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Could NASA Use Galileo to Create a Jovian Nagasaki?
The month of September 2003 will see an exciting but also scary event take place. Nobody on Earth can tell you for certain what the outcome will be, yet NASA is pushing it ahead with full steam in what could very well become a mistake of Titanic proportions for all of humankind. This is because they will hurl the spent satellite Galileo deep into Jupiter's atmosphere to a supposedly ‘safe' but fiery grave. However this planet is not Earth where our concern has been that Galileo's Plutonium could fill our lungs with radioactivity were it to reenter our atmosphere. However, this is not a concern because Galileo will be entering Jupiter's atmosphere instead. So what! Jupiter is the giant catcher of the Solar system so why should we be concerned? While it is a giant catcher, it is also the largest pressure cooker of our solar system and it could crush Galileo's 48 pounds of Plutonium. The result could be a Jovian Nagasaki with dire consequences for humankind.
The Galileo Mission
NASA's Galileo mission is coming to an end. Galileo was launched back in 1989 from the Space Shuttle Atlantis, its mission was to explore Jupiter and its moons. As September 2003 progresses, the satellite will be maneuvered into a controlled descent into the atmosphere of Jupiter; there it will find its grave on September 21, 2003. Galileo was sent out to explore Jupiter and its moons and returned some most interesting results to us.
It found water on the moon Europa, which has a global ocean under a thick layer of ice. Because this moon has a good chance of harboring some primitive life forms, NASA planned the controlled crashing of Galileo into the atmosphere of Jupiter. NASA does not want it to accidentally crash into Europa. Why is such a controlled crash necessary?
The Radioisotope Thermal Generator (RTG)
The reason for crashing Galileo into the planet Jupiter on purpose is this: Galileo since its launch has been powered by Plutonium-238 in a RTG or Radioisotope Thermal Generator. Plutonium-238 decays via alpha and gamma emission, its daughter products are also highly radioactive for a long time. An RTG catches this radiation in a heat exchanging mechanism and there converts it into electricity. Duracell doesn't make batteries like that. These RTG power units can power a satellite for many years. They powered the Pioneer and Voyager craft too or they would not have lasted three decades as they did.
Now, an independent researcher/writer by the name of J.C. Goliathan gathered some interesting information on the fuel load.
TIPS Report — August, 2003
The author believes the nuclear events reported here to be very unlikely and only remotely possible, but just as an asteroid impact with earth is remotely possible (and widely researched and reported on), the Jupiter impact issue deserves exposure also, because there is compelling evidence to suggest the feasibility of at least a temporary Jupiter ignition. Given the potential consequences of this, serious research is warranted. The author is a Geographer and Engineer, not a physicist. Further research is needed by more qualified individuals.
While Goliathan's might be a bit technical for those outside of his field of study, as a physisict, I found it to be a brilliant piece of work and so will endevor to explain the critical aspects presented in his article in layman's terms.
The Concerns Described in Goliathan's Article
Theoretically the avalanche reaction described for the Pu-238 pellets aboard Galileo can take place setting off an implosion-induced nuclear detonation. Literally no one on Earth knows for certain what will happen when Galileo plunges into Jupiter. It may even trigger an on-going fusion reaction in the abundant hydrogen supply of Jupiter, creating a mini-star. We lack the in-depth knowledge on the atmospheric structure of the planet to accurately predict the outcome, yet we push ahead on the chosen path.
Physicists at NASA will undoubtedly tell you the danger is negligible and try and push this question into the realm of conspiracy-thinking. As a physicist, I've been trained in the safe application of radiation and I think we need to pay attention here, as we may be overplaying our hand by ignoring this possibility.
In my work I do risk analysis regularly. In such an analysis I divide potentially hazardous risks into a matrix. This type of matrix sorts risks by chance of occurring (on a 1 to 6 scale for likeliness) on one scale against the severity (on a 1 to 5 scale) on the other. The product of these two is known as the hazard of the event. Anything with a product of chance and severity above 12 requires ‘mitigating action', but special attention is due also for the highest severity category, regardless of its chance.
This particular risk falls in that category of highest severity, it has to be looked at. Its chance is minute, but no matter how small the risk, its consequences are so severe and all-encompassing that we must not ignore it! I will tell a little more about why they are so great below.
This Is All About Plutonium!
Plutonium-238 is a bit of a rogue isotope in the Plutonium family. It is the "least wanted" of the isotopes of Plutonium for energy production in nuclear plants or even the use in nuclear bombs. We'll go into why that is here.
We start with a schematic buildup of a simple Plutonium implosion bomb as depicted in the figure.
It uses a sphere of a few kilograms of fissile Plutonium, which is surrounded by a layer of Uranium. Around that are a few layers of chemical explosive. This schematic is very important for what the concern among many physicists is about.
Plutonium does have spontaneous fission, I will get back to that below, it is especially interesting in this particular case. On fission, atoms fall apart in two or more chunks and a few high energy neutrons, atom shrapnel really. These neutrons are shot away in random directions. If they hit other atoms of Plutonium like a bowling ball hits the pins, these may start splitting as well because of the impact. In a sphere of only a few pounds these neutrons don't encounter enough Plutonium atoms to have enough chance of breaking one up. More Plutonium is needed for that.
Critical masses can be calculated quite accurately. The important parameters are fission cross sections, the average neutron yield upon fission, and the mass density. The latter depends heavier on the integrity of the metal lattice than on the isotopic composition, since mass differences between the different plutonium isotopes are almost negligible.
Without a neutron reflecting shield, pure Pu-239 metal has a critical mass of 10 kg , and I have calculated that for a "reactor grade" isotopic mixture this would be 18 kg. Using a 15 cm U-238 shield, the Pu-239 critical mass is only slightly over 4 kg, while for LWR -produced plutonium (65% thermal fissile isotopes, fuel burn up around 40 MWd/kg HM) this is some 7 kg.
The critical mass of Plutonium-239 to start a chain reaction without help is about 10 kilograms. With that much Plutonium, the chance of a neutron from the middle hitting an atom on its way out is high enough to keep the reaction going or even speed it up. Plutonium is easy enough to gather, but you can't control an amount like this at all. That is why in a bomb there is a layer of Uranium around it, this acts like a neutron mirror, it sends neutrons right back in for another pass through the Plutonium, only 2 to 4 kilograms of that is necessary this way. That by itself is still not enough though but keeps the Plutonium from going off by itself.
If the chemical explosive is set off exactly simultaneously, a shockwave will travel inward, compressing the Uranium shell and the Plutonium so far inward that the atoms move much closer together. So much closer in fact that the two or three neutrons coming out of every atom splitting up split at least another atom of Plutonium. The reaction becomes self-sustaining. This principle is the principle by which a nuclear power plant works. By ‘catching away' neutrons the balance point of one-splits-one is kept in-tact. The rest of the free neutrons will crash into water and gives of its energy boiling the water to steam.
If the shockwaves pushes further in still, then more than one other atom will be split by the resulting neutrons of an atom splitting up. An avalanche starts to build. This avalanche creates so much energy that a counter wave starts pushing outward overcoming the inward shockwave in an instant. The result is the notorious mushroom-shaped cloud we all so dread.
Now we get back to the special isotope of Plutonium in use in the RTGs of Galileo. From the same site there is a table listing the various isotopes and their (re-)activity.
The 3,440 neutrons per gram per seconds of Pu-238 are the real reason for concern. If you compare them to the 0.03 neutrons that spontaneously come out of one gram of Pu-239 it will become obvious that the critical mass (the mass at which the reaction inside the Plutonium becomes self sustaining) is much smaller for Pu-238 than it is for Pu-239. Neutrons travel in every direction, as a result the difference in SF rate will work in all direction too. This leads to a critical mass of roughly 200 grams for Pu-238 only. This is why NASA used 144 pellets of 1/3 pounds (151 grams) to get the 48 pounds on board of Galileo. These pellets are shielded from one another to prevent them from going out of control. The crucial question is what will happen to these pellets and their shielding when the satellite plunges into the atmosphere of Jupiter. Will the shielding hold? Will the pellets stay together or wander apart? NASA appears to hope they wander apart or quickly burn up completely (what with, as there is almost no oxygen to burn them up?).
If the pellets stay together and are compressed ever stronger by the increasing atmospheric pressure they encounter (they will keep on falling until the outside specific weight or weight per volume matches that on the inside!!) each pellet will by itself go beyond the critical point density and chain-react. The true danger is if several ones or all of them were to go supercritical together. In that case you have 48 pounds of Pu-238 going into chain reaction.