RTGs
Published on Jan 17, 2006 at 2:38 pm.
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The New Horizons spacecraft (see two posts ago) is about to launch to Pluto. Launch was delayed a day because of high winds at the Florida launch site. There have been a lot of protests about the launch of the spacecraft. There are the usual kooks who are saying that somehow the spacecraft will disturb aliens living there, and there are other kooks who think that the spacecraft will somehow alter the astrological impact of Pluto. Give me a break! But, there are a few who are protesting the power source for New Horizons.
Like all spacecraft sent to the outer solar system, New Horizons is powered by radioisotope thermoelectric generators (or RTGs for short). Some of the more gloomy and doom oriented protestors are arguing that if the rocket blows up on launch then the RTG will burst and kill us all, or something like that. Though well meaning, they are missing out on a lot of facts. So, I thought that I’d say a few things about RTGs.
First of all, what are they? RTGs are a way of getting electric power from certain radioactive materials. They are not really nuclear reactors. They work in an entirely different manner. Nuclear reactors use elements with fissionable nuclei. As the nuclei split, they release energy. This energy is then aborbed by coolant in the reactor, and the hot coolant exits the reactor, carrying the heat with it. Then, you do whatever you would otherwise do with something hot to power something. Often you make electricity. Well, RTGs are a bit different. If you have a sample of radioactive material, there are always nuclei that are spitting out energetic particles to become some other nucleus. There are specific rules for how this works, but that is beyond the scope of the blog. (I will be covering this, though, towards the end of my Physics II class this semester). Anyway, if you have something that is radioactive enough, and the particles emitted are non-penetrating, so they don’t go very far, then the energy carried by these particles is deposited either inside or near the radioactive sample. This has the effect of heating the area. There exists a type of semiconductor device called a thermocouple that produces an electric voltage when heated just right. So, mating a thermocouple with a suitable sample of radioactive material can be used to produce electricity for as long as the sample is radioactive. This is the basic idea of an RTG. The RTG used by New Horizons utilizes Plutonium-238 as its radioactive source.
OK, here is the first place that people freak out. They hear plutonium. They know that plutonium is used in nuclear weapons. So, is there any chance something can go wrong and the spacecraft might blow up? No!!!!! It is physically not possible for the RTG to blow up. First of all, it uses a different isotope of plutonium. 238-Pu is used in the RTG because it is fairly radioactive, but has a half life long enough for the RTG to continue to produce electricity for a long time. 238-Pu, though, is VERY hard to fission. A different isotope, 239-Pu, is used in nuclear weapons, and in some nuclear reactors. Yeah, it is both plutonium, but the two types give off energy in entirely different manners. It is interesting to note, though, that 239-Pu, the type used in nuclear weapons, is actually less radioactive than 238-Pu. So, the RTG can not explode like a bomb. In fact, it is even less able to explode than an automobile battery, and wouldn’t even make as big of an explosion as the battery even if something did go wrong.
The second argument is that if something goes wrong, then the RTG might break apart, or reenter the atmosphere, burn up, and distribute plutonium all over the place. Then we all die, at least according to the gloomiest protestors. Well, no. That is a bit off, too. First of all, there isn’t really all that much plutonium in the RTG. The averge person actually gets more radioactive material from table salt that you would if the RTG were to rupture. Also, a typical commercial aircraft flies high enough that the passengers and crew are exposed to cosmic rays. A typical long flight (transcontinental or transoceanic) generally subjects the passengers and crew to more radiation exposure during the flight than people at Cape Canaveral would get if the rocket lifted off, exploded and showered plutonium all over the cape. Actually, as a footnote, pilots and flight attendants regularly get as much or more radiation exposure than workers in nuclear power plants, but that would be the subject of another blog. Furthermore, the idea that a ruptured RTG would kill us all off has empirical evidence to the contrary. RTGs are not new to this mission. Many other spacecraft have used them, and a number of those have crashed or reentered. At least two of the early ones reentered and the RTGs did burn up and disperse the plutonium in them. This was over 40 years ago, and since I am still alive, and my cat who is trying to walk across my keyboard is still alive, then this argues that the plutonium exposure due to the RTG accidents did not kill everyone. Later RTGs were redesigned with containers designed to withstand reentry. There have been at least three accidents since that time, including a launch failure and two reentries, in which the RTGs were recovered intact, with no leakage of plutonium. One RTG aboard the Apollo 13 lunar module is sitting on the Pacific Ocean floor, too deep to recover, but water tests of the area show that the RTG appears to have survived reentry and decades of corrosive seawater without leaking. So, this seems to argue for the safety of the devices.
But, why use an RTG in the first place? They are rather expensive, and they are a public relations nightmare for NASA, what with all the protestors. Well, the answer is simply that they are a reliable way of providing power. Solar collectors only provide power when the Sun is shining on them. Thus, many military satellites have used RTGs, as well as the lunar modules for Apollo. But for the outer solar system, there is no other option. The farther that you are from the Sun, the dimmer the sunlight. At Jupiter, solar collectors have to be 25 times larger than they would on a satellite in Earth orbit just to provide the same electrical power, and then only if the sunlight hit the solar panels just right. At Saturn, the solar panels need to be about 100 times larger than needed for Earth orbit. And at Pluto, the solar panels would need to nearly 1600 times larger. That would be too big to even get off the ground, even if you could do it! So, this leaves RTGs as the best, and safest, way of powering spacecraft to the outer solar system.
-Astroprof
