A Scientific Critique of the Accident Risks from the Cassini Space Mission

By: Dr. Michio Kaku

Henry Semat Professor of Theoretical Physics
Physics Department
City University of New York
New York NY 10031

Note: This article is also available in a german translation courtesy of the Netzwerk Friedenskooperative (Network of the German Peace Movement).
Abstract: If we carefully re-examine, line-by-line, the physics analysis behind NASA's Final Environmental Impact Statement, we find that the FEIS has consistently underestimated the possible risks of an accident with the Cassini space mission. Originally, NASA estimated the number of cancer fatalities from a maximum credible accident over a 50 year period to be 2,300. We detail how this figure of 2,300 deaths could easily be off by a factor of 100, i.e. true casualty figures for a maximum accident might number over 200,000. Furthermore, property damage and lawsuits could be in the tens of billions. In addition, the FEIS has over- estimated the difficulty of using alternate sources of energy, such as solar and fuel cells. In line with the new NASA philosophy of faster, cheaper, better, the Cassini mission should be downsized and made into smaller, more frequent solar-powered missions to Saturn with less power requirements.

Table of Contents:

I. Introduction

The Cassini mission contains about 400,000 curies of plutonium-238, making it the largest space mission ever undertaken involving plutonium power packs (RTGs). The plutonium, about 72 pounds in weight, is distributed in 3 RTGs, with 18 modules each. If that quantity of plutonium is somehow dispersed into a populated environment, there is no question that such an accident could cause significant health effects resulting in thousands of casualties. All scientific experts are agreed on this point.

What divides the experts is:

All parties are agreed that such an event is unlikely. It may happen that the Cassini mission may be a resounding, flawless success. However, it's only a matter of time before some disaster strikes. Instead of relying on misleading computer programs which tell you what you want to hear, one should carefully examine the actual track record of accidents in the space program, with numerous booster rocket failures and space probes which malfunction.

Unfortunately, the true risks from such an accident and the consequences have been downplayed. In a democracy, the American people can make rational decisions only on the basis of scientific truth, not simplistic, misleading press releases. It is inevitable that there will be spectacular accidents with the space program, some involving casualties, and the American people have a democratic right to know what the true risks are. Thus, it is a matter of scientific interest to go over line-by-line the calculation of the FEIS.

NASA calculates in its FEIS that up to 2,300 people might come down with fatal cancer over a 50 year period from the dispersal of plutonium-238 over a populated area. More recently, it has lowered this figure to 120. However, the calculation of these figures depends on three important steps, each of which has been underestimated by NASA:

In each category, we will show that:

II. Calculation of Casualties from a Maximum Accident

A Launch Phase (Phase 1,5 and 6)

We will investigate all three steps for two crucial phases, the early launch phase and the fly-by phase.

B. Fly-by Phase (VVEJGA Phase)

The source of greatest concern, from the point of view of plutonium release, is the fly-by. The Cassini probe will be whipping around the earth at around 40,000 miles per hour, significantly faster than the escape velocity of the earth (25,000 miles per hours) and faster than many meteorites. If there is even the tiniest miscalculation of the trajectory, the Cassini may burn up in the atmosphere and spray a significant portion of land area with plutonium. There is ample experimental evidence that space probes, without heat shields, will vaporize upon re-entry. However, the FEIS again takes a low estimate of plutonium release.

III. Calculation of Risk

The analysis used by the EIS to calculate the probability of a maximum accident with the Cassini mission uses methods pioneered by the nuclear power industry (e.g. single event failures, event tree analysis, Monte Carlo calculations, etc.)

Although these methods are standard for the field, these methods have largely been discredited by the actual operating record of nuclear power accidents. Three Mile Island, for example, was a Class IX accident which was largely unforeseen by MIT's WASH-1400, the standard reference within the industry, which largely ignored small pipe breaks.

The methodology is flawed for several reasons:

  • i) Human error and design flaws.
    Most of the major accidents that have taken place in the past are beyond the simple-minded event-tree analysis of the FEIS. For example, one can design a car such that the chances of an accident approach a million to one, with air bags, anti-lock brakes, seat belts, etc. However, this does not foresee the fact that someone might drive this car over a cliff.

    The actual track record of accidents shows that computer calculations are often misleading and give a false sense of confidence:

    The real danger is that the engineers begin to believe their own computer calculations, which are only a guide, not a law of nature. Then they become overconfident and fail to foresee the inevitable.

  • ii) GIGO. There is an expression, "garbage in, garbage out." Even if you use the world's largest supercomputer, if your assumptions are faulty, then your conclusions will also be faulty. For example, one can use a supercomputer to calculate the precise number of angels that can dance on the head of a pin. But giving you this number to three significant figures is meaningless, since the original assumption was in question.
  • iii) Similarly, the basic assumption of the FEIS is that one can model accidents on the basis of single event failures, when multiple failures, common mode failures, human error, and design flaws have contributed to most accidents. Unfortunately, it is beyond the power of computers to realistically model these more complex types of accidents.
  • iv) Weakest link: the Titan IV
    A chain is no stronger than its weakest link. The weakest link is the Titan IV booster rocket, which has a failure rate of about one in 20. And booster rockets in general have a failure rate of 1 in 70 or so. Furthermore, there have been 3 failures among the 23 missions involving plutonium power packs, one which released a significant amount of radiation. In fact, everyone on the earth has a piece of the SNAP 9A satellite in their body. The SNAP 9A satellite also significantly increased the amount of plutonium-238 on the planet earth.
  • v) Where does the one-in-a-million figure come from?
    The FEIS typically has accident probabilities in the range of one-in-a-million. By analyzing the calculation, one can see where this figure comes from. One can see that most of the one-in-a-million comes from the impact of a micrometeorite on the Cassini probe. In the FEIS, very little of the probability comes from errors in transmission, errors from ground control, etc. This patently violates the actual experience with space probes.

    Meteorite damage is of a real concern, but human and technical flaws are much more likely to cause failure. For example, it has been recently estimated that the International Space Station Alpha may suffer a 50% probability of a catastrophic meteor impact during its 15 year life span. This is certainly a significant danger. But actual operating experience has shown that in almost all space missions, the real danger comes from human and technical flaws, i.e. sending the wrong instructions to space probes, failure of transmitters and solar panels to unfurl correctly, etc. These are almost impossible to model by computer.

  • vi) Furthermore, a one-in-a-million figure assumes that one million Cassini space probes have been fired into space, and only one Cassini space probe malfunctioned. This is clearly untrue. In other words, the table of probability given by NASA is just a wish list. The one-in-a-million figure is wishful thinking masquerading as reputable science.

    IV. Calculation of Alternatives

    The FEIS undertakes a half-hearted effort to calculate alternatives to using plutonium. Since only 800 watts of power need to be replaced, or the output of roughly eight light bulbs, the alternatives must be taken seriously.

    There is no question that, in deep space, there is not much sunlight. At the distance of Saturn, there is only 1% of the solar flux found on the planet earth (in watts/sq. meter). The debate revolves around whether solar/fuel cells can make up the 800 watts necessary to run the mission.

    The FEIS on p. 2-56 claims that, if the Cassini is equipped with massive, bulky solar panels, the probe will be 130 pounds too heavy for lift-off. (The Titan IV can lift 13,743 pounds of payload to Saturn). However, the calculation is incomplete, since it does not consider some simple options:

  • Downsize the craft. If the probe is 130 pounds overweight, then the obvious solution is to lose 130 pounds of equipment. This means leaving out some experiments. However, the Cassini is the Cadillac of space missions, and a few less redundant experiments will still give us excellent science. This may be the solution.
  • Conform to the new NASA philosophy. The new philosophy of NASA is faster, cheaper, better. For example, the Mars Observer was a billion dollar fiasco: bulky, costly, infrequent. The new Mars probes were correctly downsized; the new strategy is to send small space craft to Mars twice every two years. Similarly, space shots to Saturn should be downsized and made more frequent, not less frequent, and energized by solar cells.

    Cassini is therefore a left-over from the old NASA philosophy of doing big space shots once every 10 years. Since space probes were so infrequent, this philosophy resulted in space craft that were overloaded with experiments, and hence the RTGs seemed a natural solution. But the new philosophy of NASA should generate small, frequent, and cheap probes to Saturn which are well within the capability of solar power.

  • Saturn is not going away. All this will cause delays, but Saturn is not going to go away. Other windows of opportunity will open up. Given the fact that one can whip around other planets and change trajectory, windows of opportunities open up all the time.
  • Use a combination of solar/fuel cells. The FEIS only considers solar and fuel cells separately, not in conjunction. Fuel cells can be used to store energy when solar cells can no longer receive adequate energy from the sun.

    V. Conclusion and recommendations:

    We all live in a world of risks. Every day, when we enter cars or airplanes, we place our bodies at risk. Therefore, we must be careful in how risks are handled.

    But the difference with the Cassini mission is that we voluntarily put ourselves at risk when traveling. However, no one asked the American people if they wanted to put themselves in danger. NASA bureaucrats, not the American people, are making this decision.

    Second, if we are in a car accident, only a handful at most will die. But no one told the American people that thousands may die if a plutonium accident takes place.

    Similarly, the FEIS justifies the figure of 2,300 cancer deaths by stating that that figure is lost in the background cancer levels found world-wide. This is a strange argument. That same argument can be used to justify mass murder. Since thousands die violent deaths in the U.S., it makes no difference if a few hundred more die by a serial killer. They will be lost in the background noise.

    Of course, we all want a healthy, vibrant space program to explore the universe. However, it should also be made safe. Since the American taxpayers are paying for it, they have a right to know the true risks, and should be informed of the debate concerning accident risks within the scientific community.

    Unfortunately, the American people, being constantly told that the probability of an accident is on the order of one in a million or a one in a billion, will feel betrayed when a catastrophic accident does occur in space. Such a space tragedy could cause a backlash from the American people, who will correctly feel that they were lied to by NASA bureaucrats. This could be the end of the space program, which would be a disaster to science.

    Furthermore, there is no mention of property damage in such an accident. The Three Mile Accident, for example, reputedly released just 13 curies of iodine (compared to 400,000 curies in the Cassini mission) yet it generated two billion dollars in law suits.

    Even if no significant amounts of radiation are released in a plutonium accident, property values are expected to plummet. And if significant amounts of plutonium are released, then whole areas must be quarantined, earth dug up and placed in 55 gallon drums, houses hosed down with fire trucks, crops impounded, etc. That was one terrible lesson from Chernobyl. The loss to home owners and the agribusiness in the area around the Cape could amount to tens of billions of dollars.

    Therefore, the mission of a critic is to save the space program from NASA bureaurcrats.

    Unfortunately, NASA commits the worst mistake that a scientist can ever make: believing your own press release. A casual observer, reading the FEIS, may be deceived into thinking that a careful analysis has been done. But when actually reproducing the calculation, the observer will be shocked at how many guesses, hidden assumptions, and minimizations of risks there are in the FEIS.

    A true scientist carefully writes down the error bars and the confidence level he or she places in their figures. A careful scientist does not do what NASA has done:

    This borders on scientific dishonesty.

    It is no accident, therefore, that the FEIS comes up with consistently low numbers for a maximum accident.

    The simplest way to solve our problem is to use solar cells with fuel cells. This will require downsizing the space craft by at least 130 pounds. But this is also in tune with the new philosophy of faster, better, and cheaper. The Cassini mission, however, is a relic of the old thinking of slower, more expensive, less frequent.

    A new program to explore the planets would have these probes downsized and launched much more frequently, using non-nuclear energy sources.

    In the interim, this may cost more and cause some delays, but it may also have the lives of thousands, prevent law suits numbering in the tens of billions, and save the space program from NASA bureaurcrats.

    VI. Short biography

    Dr. Michio Kaku is the Henry Semat professor of theoretical physics at the Graduate Center of the City Univ. of New York. He is one of the world's leading authorities on Einstein's Unified Field Theory. He is the co-founder of string field theory. His textbooks on quantum field theory, superstring theory, quantum gravity, and conformal field theory are used by Ph.D. students in leading institutions around the world. He has lectured to the Soviet Academy of Sciences in Moscow, at Oxford Univ., Cambridge Univ., Univ. of Rome, the Univ. of Tokyo, and CERN in Geneva, Switzerland.

    He received his B.A. in physics from Harvard in 1968. He graduated summa cum laude (with highest honors), Phi Beta Kappa, and number one in his physics class.

    He received his Ph.D. at the Radiation Laboratory at the Univ. of Calif. at Berkeley in 1972. He was a research associate at Princeton University in 1973, and has been a professor at CUNY for the past 25 years. He has been a visiting professor at Cal Tech, the Institute for Advanced Study at Princeton, and New York University.

    He has published 9 books and 70 articles in the scientific literature (including Nuclear Physics, Physical Review, Physics Letters, Physical Review Letters).

    He is a Fellow of the American Physical Society, and honor held by the top 10% of physicists in the U.S.

    Reprinted online by permission of Dr. Michio Kaku.

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