October 17, 1997

The Cassini Probe

The investigator should have robust faith, and yet not believe.

Claude Bernard, 1855

The metal plutonium can be used to make atomic bombs and can be used in nuclear reactors to generate electricity. The plutonium that is used in bombs and reactors has a half life of 24,000 years.That means that if you kept a piece of it for that long, at the end of that time there would be half of its radioactivity left. That half would be halved again in another 24,000 years. Depending on the size of the the piece of plutonium, some radioactivity would be left many millions of years from now. The plutonium-238 used in space probes decays much faster. It has a half life of about 87 years,

Most elements have isotopes. Chemically, one isotope of plutonium cannot be distinguished from any other isotope of plutonium. The difference is that the weight of the atom is different and when an element loses particles from its nucleus, it changes into another isotope. In the process of changing from one isotope to another, it gives off various kinds of radiation including heat. Thus, a piece of plutonium-238 would be warm to the touch. It is this heat that makes it useful in a radioisotope thermoelectric generator(RTG) and for keeping other parts of the space probe warm. The plutonium is coupled to a solid state device that converts the heat directly into electricity.

Thermoelectricity forms the basis for devices that can be used to make refrigerators using just electricity, without pumps or coolants like freon or ammonia. It is now used in small refrigerators that can be plugged into a car's cigarette lighter socket. They have not been developed to a degree of efficiency where they can be used in a household refrigerator. If the efficiency of these devices could be increased substantially, it would mean completely noiseless and much more efficient refrigerators, without having to use those troublesome freons. One difference between an RTG and other thermoelectric devices is the source of the heat. In one case it can be electricity and a heating element, and in the other a decaying radioisotope. Although RTGs are inefficient, they are extremely reliable. There has not been a single failure in the RTGs used in the Voyager series of space explorers and in a number of other vehicles that went to the outer solar system and beyond. All of those vehicles used RTGs with about 40 pounds of plutonium.

An RTG is nothing like a nuclear reactor. A nuclear reactor depends on the controlled chain reaction identical to that of an atomic bomb. A reactor has the potential for doing immense damage and releasing very large amounts of radioactivity, as did the one at Chernoble. There is no possibility whatever of an RTG exploding. The Russians put a small nuclear reactor in orbit, which turned out to be a minor disaster when it came down. With the possible exception of the defense department, virtually all people concerned believe that putting nuclear material in orbit is courting disaster because anything placed in orbit has to eventually fall back to Earth. In contrast, a space vehicle that goes far enough away to escape the Earth's gravity, will never come back down.

Is it possible to pollute space with radioactive materials? The answer is "NO." The Sun is an immense hydrogen bomb, as is every star. Space is heavily radioactive and the closer that you get to a star,including our very own Sun, the more you will be irradiated. Astronauts and cosmonauts have probably had their lives shortened by their sojourn in space. It is an environment that is terribly hostile to life. It is our atmosphere and magnetic field that shields us from the radioactive ravages of space. What you see on Star Trek is flat-out impossible.

Is it possible to pollute the Earth with radioactive isotopes? The answer is an emphatic "Yes!" We have made a substantial start at doing just that, and many lives have been, and will continue to be, shortened by it. Medical x-ray has also shortened, as well as prolonged, many people's lives. The difference between an x-ray machine and a radioactive isotope is that when you shut off an x-ray machine, the radiation stops, while an isotope keeps on irradiating; some for a very long time.

When you read that the chance of a rocket exploding is 1 in 30 or 1 in a million, pay no attention to it. Since there are relatively few data to support those estimates, they aren't worth much. When you are told what your chances are of winning a lottery, you can believe it because the chances of winning the lottery are based upon the immutable laws of chance. Just as you know the odds of a tossed coin coming down head or tail, you know the odds in a lottery. When someone tells you that the chance of a rocket blowing up is "whatever," it is an estimate based on a small number of launches. It is not based on the laws of chance. Ten more safe launches or one failure could change the odds considerably.

The important consideration is what would happen if the rocket exploded, not what the odds are of it happening. In the case of Cassini, or any other large rocket, it could do a lot of damage if the explosion took place over a populated area. I wouldn't want to live near Cape Canaveral and I can't blame those Floridians for protesting. I do question whether the rest of the country is in any real danger, even in what they call the "worst case scenario."

What is the danger from the plutonium in the RTG? Should the rocket explode, the RTG can be expected to remain intact. It has been designed and tested to insure that it would remain intact. If it were destroyed, the plutonium would not vaporize because the isotope is in the form of ceramic (Corningware) encased pellets of plutonium oxide. Why do I believe this to be true? There have been two such explosions of rockets. In one case, the two RTGs were recovered intact. In the other, the single RTG ended up in a part of the ocean that is too deep to check. It will remain there, probably forever. The one case where an RTG exploded and metalic plutonium was vaporized, was a navy rocket that blew up in 1964. This was in the days when the Atomic Energy Commission(AEC) was irradiating the hell out of much of the population of the western states, in the name of protecting us from the Russians. That RTG was deliberately designed so that it would vaporize. The navy believed, erroneously, that small amounts of plutonium were harmless. What happened to the plutonium? It was scattered all over the Earth. We can assume that most of it fell in the ocean.

What would happen if you drank water or ate food containing plutonium? Nothing; plutonium is not absorbed by the gut. Some of that plutonium dust was probably inhaled, in which case, we can assume that those people who inhaled it were put at increased risk for developing lung cancer, some 10 or more years after inhaling it. The risk would be comparable to that incurred by smoking cigarettes. The difference is that smoking is voluntary, while exposure to plutonium is involuntary.

Can solar power be used for space probes that go past Mars? The answer is "no." For solar power to work there must be enough sunlight. While improvements to solar panels might make Jupiter and Saturn accessible to solar power, it is useless beyond those planets. It is always possible that someone may come up with something that will do the job without using radioisotopes. That doesn't exist at the present time.

I believe that the exploration of space is important. I was thrilled with the photographs that came back from all of the space probes. If you consider space exploration to be a worthless boondoggle, we have nothing more to discuss. We will have to agree to disagree.

I also believe that the danger of RTGs has been greatly exaggerated. We have been left with the impression that we are all in mortal danger. I don't believe it. The people living near Cape Canaveral have been, and are now in danger. Their danger is not from the plutonium, but from the rocket itself.

Yes, I would like the Cassini probe to be launched. If I am around 7 years from now, I would like to see the pictures that it sends back from Saturn, its rings and its moons. I am particularly interested in what the probe that descends to Saturn's largest moon will find. I also know that failures can be expected to happen frequently when one is exploring the unknown.

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