fear.
As a side, or perhaps afterthought, Fig. 2 gives a layman's reference for health risks posed by radiation exposure. Although not a scientifically rigorous chart, as noted by the creator of the chart, it is included in an effort to give some perspective to the true dangers of radiation exposure; an commonly misunderstood concept.
© Ross Allen. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Dozens Arrested in Protest of Plutonium-Fuled Space Mission," CNN, 4 Oct 97.
 E. P. Hardy, P. W. Kray, and H. L. Volchok, "Global Inventory and Distribution of Fallout Plutonium," Nature 241, 444 (1973).
 M. B. Snipes et al., "Review of John Gofman's Papers on Lung Cancer Hazard From Inhaled Plutonium," Lovelace Foundation, LF-51, September 1975
 D. L. Matson, L. J. Spilker, and J.-P. Lebreton. "The Cassini/Huygens Mission to the Saturnian System," Space Sci. Rev. 104, 1 (2002).
 M. R. Dahl, "Final Supplemental Environmental Impact Statement for the Cassini Mission," U.S. National Aeronautics and Space Administration, June 1997.
 M. Krzyzanowski, B. Kuna-Dibbert, and J. Schneider, "Health Effects of Transport-Related Air Pollution," World Health Oranization, 2005.