Risk of nuclear powered space probes

摘要

In recent years, three probes have been sent into outer space: Galileo, Ulysses and Cassini. Each has used for its electricity power source, a radioactive thermoelectric generator (RTG), heated by a source of plutonium 238. Cassini, launched in October 1997, will reach its scientific objective in late June or early July 2004, concurrent with PSAM7. The authors of this paper had the privilege of providing the final review of the Cassini safety assessments for the Office of Science and Technology of the President, before final approval by the President of the United States, as required by law and international treaty, before the launch of any nuclear device into space. In this paper, we will discuss the risk analysis carried out jointly by NASA and DOE and our assessment of their results. Since the United States has established a new initiative for space exploration using a nuclear fission device for exploration of the icy moons of Jupiter (JIMO), we believe the JIMO project can benefit from a critical review of the experience gained by past projects using RTGs, and in particular identification of those aspects which make the risk estimates plausible to the layman. There are several scenarios for possible adverse effects on public health. The probabilities for each scenario must be added to form a number for the overall risk. The probability of each scenario is a product of two basic terms: the probability of release of plutonium and the probability of an individual being hurt by the release. It is evident that the probabilities for the two terms are truly independent. For example, there is reason to believe that the probability of the space probe hitting the earth in a failed 'swing by' only has a connection with the stochastic probability of developing lung cancer by the amount of plutonium evaporated and not by the mechanism of cancer induction. Each probability is on the order of one in a million. Then it is easy to understand why the risk to an individual of a 'swing by' is less than one in 10~(11) years, although the more complex NASA-DOE calculations, which allow for a reduction in the plutonium release for many possible reentry scenarios, puts it a factor of 50 smaller. We also comment on the procedures used for calculating the probabilities and consequences of these scenarios, on presenting these calculations to the public, on errors in various public comments on the Cassini Environmental Impact Statement, and on the comparative risks from RTGs and small nuclear reactors.