by Carl Sagan
October 09, 1989

Last Updated February 1997

from Chance Website

Spanish version

An edited version of this article, amounting to cuts of about 20 percent of the text, was published as "Benefit outweighs risk: Launch Galileo craft," in USAToday, Inquiry Page, Tuesday, October 10, 1989

Late this August the Voyager 2 spacecraft flew within 3000 miles of the south pole of the planet Neptune, triumphantly concluding its exploration of all four giant planets with phenomenal interplanetary marksmanship.


Our knowledge of the solar system has been decisively rewritten. Our species has visited what is now the outermost known planet. Voyager's place in human history is secure.

Now that we have completed the preliminary reconnaissance of the solar system, it makes sense to explore selected worlds in greater depth. The next step is Galileo - to be launched aboard the shuttle, nominally in October or November. It's to be the first space vehicle to go into orbit around Jupiter, the largest planet in the solar system.


If all goes well, it will explore, in much greater detail than Voyager, multicolored Jupiter, its four large moons - one with active volcanoes, another with a possible underground ocean - and its vast magnetic field; Galileo will also drop a scientific probe directly into the atmosphere of Jupiter and radio back what it finds. It is a trailblazing mission.

However, Galileo, launched from the comparatively enfeebled present space shuttle configuration, can't simply make a beeline for Jupiter. Instead, it must execute a set of caroms through the inner solar system - first to Venus for a gravitational assist that then flings it back to the Earth, which swings it around the Sun once more to the Earth, where it receives a third boost and is finally on its way to Jupiter.


It is scheduled to begin operations there in late 1995. While it's careening past worlds, Galileo will be gathering data - about Venus, about the Moon, about two worldlets named Gaspra and Ida, about the interplanetary gas... and about the Earth. It will help determine the worldwide distribution of greenhouse gases, the present status of the ominous hole in the ozone layer over Antarctica, and the water content of the upper atmosphere - central for understanding the ozone problem. Furthermore, its investigations of the atmospheres of Venus and Jupiter promise to improve our knowledge of our own fragile envelope of air. Galileo will not only be exploring other worlds; it will help us to understand and safeguard this world. Galileo is a worthy successor to Voyager.

Because Voyager had to fly so far from the Sun (which appears virtually as a bright point of light from the distance of Neptune), it could not rely on sunlight for energy. Instead, it was powered by heat from the radioactive decay of plutonium - all this occurring safely, without the slightest mishap, in a component of the spacecraft called an RTG, for "radioisotope thermoelectric generator."

Galileo also will be powered by radioactive plutonium. There is no alternative. To power Galileo by solar panels, the spacecraft would have to be as big as a house; to power it by batteries would add so much weight that the mission would never fly - at least on any U.S. launch vehicle in existence or now under development. But plutonium can be deadly, and the Galileo RTG's have now begun to alarm many people.


A lawsuit has been filed in Federal District Court in Washington, D.C. - by the Washington-based religious affiliated Christic Institute and other organizations - to stop the Galileo launch on the grounds that it may pose a serious danger to public health. Meanwhile, the White House, after considering the dangers, has given the go-ahead for launch.

I'm a scientist working on Galileo with a long-time involvement in planetary exploration. I'm also a long-term supporter of the Christic Institute. I admired their successful suit on behalf of the estate of Karen Silkwood against the Kerr-McGee Corporation - accused of shameful negligence in protecting industrial workers from the dangers of radioactive waste. (I also admired the Christic Institute's early warnings about what later came to be known as the Iran-Contra fiasco.)


Concern about the environment and, especially, about the threat of nuclear war has been a thread woven through my life. I was a member of the team that discovered nuclear winter; I've twice been arrested at the Nevada Nuclear Test Site for demonstrating against continued American testing of nuclear weapons in the face of the Soviet unilateral moratorium; I opposed Ronald Reagan's Star Wars scheme from the moment he proposed it - on grounds that are now widely accepted; for the past decade I've been speaking out around the world to warn about greenhouse warming and depletion of the ozone layer.


At the very least, you can't charge me with uncritical acceptance of high technology. Twenty years ago, I also played a role in the NASA decisions to quarantine astronauts returning from the Moon against the unlikely contingency that they might bring back disease microorganisms. It turned out as we had expected: there was not a trace of pathogens. But we had to balance the low probability of their existence against the enormous conceivable public health danger that might follow had we been wrong and such bugs did exist. I would do the same today.

I've felt torn on the Galileo RTG issue for years. I still do. Four years ago, I arranged for the Planetary Society, the largest space interest group in the world, to commission an extensive article presenting both sides of the issue (David Salisbury, "Radiation Risk and Planetary Exploration - The RTG Controversy," The Planetary Report, May-June 1987). I believe there is nothing absurd about either side of this argument. Many people have urged me to make public my thinking on this matter, and I here take the opportunity to do so:

How dangerous is plutonium?


The authoritative Handbook of Physics and Chemistry in its various editions calls plutonium "a very dangerous radiological hazard" and "one of the most dangerous poisons known."


Robert Oppenheimer, the Director of the Manhattan Project, reminisced in February 1960:

"If the plutonium had ever caught fire, there would not have been anyone left in Los Alamos and probably in much of New Mexico, it is so terribly toxic. It burns in oxygen."

(This remark is true for plutonium the metal, but not the ceramic form aboard Galileo.)

A microgram of the stuff - a particle much too tiny to see - if breathed into your lungs may, over a period of decades, give you cancer.


Since Galileo carries 50 pounds of plutonium into space, it is hypothetically carrying a cancer fatality for everyone on Earth. This is an impossibility in fact, because it requires the plutonium to be funneled directly into the lungs of everyone on Earth, instead of being dispersed in and diluted by the Earth's atmosphere. But this is where much of the concern (including real anguish in many letters I've received) is focused. Understandably.


Why didn't we hear similar concerns voiced about the launch of Voyager or Viking (which also carried RTG's)? Because that was in another epoch - before Chernobyl, before Challenger, before the revelations about Rocky Flats, before we got serious about protecting the planet. One year before the Chernobyl disaster a Soviet Deputy Minister of the power industry announced that Soviet engineers were confident that you'd have to wait 100,000 years before the Chernobyl fission reactor had a serious accident.


Less than a year before the Challenger explosion, NASA spokesmen and contractor personnel assured us that at the then current rate of launch, you'd have to wait ten thousand years before a catastrophic launch failure. Hundreds of FBI agents descending on the Department of Energy's Rocky Flats facility in Colorado has raised justifiable fears of criminal carelessness by the U.S. government where public health and nuclear energy intersect.


The Department of Energy and the Department of Defense have systematically minimized the dangers of nuclear power and of nuclear weapons. These cases rouse valid skepticism about government-sponsored probability estimates which are intended to calm the public. Skepticism about government credibility is, in my view, healthy. You can't maintain a democracy without it. I'd like to see much more of it.

What are the actual dangers concerning Galileo's plutonium?

  1. First of all, it can't explode. Given the configuration and amount of plutonium, there is no conceivable danger of a nuclear explosion.

  2. Secondly, if the Challenger explosion happened all over again with Galileo, there would be no plutonium danger. It would fall to Earth in solid lumps contained within their protective shields. Nobody would breathe it. The danger comes when the plutonium is ground down into very tiny breathable particles, or when it's vaporized - converted into atoms.

Are there any plausible circumstances in which this could happen?

There are some failure modes - explosions just after launch, for example, in which pieces of metal, improbably, go sheering through the protective graphite shields and iridium clads that surround the lumps of plutonium - that I'll ignore here because they release much less plutonium than the most worrisome potential failure: the possibility that the plutonium is vaporized during a fiery accidental reentry of Galileo into the Earth's atmosphere.

On its second pass by the Earth, Galileo is scheduled to miss our planet by as little as 200 miles.


What if the trajectory is a little bit off and it hits the Earth?


Then, entering the Earth's atmosphere at 30,000 miles per hour, it might burn up; it's not guaranteed, it may even be unlikely, but there's a chance that all 50 pounds of plutonium would be vaporized. Some of the plutonium would quickly settle out; some of it would be carried widely by the winds and the general circulation of the Earth's atmosphere. It would be enormously diluted in the air.


Some people would breathe in more plutonium and some less over the next 50 years, but no one is likely to get as much radiation from this source as in a single dental X-ray. But there's a tiny chance that you can get cancer from such an X-ray. In our ignorance, we don't know what these low radiation doses would do. In the worst case, you might have an incremental chance of around one in 10 million of getting cancer were all of Galileo's plutonium to vaporize in the upper air. That's the equivalent of producing bone and other cancers in roughly a thousand people worldwide.


Or there might be no health effects at all. We simply don't know. (Remember, these people are at risk only if, improbably, Galileo burns up in the Earth's atmosphere on its way back from Venus.)

There are two ways of looking at this: One chance in 10 million is very long odds - safer over 50 years, for example, than taking a single commercial airline flight is for a few hours. By such a standard, the risk is negligible. But when I fly on an airplane, I do so voluntarily and presumably fully aware of the dangers. It's no business of the government, or some Jupiter-obsessed scientists, to diminish my life expectancy without even consulting me.


Roughly 1000 deaths, over 50 years, in a world population that will by then be 10 billion people, seems very small. But if anyone dear to me is one of those people, I no longer find the odds comfortably small.


So then I have to ask myself: why should it matter whether it's someone dear to me? Shouldn't I have the same concern for the health of everyone on Earth?

But we haven't yet asked how likely it is that Galileo, instead of swinging by the Earth, will accidentally collide with it. Here I believe the probability estimates are reliable.


They are not made by the Department of Energy or NASA contractors, but by NASA's Jet Propulsion Laboratory (JPL), run by the California Institute of Technology.

  • On the one hand, JPL - responsible for the Galileo project - has an enormous vested interest in seeing the spacecraft successfully launched.

  • On the other hand, JPL's record on risk assessment is excellent.

These are the people responsible for Voyager and most other American robotic missions to the planets, the people with the most experience on Earth in interplanetary navigation and the inventors of the gravity assist.


The safety program for containing the plutonium in the Galileo RTG's and for understanding the risks has cost NASA about S50 million.

The JPL engineers have listed the remote contingencies: The spacecraft might be hit by a meteorite in interplanetary space and by accident redirected towards the Earth. There might be a programming error so the spacecraft veers much closer to the Earth than had been planned. There might be an accidental firing of the onboard rocket motor that would have the same effect. There are many possibilities. Every one of them is extremely unlikely. Even if they occur, there is little danger, because unless Galileo itself is crippled in some way, the spacecraft can be commanded to alter its trajectory.


When the JPL engineers add up all conceivable sources of trajectory error and their probabilities, plus the likelihood that the error will make the spacecraft hit the Earth rather than miss it by a bigger distance, plus the probability that simultaneously the spacecraft will be unresponsive to commands from the Earth, they derive an overall estimate of the probability of accidental impact. This number is 1 chance in 2 million.

So there's only 1 chance in 2 million that instead of swinging by the Earth and being flung on to Jupiter, Galileo will plummet in flames into the Earth's atmosphere, fragment, burn up and release its fuel as plutonium dioxide vapor into our atmosphere. If that happens, only then is there a chance that around 1000 people would get cancer over the next 50 years - although, in our ignorance, it might be that not even one person is injured.

There is no such thing as absolute safety. To assess risks, we are required to assess probabilities. If there were a 50-50 chance that even one person would die because of the Galileo launch, I would be against it. But there must come some point where I conclude that the risk is so minimal that it becomes acceptable. Different people may well draw that line in different places. One chance in a million that 1000 people would die is, in a certain sense, like one chance in a thousand that one person would die.


This is somewhere around my threshold. That's why I find the Galileo decision so agonizing. But taking account of the past history of government incompetence or worse in matters of public health, considering the likely scientific findings (including the possibility that many more lives might be saved because of Galileo's findings), and evaluating the low magnitude of the risk, my personal vote is to launch.

My assessment for spacecraft in Earth orbit is quite different. Here sunlight is strong enough to provide power. Here chemical sources of energy can be carried up. And here - where the plutonium is guaranteed to come down sooner or later - lies the greatest danger. The attitude of the space-faring nations on this issue has often been irresponsible. In 1964 a U.S. Department of Defense satellite carrying an RTG did enter the Earth's atmosphere and dispersed plutonium-238 at high altitude; but this was no accident - it was designed to disperse its plutonium worldwide.


So no protective covering was included to minimize the plutonium dispersal. No official thought seems to have been given to the possibility that it might be a bad idea to distribute deadly plutonium all over the planet. An even more serious danger than RTG's is power reactors - in which nuclear fission is occurring in Earth orbit. The chief offender here has been the Soviet Union, especially its radar satellites designed to follow the activities of U.S. warships worldwide. Their failed Cosmos 954 satellite distributed plutonium pellets all over Western Canada.


The Washington-based Federation of American Scientists (FAS), the Moscow-based Soviet Scientists Against War and the Nuclear Threat, and House bill H.R. 966, introduced this year, all propose, in the words of the House bill, a,

"ban on the use of nuclear power sources in orbit around the Earth," although "nuclear power sources for a Moon base or deep space scientific and exploration missions should not be curtailed."

The FAS, the Chairman of the Soviet group, and Rep. George Brown, sponsor of the House Bill, unanimously support the launch of Galileo.

I end with a plea for consistency. There are issues -including nuclear war (accidental and deliberate), greenhouse warming, depletion of the ozone layer, AIDS, social and economic injustice and the world population crisis - where the combination of probability and consequence are enormously more dangerous than for Galileo's plutonium.


I would like to urge everyone concerned about the Galileo RTG - including the scientists, engineers and government officials who for the first time have been forced to think seriously about this matter because of public protest - to devote a proportionate amount of passion, wisdom and hard work to those activities (and inactivities) that really jeopardize the human family.