Cassini by Dr. Petzke
Kai Petzke holds a Ph.D. in Physics from the Technical University of Berlin, TU Berlin.
This article is reproduced intact from http://troubadix.physik.tu-berlin.de/~petz0433/ewelcome.html with the author's permission.

Humans have always been exploring the world. Sometimes, they find new countries that way. Other explorations end in disaster. If people boarded big ships to sail the unknown, the danger was limited to those people, though. If we put satellites in space, that contain radioactive material, the danger is carried by all humans.

Cassini is designed to explore the rings and moons of Saturn and Saturn itself. Because the sun shines very weak at Saturn, mission designers decided to use so-called RTGs to supply the probe with electricity. RTGs contain radioactive material. While this material decays, it creates heat. The heat is then converted to electricity.

The risk

There are two dangerous points in the Cassini mission. The first one is the launch, especially due to the danger of the main booster rocket exploding. The radioactive plutonium on board Cassini is sealed. There is one satellite from NASA, where the rocket exploded shortly after takeoff. The sealed Plutonium fell into the ocean and could be recovered. No radioactivity was set free.

The second dangerous point of the Cassini mission is, that it will come back close to earth about two years after launch on August, 16th, 1999. The space engineers call such operations flybys, swingbys or gravity assist. Such flybys are the very trick on deep space mission. If correctly designed, each flyby can make the spacecraft faster. Cassini uses four flybys, one of them by earth, to gain up enough speed to go all the way to Saturn. Without that extra speed, Cassini would roughly make it to Mars, not even one sixth of the way to its destination. If something goes wrong with the flyby, it might end up in a crash instead. As it is already the third gravitational assist operation in a series of four, Cassini will already be very fast, over 19 kilometers per second, more than twice the speed of the Space Shuttle! No shielding can be designed, that will be able to keep all of the Plutonium safe under all possible circumstances. In the beginning of the use of Plutonium power packs, they were even designed to burn up in higher atmosphere in case of an adverse reentry. That was the fate of the SNAP 9A power source on board the TRANSIT 5BN-3 satellite, whose liftoff rocket failed. In NASA's document, that is referred to as: "Mission aborted - burned up on reentry as designed". A nice way to describe the failure to protect people from radiation hazard.

The most dangerous scenario is a direct hit. At the speed of 19 kilometers/second, Cassini would need just two seconds to travel through our atmosphere. I cannot answer the question, though, if the spacecraft (or at least part of it) could survive the extremely high temperatures and pressures for those two seconds. If yes, it would come down in one big BANG. The energy of the impact would be enough to vaporize the complete craft. The alternative is, that things break apart during the rush through atmosphere. In that case, we are confronted with many tiny little pieces, some of them harmless, some very, very toxic. This second scenario is the one expected by NASA. They further claim, though, that almost all the plutonium filled heat source modules will safely reach earth's ground. This figure is doubted by many.

How big a risk is it?

Basically, there are two dangers: the relatively high danger of a rocket failure during takeoff, resulting in no or moderate release of Plutonium, and the low danger of a crash during the flyby, resulting in a very high release of Plutonium. Even the NASA admitted in their safety report from 1995, that 32% to 34% of the Plutonium will be released at high altitudes in such an event.

Cassini has more Plutonium on board than any mission before. The isotope is called 238Pu. Because of its shorter life time, it is about 280 times more radioactive than the well known bomb material 239Pu. During atmospheric testing of nuclear weapons, tons and tons of the 239Pu were released into atmosphere. But because the 238Pu is so much more dangerous, it would more than double the man-made Plutonium activity in the atmosphere, if the 400,000 Curie on board Cassini were set free!

I live in Germany, over 1,000 Miles away from the Chernobyl nuclear power plant, that exploded in 1986. When the radioactive cloud came, it was suggested by state agencies, that people not eat fresh fruit or vegetables, not drink fresh milk, and a couple more things. The areas where it rained, while the cloud was over them, are the most heavily contaminated. Milk powder that was produced at a dairy in the first weeks after the Chernobyl accident was so radioactive that it had to be treated at a (German) atomic power plant, where they isolated the radioactive components from the non-dangerous rest.

It turned out that most of the radiation problems in Germany were created by just one component of the radioactive cocktail that had left Chernobyl after the explosion: 137Cs. This is a radioactive form of the metal caesium. In some growings, especially mushrooms, the Caesium radioactivity can still be so high, that eating them has to be considered a health hazard.

Germany got out of the Chernobyl disaster with a "black eye" only. There was some radiation, but nobody found significantly increased rates of death. This is not true for Russia. Over 400,000 people had to be evacuated from polluted areas. In 1986, long after the accident, Ukraine had to mourn 3178 deaths, that are a direct consequence of Chernobyl. This figure is for just one of the countries of the former Soviet union and for just one year. The total number of deaths is much higher. A lot of information has been collected by an initiative to help the children that still live in contaminated areas.

The amount of 137Cs released during the Chernobyl disaster has been estimated and calculated to be between 1,000,000 and 2,400,000 Curie. Before we compare that to the 400,000 Curie on board Cassini, we have to take into account the strength of the radiation. The α particles created by Plutonium decay carry an energy of about 5 MeV each. The β particles from Chernobyl's 137Cs have only 1.2 MeV. So each Curie from Cassini is as dangerous, as four Curies from Chernobyl. There is another thing that we shouldn't forget: once the radiation entered the body, α particles do ten times the biological damage than β particles.

In other words: if all of the Plutonium is vaporized and spread widely, we can expect a radiation damage, that is in the same order of magnitude as that of Chernobyl.

The figures, once again

Nonetheless, NASA claims, that the Plutonium is mostly harmless. But this is true only as long as it is still in one piece. The α particles, that are emitted by Plutonium, can be shielded by a piece of paper or the human skin, but only, if they are outside the body. Once inhaled or eaten, they get dangerous. A reasonable estimate of the dangers of the weapons grade 239Pu says, that if a kilogram of it is vaporized and spread over a city and then inhaled, it could cause up to 1000 cancers. This figure will be a lot lower, if people can hide away from the contaminated air indoor, but higher, if there is no cleanup and the fallen Plutonium particles are blown up again by the wind. So a city in an industrialized country might get away with few cases of cancer - because people can hide indoors, and there is a fast cleanup. A third world city might see a different scenario: no hiding in poorly build houses and slow or even no cleanup.

But let's not forget, that the figure above was normalized to 1 kilogram 239Pu. On board Cassini, we have 32.8 kilograms instead of 1 kilogram and we have the over 200 times more radioactive 238Pu. It is left over to you to do the math: 32.8 kg * 200 * 1000 deaths/kg.

Of course, the high number of deaths will only occur, if we have double bad luck: the Cassini craft has a failure during the earth swingby, and it crashes into a highly populated place instead of into the Ocean.

Do we really have to take it?

The question is nonetheless, if we really need and want to accept that danger. Do we want to put a radiation source in space, whose dangerous potential is in the range of that of Chernobyl? Aren't there any alternatives? Why not generate the electricity using solar panels, as done with most satellites in space? Sun is faint at Saturn, but modern solar power devices can create electricity even in deep and cold space. See The Solar Solutions for Cassini.

The thing, that puzzles me most, though: if it has to be nuclear power, why don't use a real nuclear reactor? 100 grams of nuclear fuel like 235U (Uranium) would suffice to power Cassini for 20 years. At normal enrichment, we'd need around 5 kilograms (11 pounds) of Uranium total, as compared to the 72.3 pounds of Plutonium used now. But most of all, the radioactivity of 235U is about 10 million times less than that of 238Pu. Nice increase of the safety margin, isn't it?

Of course, the radioactivity increases dramatically, once the reactor has been ignited or made critical, as the experts say. But can't we easily defer use of the reactor to the point, where we have successfully completed the earth swingby and are heading for deep space? In the beginning of the mission, we don't get far away from the sun, and we don't perform much research. All we need is some basic power to run the navigational computer and operate the thrusters to keep the spacecraft on the right course. A small array of solar cells is definitely enough to create that power. Once we are safe away from earth and enter deep space, the nuclear reactor could be activated.

A nuclear reactor might even have further advantages over the RTGs. Because of their heat source, the radioactive decay, RTGs cannot be turned off. Before take off, they have to be constantly cooled. This complicates the takeoff procedure, increasing the danger of failures due to human error. Just three days, before I wrote this page, a cooling accident happened. On September, 3rd, NASA told the press, that a cooling device damaged the Huygens probe by blowing air through it at too high a speed. Huygens is part of the Cassini mission.


I do not call for the nuclear power reactor alternative, because a solar only mission is also likely to be possible. But the reasoning given above should allow any person to understand, that NASA's statement "RTGs are the only feasible power system for the Cassini mission." is wrong. I do not object Cassini. I want space missions to be made, because there is a lot to learn and research, a lot of very, very interesting things. But I want space missions to be as safe as possible. I believe that researchers should be on the leading edge when it comes to safety.

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