Plutonium-238 has a far shorter half-life--87.8 years--as compared to Plutonium-239 with a half-life of 24,500 years. An isotope's half-life is the period in which half of its radioactivity is expended.
As Dr. Arjun Makhijani, a nuclear physicist and president the Institute for Energy and Environmental Research, has explained, Plutonium-238 "is about 270 times more radioactive than Plutonium-239 per unit of weight." Thus in radioactivity, the 10.6 pounds of Plutonium238 that is to be used on the Mars Science Laboratory Mission would be the equivalent of 2,862 pounds of Plutonium-239. The atomic bomb dropped on Nagasaki was fueled with 15 pounds of Plutonium-239.
The significantly shorter half-life of Plutonium-238 results in it being extremely hot. This heat is translated in a radioisotope thermoelectric generator to electricity.
The pathway of greatest health concern is breathing in a plutonium particle. A millionth of a gram of plutonium can be a fatal dose. As the NASA Environmental Impact Statement says: "Particles smaller than about 5 microns would be transported to and remain in the trachea, bronchi, or deep lung regions." The plutonium particles "would continuously irradiate lung tissue."
The NASA Environmental Impact Statement lists "secondary social costs associated with the decontamination and mitigation activities" as: "Temporary or longer term relocation of residents; temporary or longer term loss of employment; destruction or quarantine of agricultural products including citrus crops; land use restrictions which could affect real estate values, tourism and recreational activities; restriction or bans on commercial fishing; and public health effects and medical care."
Meanwhile, as to Juno, Aviation Week and Space Technology reports: "The unique spacecraft will set a record by running on solar power rather than nuclear radioisotope thermoelectric generators previously used to operate spacecraft that far from the Sun."
Juno--66-feet wide--will be powered by solar panels built by a Boeing subsidiary, Spectrolab. The panels can convert 28 percent of the sunlight that reaches them to electricity. They'll also produce heat to keep Juno's instruments warm. This mission's cost is $1.1 billion.
Accidents have happened in the U.S. space nuclear program. Of the 26 space missions that have used plutonium which are listed in the NASA Environmental Impact Statement for the Mars Science Laboratory Mission, three underwent accidents, admits the document.
The worst occurred in 1964 and involved, it notes, the SNAP-9A plutonium system aboard a satellite that failed to achieve orbit and dropped to Earth, disintegrating as it fell. The 2.1 pounds of plutonium fuel dispersed widely over the Earth, and Dr. John Gofman, professor of medical physics at the University of California at Berkeley, long linked this accident to an increase in global lung cancer. With the SNAP-9A accident, NASA switched to solar energy on satellites. Now all satellites--and the International Space Station--are solar-powered.
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