would be dispersal of the plutonium after collision with a meteor—a risk that applies only to the options involving continuing orbit around the sun, rather than escape from the solar system or disposal in the sun itself. The 1980 study estimated that the probability of a collision that would release even 0.2 percent of the payload in small particles would be about 4 parts per billion per year, while complete fragmentation (meaning collision with a larger meteor) would be 100 times less probable.^31 Thus, unless the payload were extremely unlucky, the plutonium would have completely decayed to uranium by the time a collision with a meteor might occur.
If the payload were in a circular orbit at 0.85 astronomical units (AU) (85 percent as far from the sun as the earth) the study estimated that 0.12 percent of the fine particles resulting from such a collision would fall to earth, taking an average of 100,000 years after the collision to do so. In contrast, if the payload were placed beyond earth's orbit, at 1.19 AU, 6.7 percent of the small particles produced in a collision will intercept the earth, taking an average of only 50,000 years to do so.^32
The concept of launching the material directly into the sun would eliminate any risk that the plutonium would re-encounter the earth, but it would require a higher velocity (more than 18 kilometers per second) than any of the other options.
Cost
The cost of this approach would depend on the mass of the material that had to be launched, the velocity that the material had to reach, and what new systems had to be developed.
The mass that needs to be launched would be much more than the mass of the plutonium itself. First, one must consider that the plutonium should be in a form that will reliably be noncritical even if, for example, an accident results in it being immersed in ocean water. Thus, the plutonium must be combined with some neutron-absorbing material. Detailed calculations show that PuOa x 3B is subcritical in any quantity, density, or configuration (using boron of normal isotopic composition); it is 81 percent plutonium by weight.^33 The reentry vehicle (RV) needed to ensure integrity in the event of an accidental explosion or reentry from orbit would roughly double the mass of the composite. Thus, the overall RV mass would total 2.5 times the mass of the contained plutonium.
31
Analysis of Nuclear Disposal in Space, op. cit.
"This major difference arises from the Poynting-Robertson drag on the small fragments, which encounter more solar radiation on their leading face than on the trailing face, thereby experiencing a
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