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Radioisotopes in Space
Coasting through space, perhaps somewhere near Neptune, a spacecraft lacks the warm touch of the sun's rays; in case an instrument needs an adjustment or fuel needs to be restocked, human hands cannot help. How does one formulate the design basis for this harsh environment? Radioisotope thermoelectric generators (RTGs) are a type of power system that, despite the hazard of radioactivity, has long been used by NASA to provide power in the coldest, deepest parts of the universe. Of course, there are many risks and issues involved in utilizing radioisotopes, including safety, expense, and a history of accidents and radioactive contamination. Nevertheless, radioisotope thermoelectric generators are, overall, a practical choice to supply power in remote locations
Radioisotope thermoelectric generators provide power for spacecraft using the heat created by the radioactive decay of a radioisotope. As the radioisotope decays, a thermocouple consisting of the metals silicon and germanium converts heat into electricity. The chemical differences between these two metals cause energy to transfer from the "hot" side of the thermocouple to the relatively "cold" side, which creates voltage. RTGs may be used alone or with other power systems, like solar panels, and they can produce up to a few hundred watts of power.
There are several versions of the RTG that use a few different radioisotopes, but the most common source used in these power systems is plutonium-238, which has a half-life of 87.7 years. The heat that plutonium-238 produces results from alpha decay into uranium-234. Alpha decay is easy to shield against--with cardboard, for instance--since alpha particles do not penetrate objects very well. Plutonium-238 is a fissionable, rather than fissile, radioisotope, meaning that it is incapable of undergoing a chain reaction. RTGs lose power over time as the radioisotope decays, but they produce more radiation as they age because of the build-up of the daughter nuclides. Daughter nuclides, like the uranium-234 that results from plutonium-238 decay, will continue to decay until they obtain a stable state.
RTGs have a number of characteristics that make them a practical power source. Firstly, they are compact for use in small spaces, so they decrease the launch mass of a spacecraft. The less space and mass the power source takes up, the more room there is for scientific experiments and other necessities. RTGs are also long-lasting, useable for 10 years or more, but they do not need refueling. Additionally, radioisotopes are very reliable, so when an RTG is many miles away, it can still function--and it functions continuously as the radioisotope keeps decaying. They contain no moving parts that have the potential to wear down and fail. In terms of spacecraft in particular, not only does the RTG power the spacecraft, but any excess heat from the radioactive decay benefits the scientific instruments on board by keeping them warm in the depths of space. Also, according to NASA, radioisotopes are mostly unaffected by other sources of radiation, like cosmic rays.
In use since 1961, the RPS has been utilized on many NASA missions, including Apollo (Moon), Viking (Mars), Voyager (outer solar system), Galileo (Jupiter), and, more recently, New Horizons, which is headed towards Pluto; and the system is expected to be used in future deep space missions. In one report, NASA proclaims that even if other technologies were developed in the future, the RPS will remain the "mainstay for deep space exploration."
Risk is involved in any project destined for a remote location, regardless of whether radioactivity is a factor or not, so the best we can do is prepare as much as we can in order to mitigate the possibility of the worst-case scenario. For instance, the plutonium used in RTGs is coated with ceramic, which reduces the radioisotope's ability to react chemically with other elements. Also, the radioisotopes come as individually shielded modules. Furthermore, RTGs are designed to withstand spacecraft accidents so that the risk of radioactive leaks is minimized. However, if a satellite containing an RTG collided with another satellite, certainly it must be possible for a breach of the radioactive shielding. But, as mentioned earlier, the radioisotopes used in RTGs cannot explode violently, and many spacecraft equipped with RTGs travel beyond Earth where they are no longer a risk anyway.
Despite our preparedness, sometimes accidents happen. In the United States, RTGs have yet to actually cause an accident, but they have been involved in some precarious situations. One of these instances was Apollo 13, which returned to Earth without jettisoning the lunar module--as it normally would have--that carried plutonium to be used in experiments on the moon. Now a cask of plutonium sits at the bottom of the ocean, supposedly not at risk of leaking. Nevertheless, the Environmental Protection Agency reports that it has partnered with NASA and the DOE to create "contingency emergency plans" in case of contamination during the launch or re-entry of spacecraft. For instance, during both the launches of Galileo and New Horizons, which had RTGs on board, the EPA was prepared to provide "monitoring for radiation; sampling air, water, and soil; determining levels of contamination; [and] advising Florida on managing contaminated areas." When dealing with radioisotopes, accidents are always possible, but many safety systems are in place to prevent radioactive contamination or to clean up if contamination occurs.
If we say that RTGs are too much of a risk or create too many issues in comparison with the benefits, what are our other options? Solar-powered spacecraft become rather ineffective if the panels are not directly facing the sun or as a spacecraft travels into the deepest parts of space where there is little sunlight. Solar panels also take up a lot more space than an RTG. However, since they cannot explode like a weapon and because they are secured, RTGs are a very practical alternative to solar panels. RTGs also last longer than batteries, which cannot constantly provide the huge amounts of power that a spacecraft needs, and they last longer than fuel cells, which eventually need refueling. Furthermore, RTGs are more efficient in converting heat into electricity than other thermoelectric devices. Certainly there are other ways of generating power in remote locations, but the use of radioisotopes is quite practical, especially for spacecraft that travel deep into the solar system.
Radioisotope thermoelectric generators have literally provided us with the power to send equipment to the most remote places in the universe, allowing us to venture places we never thought we could reach. Without RTGs, it would otherwise be quite difficult to find another power source that is as compact, reliable, and long-lasting.
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