Life Span
Most RTGs use 238Pu, which decays with a half-life of 87.7 years. RTGs using this material will therefore diminish in power output by 0.787% of their capacity per year. 23 years after production, such an RTG will have decreased in power by 16.6%, i.e. providing 83.4% of its initial output. Thus, with a starting capacity of 470 W, after 23 years it would have a capacity of 392 W. However, the bi-metallic thermocouples used to convert thermal energy into electrical energy degrade as well; at the beginning of 2001, the power generated by the Voyager RTGs had dropped to 315 W for Voyager 1 and to 319 W for Voyager 2. Therefore in early 2001, the RTGs were working at about 67% of their original capacity instead of the expected 83.4%.
This life span was of particular importance during the Galileo mission. Originally intended to launch in 1986, it was delayed by the Space Shuttle Challenger accident. Because of this unforeseen event, the probe had to sit in storage for 4 years before launching in 1989. Subsequently, its RTGs had decayed somewhat, necessitating replanning the power budget for the mission.
Read more about this topic: Radioisotope Thermoelectric Generator
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