A radioisotope thermoelectric generator, RTG for short, is a small, very long lasting power source – an outgrowth of the nuclear power program. About the size of a thermos bottle, it can supply useful power for very long periods of time. Because it has no moving parts, it is more reliable than most other power sources, and because it is not dependent upon any outside element, it can supply power where nothing else will work.
RTGs are incredibly simple and safe. Certain substances are naturally radioactive – the atoms of these substances spontaneously change from time to time into simpler substances. When these changes happen, one or more relatively high energy neutrons are released. Because of their high energy, these neutrons can be harmful to living matter, just as bullets or forcefully thrown rocks can be. This natural process of atomic change is called radioactive decay.
One group of substances generates an especially large number of neutrons during this decay process – plutonium and some of its oxides. This is why plutonium is used in nuclear explosives. When a sufficiently large amount of plutonium is concentrated in a special way, neutrons can be forced into a very rapid chain reaction where each neutron creates more neutrons, so that in a fraction of a second incredibly high energies are produced.
Under normal circumstances this cannot happen. In an RTG, a small amount of plutonium dioxide (essentially plutonium rust), far less than the critical mass for a bomb or even that needed to sustain a reactor, is placed inside a container designed to trap the emitted neutrons as heat. With the plutonium, a simple electrical device having no moving parts, called a thermocouple, produces electricity when it gets warm. Since the plutonium can produce heat for a hundred years or more, RTGs can produce electricity that long.
RTGs are especially useful in deep space probes. Solar power could be used, of course, but the collector panels are less efficient, are subject to damage from external sources, and only work well near the sun.
In the mid-1990s, several “environmental” groups expressed serious opposition to using RTGs in space. They publicly decried the “dangers” of nuclear energy, and they insisted on keeping space free of such “dangers.” On one hand they seemed to know nothing of the true nature of the space environment: the sun is a continuously exploding hydrogen bomb millions of times the size of the whole Earth; our solar system is filled with high-energy particles, neutrons, cosmic raysat times it is like the inside of a nuclear reactor. Nothing humans could ever do would have the slightest effect on this environment.
On the other hand, they seemed equally ignorant of the benign character of RTGs. The only possible risk from RTGs could be scattering the thimbleful or so of plutonium into the atmosphere. This is a non-problem for two reasons: RTGs are designed to withstand an uncontrolled return through Earth’s atmosphere without breaking up; and even if one did break up, the amount of plutonium is so small that it is extremely unlikely it could even be detected, let alone harm anybody.
The irony is that the focus of ULYSSES, the European Space Agency space probe these groups were objecting to, was solar variability and its effect on Earth’s atmosphere, most notably the greenhouse effect. This is a crucial concern to everyone on Earth and a special interest to many environmental groups, including those protesting.
Once again, in October 1997, anti-nuclear groups tried to prevent the “contamination of space with nuclear energy,” this time objecting to the launching of the Saturn-bound CASSINI spacecraft. Scheduled for launch on October 6, 1997, CASSINI was powered by three medium-sized RTGs. Since this $3.4 billion spacecraft was scheduled to travel near Saturn, almost a billion miles from the sun, RTG power was unequivocally the best choice for this craft. Following its arrival in the vicinity of Saturn in 2004, CASSINI would study Saturn and its satellites for at least four years and parachute a small probe dubbed HUYGENS onto the surface of Saturn’s largest moon, Titan. It was the twenty-fourth U.S. space mission to carry RTGs – including the manned Apollo lunar landings.
The problem here is not the twenty-three eminently successful prior RTG-powered missions, nor the CASSINI mission, but the incredible ignorance demonstrated by the leadership of the anti-nuclear environmental coalition that continues to resist these launches. Their legal interference in the launch of ULYSSES delayed its deployment and nearly doubled its cost. The same kind of legal mischief prior to CASSINI’s launch in 1997 delayed that launch by about one and a half weeks.
In a free society, even uninformed people have the right to express their opinions. The leaders of the antinuclear environmental coalition have failed to educate themselves even to the most rudimentary level of knowledge regarding RTGs and the space environment. Fortunately, the courts recognized this failure in finding for NASA and the government. Unfortunately, the courts did not place responsibility for the financial cost of the proceedings and the subsequent launch delays where it belonged: Greenpeace, a coalition of Green parties, and other environmental groups.