Radioisotope Generator RTG Decay Calculator
Deep-Space Power Engineering: Predict Plutonium-238 thermal decay ($Q_{\text{th}}$), thermocouple degradation, and net electrical power ($P_e$) over decades.
RTG Architecture & Fuel Inventory
Current Electrical Power & Thermal Output
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Tested hardware and components for high reliability
Physics of Radioisotope Thermoelectric Generators (RTGs)
Radioisotope Thermoelectric Generators convert the spontaneous radioactive decay heat of Plutonium-238 ($\text{PuO}_2$) directly into continuous electricity via the Seebeck effect, enabling deep space exploration where sunlight is negligible.
1. Radioactive Decay Kinetics
The thermal heat output $Q_{\text{th}}(t)$ decays exponentially according to the half-life of $^{238}\text{Pu}$ ($T_{1/2} = 87.74\text{ years}$):
Q_th(t) = Q₀ · exp( - [ln(2) / 87.74] · t )
2. Electrical Power Output
Net electrical power $P_e(t)$ is the product of thermal heat output and thermocouple conversion efficiency:
P_e(t) = Q_th(t) · [ η₀ · (1 - r_deg)^t ]
where $\eta_0$ is the Beginning-of-Life thermocouple conversion efficiency ($5.5\text{--}7.0\%$) and $r_{\text{deg}}$ is the annual thermocouple degradation factor ($0.6\text{--}1.0\%/\text{year}$).
Frequently Asked Questions
Why is Plutonium-238 the preferred isotope for deep space RTGs?
Plutonium-238 emits pure alpha particles ($5.5\text{ MeV}$) which are completely stopped within the ceramic fuel pellet itself, converting kinetic energy entirely into thermal heat without dangerous penetrating gamma rays or high neutron flux. With a half-life of $87.74\text{ years}$, it provides multi-decade longevity for missions to the outer planets (Voyager, Cassini, New Horizons).
What causes the power output of an RTG to degrade over time?
RTG power decline is caused by two compounding factors: (1) radioactive decay of the fuel ($0.787\%\text{ per year}$ reduction in thermal heat $Q_{\text{th}}$), and (2) thermoelectric couple aging ($0.5\text{ to }1.0\%\text{ per year}$ degradation) caused by high-temperature dopant migration, sublimation of semiconductor materials (e.g. PbTe / TAGS), and increased electrical contact resistance.
How much electrical power are the Voyager probes still producing?
Launched in 1977 with approximately $470\text{ W}_e$ of electrical power from three MHW-RTGs, the Voyager 1 and 2 probes are still operating in interstellar space nearly 50 years later, producing roughly $220\text{ W}_e$ (about $47\%$ of initial launch power).