I remember the protesters, and I think about them every time more pictures come back from Cassini. At the time I thought they were being a little bit hysterical. I just did a quick search for Michio Kaku and Cassini, and found this. It’s the text of a speech Dr. Kaku made protesting the Cassini launch because of the Plutonium power supply. I don’t know if I agree with him that it should never have been launched, but he makes a couple of good points. If something had gone wrong, it looks like it could have been very bad.
I haven’t heard of any protests like this since the Cassini launch. Have they found a better, safer design that doesn’t make people so nervous? Or do they routinely launch space probes with Plutonium batteries these days?
The power source for the Cassini-Huygens consists of three radioisotope thermoelectric generator (RTG), a device that converts thermal radioactive decay products into electric energy. The model of those use on Cassini is the General Purpose Heat Source RTG (GPHS-RTG) a standard design that has been extensively tested and successfully used on a number of missions including Galileo, Ulysses, and New Horizons, which themselves were derived from previous designs used on the Pioneer and Voyager spacecraft and Viking landers, as well as on the Apollo Lunar Module (the “odd-number” SNAP RTGs). The GPHS-RTG is no longer in production and is being replaced by the Multi-Mission RTG (MMRTG) because of part obsolescence and increased modularity. The Mars Science Laboratory ("Curiosity’) will use a coule of MMRTGs, as will several future and proposed trans-Jovian missions. RTGs are used on these missions because they are the only effective way to provide long duration power to a spacecraft that cannot use or rely upon solar power; this is true for all missions that go beyond Mars orbit, or are deployed on a planet’s surface where solar power may not be available.
As for the hazards, here is a NASA white paper on Cassini’s power source (PDF) and an evaluation of the risks that it presents. The claims that unintended re-entry and loss of containment of the plutonium ([SUP]238[/SUP]Pu) core would cause mass death and destruction are beyond hyperbolic. The plutonium core is designed to survive re-entry intact, and even if it does fracture it should come apart in large chunks, which invalidates the basic assumption by doomsayers who assume an even cloud-like distribution of plutonium dust across the entire Northern Hemisphere. While plutonium is hazardous and persistent, it really has to be inhaled or injected in order to result in a genuine health hazard; plutonium outside the body releases alpha particles which can’t even penetrate the dead layers of the dermis, and even injesting it will cause it to pass through the intestinal tract with virtually no absorption. The US has had three incidents of RTGs re-entering the atmosphere, only one of which resulted in measurable radiation escape. Two of the Soviet Kosmos satellites (not a design series per se, but a designation for a number of different satellite designs and missions) both of which released measurable radiation. None of these incidents resulted in direct casualties, and while estimates of life-hours reduced vary wildly, it is clear that the level of hazard isn’t great even should radioactive material escape.
To put this in perspective, between the late 'Fourties and the implementation of the Partial Test Ban Treaty in 1963, the United States, the Soviet Union, and the United Kingdom performed over four hundred and fifty above ground nuclear tests with highly particlized fallout, much of it plutonium-based weapons. While there are measurable instances of elevated mortality from cancer, leukemia, and other cancer-related deaths, there was no mass dying of radiation victims, and even the smallest nuclear weapon is going to put out more plutonium and in finer form that what is found in an RTG.
In other words, the fears of massive death and dying were nothing more than ignorant fear-mongering. And there is no practicable alternative for powering deep space missions for the foreseeable future.
Stranger, thanks for the information. I seem to remember that some of the fear had to do with the probe coming back toward Earth for a gravity-assist flyby thing - obviously I’m no expert on the subject. But considering the speed it was traveling when it passed by the Earth, if it did happen to come too close and slam into the atmosphere, would there not have been a danger of the whole thing vaporizing, including the Plutonium, and people then breathing it in?
The odds on it intercepting the Earth during the swing-by maneuver were lower than six standard deviations–literally a one in a million chance–and that using worst case assumptions of trajectory parameters. NASA has never lost a craft or even seen a serious deviation from planned course during a swing-by encounter of Earth or another planet, and in fact Cassini performed dozens of swing-bys in the Saturnian moon system without ever once deviating off planned course by more than two standard deviations (less than that, actually, but I can’t find the charts offhand).
So what if we did have that million in a one shot, and Cassini re-entered, and the cask holding the plutonium burned up in the atmosphere releasing particulate [SUP]238[/SUP]Pu? Well, about the same thing as what happened when we detonated large plutonium-based nuclear weapons in the atmosphere one hundred and fifty miles northwest of Las Vegas. (This was, admittedly, mostly [SUP]239[/SUP]Pu and [SUP]240[/SUP]Pu, but the biological impact is considered similar.) What is the effect of plutonium on the human body? Let’s take a look at the Argonne National Laboratory Human Health Fact Sheet, 2005: “As a note, for inhalation (the exposure of highest risk), breathing in 5,000 respirable plutonium particles of about 3 microns each is estimated to increase an individual’s risk of incurring a fatal cancer about 1% above the U.S. average.” Remember, that’s a lifetime risk. You’re in more danger from increased lifetime radiation damage if you smoke, drink a lot of calcium-fortified dairy products, or live in an area with a high basalt content. Those granite countertops you paid a premium for are far more likely to shorten your life than the vanishingly rare chance that an RTG-equipped space probe will burn up in the atmosphere and cause you to inhale a few plutonium particles.