At no point will Juno (or the other solar spacecrafts) be a threat to life on Earth. This includes Juno posing no danger when in 2013 it makes a flyby of Earth. Such flybys making use of Earth's gravity to increase a spacecraft's velocity have constituted dangerous maneuvers when in recent years they've involved plutonium-powered space probes such as NASA's Galileo and Cassini probes.
Curiosity is a return to nuclear danger.
NASA's Final Environmental Impact statement admits that a large swath of Earth could be impacted by plutonium in an accident involving it. The document's section on "Impacts of Radiological Releases" says "the affected environment" could include "the regional area near the Cape Canaveral Air Force Station and the global area."
"Launch area accidents would initially release material into the regional area, defined"to be within "62 miles of the launch pad," says the document. This is an area from Cape Canaveral west to Orlando.
But "since some of the accidents result in the release of very fine particles less than a micron in diameter, a portion of such releases could be transported beyond"62 miles," it goes on. These particles could become "well-mixed in the troposphere"--the atmosphere five to nine miles high--"and have been assumed to potentially affect persons living within a latitude band from approximately 23-degrees north to 30-degrees north." That's a swath through the Caribbean, across North Africa and the Mideast, then India and China Hawaii and other Pacific islands, and Mexico and southern Texas.
Then, as the rocket carrying Curiosity up gains altitude, the impacts of an accident in which plutonium is released would be even broader. The plutonium could affect people "anywhere between 28-degrees north and 28-degrees south latitude," says the NASA document. That's a band around the mid-section of the Earth including much of South America, Africa and Australia.
Dr. Helen Caldicott, president emeritus of Physicians for Social Responsibility, has long emphasized that a pound of plutonium if uniformly distributed could hypothetically give a fatal dose of lung cancer to every person on Earth. A pound, even 10.6 pounds, could never be that uniformly distributed, of course. But an accident in which plutonium is released by a space device as tiny particles falling to Earth maximizes its lethality. A millionth of a gram of plutonium can be a fatal dose. The pathway of greatest concern is the breathing in plutonium particle..
As the NASA Environmental Impact Statement puts it: "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."
"A small fraction would be transported over time directly to the blood or to lymph nodes and then to the blood," it continues. Once plutonium "has entered the blood via ingestion or inhalation, it would circulate and be deposited primarily in the liver and skeletal system." Also, says the document, some of the plutonium would migrate to the testes or ovaries.
The cost of decontamination of areas affected by the plutonium could be, according to the NASA statement, $267 million for each square mile of farmland, $478 million for each square mile of forests and $1.5 billion for each square mile of "mixed-use urban areas."
The NASA document 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 bands on commercial fishing; and public health effects and medical care."
As to why the use of a plutonium-powered rover on Mars--considering that NASA has successfully used solar-powered rovers on Mars--the NASA Environmental Impact Statement says that a "solar-powered rover"would not be capable of operating over the full range of scientifically desirable landing site latitudes" on this mission.
There's more to it. For many decades there has been a marriage of nuclear power and space at NASA. The use of nuclear power on space missions has been heavily promoted by the U.S. Department of Energy and its predecessor agency, the U.S. Atomic Energy Commission, and the many DOE (previously AEC) national laboratories including Los Alamos and Oak Ridge. This provides work for these government entities. Also, the manufacturers of nuclear-powered space devices--General Electric was a pioneer in this--have pushed their products. Further, NASA has sought to coordinate its activities with the U.S. military. The military for decades has planned for the deployment of nuclear-powered weapons in space.
Personifying the NASA-military connection now is NASA Administrator Charles Bolden, a former NASA astronaut and Marine Corps major general. Appointed by President Barack Obama, he is a booster of radioisotope thermoelectric generators as well as rockets using nuclear power for propulsion. The U.S. has spent billions of dollars through the years on such rockets but none have ever taken off and the programs have all ended up cancelled largely out of concern about a nuclear-powered rocket blowing up on launch or falling back to Earth.
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 accident, 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.