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The RTG Debate
The Cassini mission has been dogged by controversy, especially in the months leading up to its October launch. Many anti-nuclear activists claim that Cassini represents a threat to the lives of thousands of millions of people with its radioisotope thermoelectric generators (RTGs), devices that use plutonium to generate electricity. While NASA and other experts note that the RTGs are carefully designed to stay sealed even in a launch accident, and that the plutonium inside the RTGs is not as toxic as some people claim, anti-Cassini activists discount these statements. As the debate comes to a head in the weeks before launch, SpaceViews provides a look at what RTGs are and provides links to resources on both sides of the issue.
What Are RTGs?
![[image of RTG assembly]](RTG1.gif)
RTGs are devices that use the decay of radioactive materials to generate electricity for spacecraft missions. RTGs are not nuclear reactors; they use the natural decay of elements like plutonium to generate energy. The decay generates heat, which is used by equipment in the RTG to generate electricity.
Cassini uses 33 kg (72 lbs.) of largely plutonium-238, a non-weapons-grade isotope of the element. The plutonium is not placed is pure form in the RTGs but is installed as bricks of plutonium dioxide (PuO2), a ceramic which, if shattered, breaks into large pieces rather than smaller, more dangerous dust. The plutonium dioxide is encased in layers of materials, including graphic blocks and layers of iridium. Both materials are strong and highly heat resistant, which protect the plutonium bricks in the event of a launch explosion.
Still, the plutonium dioxide in RTGs is radioactive. Most of the radiation is emitted as alpha particles, which are nuclei of helium atoms. Such particles can be easily shielded against: a sheet of paper is sufficient to screen out the particles. Inside the body, however, alpha particle can damage cells and instigate cancer. Thus, an emphasis is placed on minimizing any release of plutonium in the form of easily respirable (breathable) particles, like dust. Plutonium dioxide is also largely insoluble in water, making it difficult for it to enter the food chain.
RTGs in Past Missions
The Cassini mission is not the first time RTGs have been used on a spacecraft. RTGs have been used nearly two-dozen times in the last 30 years. Some of the missions which included RTGs have been a number of the manned Apollo missions (for use as a power supply for experiments left behind on the lunar surface), Pioneers 10 and 11, Voyagers 1 and 2, and the Galileo mission to Jupiter. No approved NASA missions after Cassini use RTGs, although some missions in the planning stages, such as the Pluto/Kuiper Express mission and proposals for missions to Jupiter's moon Europa, have designs which include RTGs.
Not all of these missions have been successful. The Apollo 13 lunar module still had its RTG attached when it burned up over the Pacific Ocean upon its return to Earth after its aborted 1970 mission. The RTG was targeted for the Tonga Trench, one of the deepest points in the Pacific. No radioactivity was measured in the area of impact. In 1968 a Nimbus weather satellite fell into the Pacific just off the California coast when a launch failed from Vandenberg Air Force Base. The SNAP RTGs were later recovered and used on a later mission. Another early version of the SNAP RTG was lost when a satellite burned up in the Earth's atmosphere in the mid-1960s; by 1970 small levels of Pu-238 had been detected in the northern and southern hemispheres. These amounts were generally smaller than the Pu-238 from nuclear weapons tests (except in the southern hemisphere, because the RTG burned up over the southern hemisphere and nuclear tests took place in the northern) and far smaller than levels of Pu-239 and Pu-240 from tests.
Why Use RTGs?
Most NASA missions, including all those that fly through the inner solar system, use solar panels to generate power. These are cheaper and lighter than RTGs and do not carry the same concerns about safety that RTGs do.
However, for these to work, they require strong sunlight that can be converted to electricity. In the outer solar system, many times farther from the Sun than the Earth, strong sunlight does not exist. To compensate, the solar panels used would have to be many times larger to capture enough sunlight. For the Cassini mission, the solar panels used would have to be large enough to cover one-fourth of a football field. Panels that large would be very heavy and difficult to control.
The above calculation, though, assumes solar panels have the same efficiency. The European Space Agency has developed panels which, in the lab, has a much higher efficiency; thus, in theory, could be used to make smaller solar panels for Cassini. However, these advanced solar panels have not been used in practice and, in lab tests, are rather heavier than conventional panels, leaving the mass of the panels still an issue.
RTG Resources
There are a number of Web sites which provide perspectives on both sides of the issue, as well as sites which try to provide a neutral viewpoint. Spaceviews has collected these sites in a separate page.