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Finding a Power Supply for Space Probes - radioisotope thermoelectric generators, or RTGs.

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Finding a Power Supply      

Finding a Power Supply

When you imagine a 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 space bound probe will not use solar panels as its primary 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 generators, or RTGs.

RTGs work by converting the heat given off by a radioactive isotopes into electricity. To understand this process, it helps to understand what a radioactive isotope is. Radioactive isotopes come from elements which of course are non-renewable, these isotopes have extra neutrons and want to get rid of them. In doing so, they become radioactive, releasing protons, neutrons, and energy in a process called decay.

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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 . 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.

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Engineers have realized with this kind of potential, they don’t have to limit 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 because of their longevity .

Nevertheless, NASA and the Department of Nuclear Energy continue to improve the design to (the still in development) ASRGs or advanced stirling radioisotope generators. These new generators will use less plutonium cells and weigh less when successfully deployed.

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.

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