Explaining The Risks of The Cassini Mission PDF


The Cassini mission is an international venture that involves launching a spacecraft in October, 1997 to conduct a four-year, close up study of the Saturnian system, including Saturn's atmosphere and magnetosphere, its rings and several moons. As with all planetary missions, there are risks and benefits associated with the mission, and it is the purpose of this fact sheet to explain how NASA ensures that the risks to human health from the Cassini mission, specifically the risks of fatal cancers, are kept extremely low. The hazard of concern with the mission is the potential for a release of some of the approximately 33 kg (about 72 lb) of plutonium dioxide in the RTGs.

The Cassini spacecraft will carry three radioisotope thermoelectric generators (RTGs) that convert the heat from the decay of plutonium directly to electricity for use by the spacecraft and its instruments. The plutonium is in the form of a ceramic, plutonium dioxide, and is not suitable for weapons. RTGs used by U.S. spacecraft have been designed by the Department of Energy to contain their plutonium dioxide in a wide range of launch and orbital reentry accidents, recognizing that accidents can happen. In addition to the safety design, RTGs have been subjected to years of extensive safety testing and analyses. This testing and the outcomes from real accidents involving RTGs have demonstrated they are extremely robust. For example, since 1965, when RTGs were redesigned to prevent the release of radioactive material rather than disperse it, there have been two accidents (1968 NIMBUS-B satellite launch and 1970 Apollo 13 lunar module reentry) where RTGs were on-board spacecraft. Neither of these accidents were caused by the RTGs, and in both accidents, the RTGs worked exactly as their design and testing had predicted, fully containing their plutonium dioxide.

Likewise for the Cassini mission, extensive analysis has predicted that with the vast majority of launch accidents there would not be a release of plutonium dioxide from the RTGs. However, there are some low probability launch accidents and accidents from low Earth orbit, and one very low probability accident known as an inadvertent Earth swingby reentry, that could possibly cause the release of some of the plutonium dioxide. This fact sheet will describe these unlikely accidents and explain why the risks to people from RTG plutonium dioxide releases are low.

Plutonium Dioxide Health Effects

It is often quoted that "plutonium is the most toxic substance known" and that exposure to a tiny amount of plutonium will lead to death. Though plutonium is a hazardous material that should be handled carefully, the actual risks to human health from plutonium exposures depend on the form of the plutonium. The hazard from plutonium dioxide is presented by the alpha radiation that it emits. Alpha radiation can travel only a few inches in air and is effectively stopped by clothing or even by the outer layer of skin. Thus, plutonium dioxide only can become a serious health hazard if it is reduced to very small particles and then delivered into the human body and stays there, causing internal organs to be exposed to the alpha radiation emitted by the plutonium dioxide over a long period of time.

Even assuming a release of plutonium dioxide from RTGs, the possibility and amount of an internal exposure is kept low because of the ceramic form that is used. This ceramic form, similar to a coffee cup or dinner plate, makes the plutonium dioxide highly insoluble in water and limits the amount of respirable particles. Limiting the generation of small particles keeps to a minimum the amount of material that could be inhaled. These factors reduce the potential radiation exposure from inhalation or other routes. The insolubility keeps the plutonium dioxide from being widely distributed within the human body and also inhibits the released plutonium dioxide from moving through the environment into water supplies and the food chain.

It is important to understand that exposure of a person to radiation does not mean they will get cancer. People are exposed to radiation on a daily basis, mainly from natural sources in the environment and to a lesser extent from human activities such as medical X-rays. This radiation exposure is measured in units of dose called millirem. Natural sources of radiation include radon, other naturally occurring radioactive material in the Earth, cosmic rays, and even some radioactive materials that naturally occur in a person's body (see Figure 1). All of these radiation sources contribute to what is often referred to as "background radiation." Over the course of a year, the average person will be exposed to a total of about 360 millirem of background radiation, with about 300 millirem of that total coming from natural background radiation (that is radon, cosmic rays, and rocks and soils). Over 50 years, the average person will be exposed to about 15,000 millirem of natural background radiation.

Scientists use what is called a health effects estimator to predict how many people in a population who are exposed to radiation would be expected to die from cancer. The number of fatalities increases with the amount of radiation; the more radiation to the same size population the more health effects would be predicted. As an example, people living in Denver, at an altitude of 1,500 meters (5,000 feet), receive an annual radiation dose that is higher than the average person who lives near sea level. The extra radiation dose from cosmic radiation contributes about an extra annual 30 millirem to each person in Denver. Using the health effects estimator, a scientist would calculate a slightly higher estimate of health effects in Denver than in the same sized city near sea level because of this extra 30 millirem.

This scaling method of estimating cancer fatalities from radiation dose may overestimate the number of expected fatalities from low level radiation. This is because some scientists have evidence suggesting there may be a minimal threshold of radiation exposure necessary for a cancer fatality to be possible. These scientists reason that the human body may repair the small number of cells that may be damaged from low level radiation exposure. NASA used the more conservative approach for assessing the potential consequences of a Cassini accident.

Figure 1. Average Annual Dose From Background Radiation About 360 Millirem (mrem)

Explanation of Risk

When analysts say a "risk" is low, they mean that the probability of an event multiplied by its potential consequences is low. For example, it has been estimated that there is about a two in one million chance in any given year that the Earth will be hit by a large asteroid, about 1.5 kilometers (1 mile) in diameter, that could result in about 1.5 billion deaths. Therefore, the risk we face each year from this potential event would be two in one million (or 0.000002, the probability of impact in any given year) times 1.5 billion (the expected number of fatalities) which equals a risk factor of 3,000. This risk factor is a number that gives us an impression of the frequency and magnitude of the hazardous event. This allows us to compare risks of dissimilar events in a relative way, for example event A is twice as risky as event B.

Having this relative measure called a risk factor allows us to compare infrequent but large consequence events like the one described above to more frequent events that may have fewer consequences. For example, we can calculate the risk factor due to a smaller but more likely asteroid impact. It has been estimated that there is a one in 250 chance in any given year that the Earth will be hit by an asteroid that is about 50-300 meters (150-1,000 feet) in diameter. Such an impact could result in about 5,000 fatalities. Therefore the risk factor we face each year from the impact of this size asteroid would be one in 250 (or 0.004, the probability of impact in any given year) times 5,000 (the expected number of fatalities) which equals a risk factor of 20. Thus, the risk from the larger asteroid is higher (about 150 times greater) than the risk from the smaller one. This is true even though the likelihood of the Earth being struck by the larger asteroid is much smaller than the likelihood of being struck by the smaller asteroid.

It is interesting to note, however, that the risk from asteroids does not always increase with an increase in asteroid size. For instance, the risk factor for the Earth being struck by an asteroid similar to the one that is theorized to have killed the dinosaurs (an asteroid about 10 kilometers [6 miles] in diameter or greater) would equal about 50 (a one in 100 million annual probability times an expected 5 billion fatalities). So, the risk factor is actually higher for a one mile wide asteroid than for a 6 mile wide asteroid, even though this larger asteroid would be expected to cause more fatalities if it struck the Earth. [See Table 1]

Launch Accident Risk

Some people have expressed concerns about the possibility of a launch accident involving the Titan IV rocket that will be used to launch the Cassini spacecraft, and about the consequences that would be expected from launch accidents involving RTGs. As mentioned earlier, RTGs are extremely rugged and have demonstrated that they will not release plutonium dioxide with impacts on water or soft dirt or sand in launch accidents. It is worth noting that almost the entire launch trajectory is over water.

This is why the probability of a release of plutonium dioxide during a launch accident is so low, even when assuming a launch vehicle failure. For example, even though the historical failure rate of a Titan IV is about one in 20 (an estimate based on historical data, for the Titan IV with 19 successful launches out of 20 attempts as of February, 1997), the analysis estimates that the probability of a Titan IV launch accident releasing plutonium dioxide is about 1 in 1,400.

Assuming there is a launch accident that releases plutonium dioxide, most of the ceramic material would be in the form of large chunks. This would prevent almost all of the plutonium dioxide from traveling far from the accident site, so it could be cleaned up. These large chunks would not be breathable. The amount of material that could potentially be breathed in by people would be very small, and thus the expected radiation dose to a person in the exposed population (estimated at 100,000 people) would be less than 2 millirem total over 50 years. For this event, 0.1 fatalities have been calculated, therefore no cancer fatalities are anticipated.

Again, regardless of the assumptions, the risk due to launch accidents is very small. The risk factor, using the assumption that low level radiation may cause cancer, would be one in 1,400 (or 0.0007, the probability of an accident releasing plutonium dioxide) times 0.1 fatalities, and thus would equal about 0.00007.

Similarly, the risks due to a late launch accident or reentry from Earth orbit are low. The probability of such an accident that would also release plutonium dioxide is about 1 in 476. If such a release were to occur the expected radiation dose to a person in the exposed population (estimated at 5,000 people) would be less than 20 millirem over 50 years and expected to result in about 0.04 fatalities (thus no fatalities would be anticipated). The risk factor would be one in 476 (0.002, the probability of an accident releasing plutonium dioxide) times 0.04 fatalities, and thus would equal about 0.00008.

The Cassini Mission Inadvertent Swingby Reentry Accident Risk

The Cassini mission has been designed to ensure that the chance of an inadvertent reentry during Earth swingby is less than one in one million. NASA has conducted an in-depth analysis, which incorporated human error and historical spacecraft reliability data, and has determined that the probability of an inadvertent Earth reentry is less than one in one million.

In the extremely unlikely event that a Cassini inadvertent Earth reentry has occurred, some plutonium dioxide could be released into the atmosphere. The fine particles of plutonium dioxide that are potentially hazardous to people would remain high in the atmosphere for a long period of time. This would result in the particles being spread very thinly across the world and eventually making their way to the surface, mostly the oceans. Since the material is highly insoluble, once it reaches the surface most of it would become trapped in the oceans or soils and not pose a health hazard. Thus, most of the released material would not be breathed in by people. The small amount of released material that would be breathed in would be distributed over much of the world. Since the amount to be breathed in is so tiny, the radiation dose that a person would be expected to receive is less than one millirem total over 50 years. This small radiation dose is indistinguishable when compared to the 15,000 millirem dose an average person will receive (over that same 50 year period) from natural background radiation.

Using the conservative approach to estimating consequences from low level radiation exposures, it has been estimated that this type of accident could result in about 120 cancer fatalities worldwide over 50 years. However, it is quite possible that such a low radiation dose (less than one millirem over 50 years) may not be capable of causing cancer in a person.

Regardless of the assumptions, the risk from an inadvertent Earth reentry is low. The risk factor, assuming there would be fatalities, would be one in one million (0.000001, the probability of inadvertent Earth reentry) times 120 (fatalities), and thus equal about 0.0001. This risk factor is much smaller, at least 200,000 times smaller, than any risks discussed above from asteroid impacts.

Risk Comparison

Table 1 summarizes the annual risks people face due to asteroid impacts and the one time risk people face due the Cassini mission. Though the likelihood of a swingby accident is roughly comparable to the Earth being hit by a large asteroid, the consequences from a Cassini accident would be minor in comparison; thus, the risk is considerably lower. Not only is Cassini's risk much lower (about 10 million times lower) than the risk from a one mile diameter asteroid hitting the Earth, but it is also a one-time risk since there will only be one Cassini mission; and once the spacecraft passes Earth, the risk is over, while humanity will continue to face the risk from asteroid impacts every year.

Table 1
Potential Event Probability Estimated Fatalities Risk Factor
10 Kilometer (6 Mile) Diameter, or
greater, Asteroid Hitting the Earth
1 in 100 million 5 billion 50
1.5 Kilometer (1 Mile) Diameter
Asteroid Hitting the Earth
2 in 1 million 1.5 billion 3,000
50-300 Meter (150-1000 Feet)
Diameter Asteroid Hitting the Earth
1 in 250 5,000 20
Cassini Inadvertent Earth Swingby
Reentry with Plutonium Dioxide Release
1 in 1 million 120 0.0001
Cassini Early Launch Accident with 1 in 1,400 0.1 0.00007
Cassini Late Launch or Reentry
from Earth Orbit Accident with
Plutonium Dioxide Release
1 in 476 0.04 0.00008


National Aeronautics and Space Administration (NASA). 1997. Draft Supplemental Environmental
Impact Statement for the Cassini Mission. Washington, D.C., April 1997.

Clark R. Chapman and David Morrison. 1994. "Impacts on the Earth by Asteroids and Comets:
Assessing the Hazard" in Nature, January 6, 1994 (Volume 367).

Last Updated : July 8, 1997