Welcome to AAC's series of articles celebrating the Voyager missions! Please check out the rest of the series to learn more:
- Long Distance Communications
- Command, Data, and Attitude Control Computers
- Cameras, Polarimeters, and Magnetometers
The Deep Space Problem
One of the most important mission parameters of the Voyager probes was to send back scientific data and images of Jupiter and Saturn. However, power generation from such a distance from the sun is a serious problem. Assuming the best case scenario, the distance between the Sun and Jupiter is approximately 5.2AU and with the energy intensity from the Sun on the earth being ~ 1300W/M2 then the approximate energy intensity from the Sun at Jupiter is ~ 50W/M2. This dramatic drop in solar intensity, despite only Jupiter being 5 times further from the Sun than Earth is to the Sun, is due to the inverse square law where the intensity at distance r is proportional to the inverse square of that distance. For example, if the distance from the sun is doubled then the intensity is reduced by a factor of 4. Jupiter, being 5AU from the sun, has an intensity 1/25 of that found at earth.
Solar panels are great... if you're close to the sun! Image courtesy of NASA
So with only 50W/M2 of energy and assuming a solar panel efficiency of 15% (this figure is debatable), the energy per square meter of solar panel becomes 7.5W. Considering that the probe was going to need a power supply close to 400W, the total solar panel surface area would have been 53.3 M/2. This calculation does not even take into account a Saturn encounter (a distance of 10AU), which would only have 10W/M2 of solar radiation. Therefore, a better power source was needed and so the engineers at NASA decided to go for a Radioisotope Thermoelectric Generator.
Using Radiation for Power
"Radiation” is a term that describes several processes of transferring mass or energy out of an atom. Electromagnetic radiation is created when electrons move from high-energy to lower-energy orbitals and photons are produced—this type of radiation allows Voyager to wirelessly communicate with Earth. Nuclear radiation occurs when the nucleons (protons and neutrons) rearrange themselves to move from a high-energy to a lower-energy state—particles and photons are produced and captured to create heat—this type of radiation provides Voyager with power.
The electromagnetic spectrum. Image courtesy of OSHA
Ionizing radiation penetration depths. Image courtesy of Mirion
The RTGs on the Voyager probes rely on Plutonium-238, which is primarily an Alpha emitter. Alpha radiation is the most ionizing type which means that it has a strong ability to strip other atoms of their electrons. But, Alpha is also the easiest to shield against.
Radioisotope Thermoelectric Generator – How They Work
The basic RTG consists of a radioactive material (preferably one with a short half-life such as Plutonium 238 which has a half-life of 83 years), and a thermocouple. Radioactive materials release thermal energy when they decay and this heating effect is, naturally, called decay heat. When an atom undergoes radioactive decay the kinetic energy of the emitted particles is converted into thermal energy. This thermal energy is then applied to the “hot side” of a thermocouple and the “cool side” is kept as cool as possible (with the use of a heat sink). The result is the generation of an EMF across the thermocouple which can be directly used as an electrical source.
While the basic RTG may seem simple, there are many factors that have to be considered. One of the most important considerations is safety to people and the environment. For an RTG to produce a useable amount of power (hundreds of watts for example), the radioactive material used must be able to generate a sizeable amount of heat (several kW). Materials that have a short half-life undergo more radioactive decays per second than a material with a longer half-life. The heat generated by a radioactive material is inversely proportional to its half-life and therefore materials with short half-lives generate the most heat. This is why elements such as Plutonium-238, Strontium-90, and Polonium-210 are used.
Radioisotope Thermoelectric Generator – The Issues
RTG fuels are highly radioactive and thus pose a credible threat to the environment if released. This is why the Cassini probe is being crashed into Saturn as the RTG onboard could potentially harm undiscovered life on the moons of Saturn. Therefore, RTGs contains many layers of protection to prevent the release of radioactive material. The Voyagers RTGs consists of a fuel assembly (referred to as the heat source), and the actual generator itself.
The radioactive material is split up into many pellets (as to prevent critical mass which would result in a fission reaction), with each pellet being housed in a small sphere. The sphere consists of two layers, an impact shell, and a post impact shell, which help to prevent the release of radioactive material in the event of a catastrophic failure. These spheres are then placed into a container which is then placed inside the generator.
The generator consists of many Si-Ge uni-couples which generate electricity and the uni-couples are also connected to fins which help to dissipate heat on the cool side of the thermocouples through radiative cooling.
A second, more bizarre consideration must be taken with RTGs which is interference. Radioactive materials can undergo two kinds of decay; alpha decay and beta decay. Regardless of which decay takes place, a gamma particle is also emitted (due to the “leftover energy” in the nucleus). While the alpha and beta particles are readily absorbed by the RTG the gamma particles will easily pass through. This can be a serious problem for any sensitive instrumentation and/or circuitry as bits can be flipped and false detections can be made by detectors.
Therefore, RTGs need to be kept away from such devices and this is why the RTGs on the Voyager probes are placed on a boom far from sensitive electronics. The orientation of the electronics is at 180 degrees to the RTG because the probe itself can mitigate against some gamma emissions and that the RTG emits more gamma rays on its long edge than its ends.
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Thanks to the radioactive decay of certain elements the Voyager probes continue to operate well beyond their original mission parameters. While the RTGs today only operate at approximately 50% power (due to the decay of almost half the plutonium-238), they are still able to provide power for the transmission of data. By 2020, the onboard instrumentation on the Voyager probes will be turned off one by one in an attempt to conserve power and keep the probes in communication with radio telescopes on Earth. By 2025, there will not be enough power to operate the onboard scientific equipment and thus ending the scientific operation of the Voyager probes.
RTGs provide probes going to the outer solar system with power when solar energy becomes too little and they can provide power for a long time. Until more exotic power generation techniques become available RTGs will still be the favorite for long-term deep space exploration.