SciTech

How Things Work: radioisotope thermoelectric generators

When you imagine a deep-space probe, what do you see? A big metal box, a forest of antennas, and a pair of solar panels, right? Wrong. Contrary to the stereotype, almost any probe traveling into space will not use solar panels as its power source. The energy from the sun at this distance is simply too small. An alternative is needed: something that is reliable, has a long-life, and is lightweight. What can we do? American engineers answered this very question in the early 1960s with the radioisotope thermoelectric generator, or RTG.
RTGs work by converting the heat given off by a radioactive isotope into electricity. To understand this process, it helps to understand what a radioactive isotope is. One might say they are out of their element! (Hah.) Seriously though, they have extra neutrons. Such elements feel these neutrons are like annoying cousins: they want to get rid of them. In doing so, they become radioactive, releasing protons, neutrons, and energy in a process called decay. Eventually the isotope will turn into other elements and disappear.
At first glance, this might seem like the same process that takes place in nuclear reactors, but it isn?t. Nuclear reactors generate heat by fission, a process in which atoms are split by a bombardment of neutrons. In an RTG, heat is generated only by the natural radioactive emissions of the isotope contained in it. This is a much slower process, but the advantage is its stability; it can never go out of control and melt down.
Another advantage of RTGs is that they contain no moving parts. This is beneficial because moving parts tend to break and wear out over time. RTGs make their electricity by using thermocouples. To understand how a thermocouple works, let?s take a trip back to 1821 to the great country of Estonia. There, scientist Thomas Seebeck found that a temperature difference between two dissimilar metals produces a voltage. Like a true scientists, he discovered this accidentally, and named the phenomenon after himself ? the Seebeck effect.
The Seebeck effect is quite useful in an RTG. If one end of a thermocouple is placed next to a hot radioisotope and the other is connected to a heat sink in the cold, hostile void of space, a temperature difference is created, providing a continuous source of energy limited only by the isotope?s decay rate.
This brings up an important point: What isotopes are good candidates for powering an RTG? Firstly, a good isotope should decay at such a rate that it gives off a usable amount of heat, but does not decay so fast that it disappears quickly. The decay rate is measured by a factor called half- life, the time it takes for half of an isotope?s mass to disappear (not the video game by Valve Software). The half-life for isotopes used in most RTGs is between 20 and 100 years.
Secondly, the isotope should have a high energy density. This is especially important in space, when every gram of weight is at stake. The higher the energy density, the smaller the RTG can be.
Thirdly, the isotope should emit only alpha-type radiation. Alpha radiation is easily absorbed and can be stopped by as little as a sheet of paper. This means that it requires the least amount of shielding to protect people and the environment from ionizing radiation. Since shielding equals weight, the less the better. Some isotopes emit beta and gamma radiation, both of which are more penetrating than alpha particles. This makes such isotopes less than ideal candidates, even if they have good energy output.
With these principles in mind, a continuous, reliable source of energy can be created that will last for decades. The Voyager 1 and 2 satellites, which were launched in 1977, were only designed to operate for five years. Their RTGs, however, have been running for 28 years and are projected to run for many more.
Engineers have realized this kind of potential, and they have not limited the use of RTGs to the outer reaches of space. Many RTGs have been used to power remote lighthouses, buoys, and other hard to reach places. There were even tiny RTGs made to power pacemakers, but unfortunately, these plutonium-powered models never caught on.
Nevertheless, NASA and the Department of Energy continue to improve the design of current RTGs and find new applications. Who knows? Someday we might have RTGs powering our MP3 players and cell phones. Just make sure to slip them in your lead-lined pants pocket!