This is a fax sent to Clinton Science Advisor Dr. John Gibbons by Geoffrey Sea, radiological health physicist and director of the Atomic Reclamation and Conversion Project of the Tides Center.


6 October 1997

John Gibbons
Science Advisor to the President
Subject: Cassini Space Probe

Transmitted via facsimile: 202-456-6021


Dear Dr. Gibbons,

I am a radiological health physicist and director of the Atomic Reclamation and Conversion Project of the Tides Center, which conducts research and education on issues of radiation, health and the environment. For eighteen years I have studied health problems associated with exposure to radioisotopes. I will not recount the many arguments made by other scientists and physicians that warrant against launch of the Cassini space probe. But I believe some important issues have been neglected in the debate:

  1. The type of analysis used by NASA to estimate the probability of a catastrophic plutonium release was never intended and is not suited to this type of calculation. "Fault/Event" modeling of the type employed by NASA was designed to determine the relative probabilities of various types of failures for the purpose of directing redesign and maintenance operations, not determining the absolute probabilities of a multiple-failure accident. The fact is that there is no means to determine such absolute probabilities because the probabilities of certain known accident scenarios (like terrorist attacks) and unknown accident scenarios (like mistakes in materials production or supply) are impossible to determine. Rather than admit that no reliable estimates could be provided, NASA simply misappropriated a method that looks as if it is yielding accident risk estimates, a common practice among technologists asked by policy makers to provide unavailable data. All of the accident risk estimates are meaningless. Policy makers must assess Cassini on the basis of the record of spacecraft accidents, which is not reassuring.
  1. Most toxicity data that has been accumulated for plutonium is specific to Plutonium-239, the isotope most frequently encountered in the weapons complex. Most of the plutonium carried by Cassini will be Plutonium-238. One key difference is that Pu-238 decays about 270 times faster than Pu-239. In the case of Pu-239, the ingestion pathway is usually overlooked because most of the plutonium will have been excreted by the time it decays. But the ingestion pathway for Pu-238 has not been adequately studied. It may well be as important as the inhalation pathway because of the much faster rate of decay.
  1. Another reason that the ingestion pathway has been dismissed is the insolubility of the plutonium. But nearly all studies on the biological absorption of plutonium from the gut have been performed under laboratory conditions, dissolving the plutonium in distilled water. When plutonium is dissolved in chlorinated or fluoridated water, however, it forms soluble compounds with the chlorine or fluorine and its absorption is increased by up to four orders of magnitude. Since most sources of drinking water in the United States contain chlorine or fluorine or both, ingestion should be studied as a pathway of principal concern in the event of a catastrophic release of plutonium.
  1. There are important sovereignty questions here. Suppose Libya or North Korea decided to launch a plutonium-powered probe into space, or a nuclear powered rocket for that matter, certified as safe by their own space agency. No country has the right to deliberately jeopardize the citizens of other nations.

Finally, and most importantly, I believe that the Cassini mission will be a failure, even if there is no catastrophe. It will be a failure because it will associate the space program with raising a Sword of Damocles over the head of every person on the planet. NASA runs a real risk of having the pictures that come back from Saturn viewed as a kind of violent pornography.

The space program was supposed to be about that sense of awe and wonder we feel, as children and as ever-curious adults, as we look to the skies and realize that they, in their purity and perfection, reflect our best image of ourselves, making us feel more at home in the universe. These are not rational feelings, nor should they be. If Cassini is launched, and comes, a toxic terror, back toward Earth in a head-long rush on its "swing-by," it will be as if we have shattered the perfection of the universe with our most vain and errant indulgences, as if we are intent on putting our worst poisons and impulses into space, as if we are impelled to make our home in the universe as dangerous and dysfunctional as our earthly homes are fast becoming.

Whatever the actual statistical risks, the idea of a cosmic game of chicken is scaring people half to death. The skies, as a metaphorical reflection of life on the ground, always evoke such feelings, and the irrationality of these emotions makes them no less real, as NASA certainly knows. In short, the social and psychological impact of Cassini, including its chilling effect on public support for the space program, may outweigh any potential health risk from the plutonium, even if everything goes according to plan. A Cassini scientist has told me that the controversy surrounding Cassini has already doomed any future plans to put plutonium in space. That is as it should be.

If the President intervenes to cancel Cassini he will fulfill his obligation of public protection and will also send a vital message: that the juggernauts of big science and technology are not unstoppable; that managers and engineers are not free to pose and accept any risk on our behalf; that the government will not always subordinate the public interest to corporate and bureaucratic imperatives; that our exploratory impulses are not exclusive of the virtues of patience and prudence. The President can score a victory for accountability over acceleration, and that alone would justify the billions already spent on Cassini.

But the benefits of the innovations that went into the Cassini probe will still be with us. In addition, future technological projects will be improved by the awareness that large expenditures do not in themselves make project completion inevitable. And Saturn will be explored, in good time, by smaller, cheaper, better machines that do not threaten our conception of the skies and of ourselves.

We urge the President to cancel the Cassini launch.



Geoffrey Sea, director



The Atomic Reclamation & Conversion Project is an activity of the Tides Center,

a nonprofit, public charity. Contributions are tax deductible to the extent permitted by law.

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