American Scientist

To the Editors:

The extended survival of the robotic Voyager probes has been unexpected and welcomed by those interested in space exploration ("The Voyagers’ Odyssey,"  July–August 2015). It should be noted that the power loss suffered by the radioisotope thermoelectric generators is not as dire as the authors indicate. Since the decay of plutonium-238 is exponential rather than linear, the available power remaining in 2015 (38 years after launch) should be 344 watts rather than the 264 watts reported. Vive les Voyageurs!

Brian MacPherson
Burlington, VT

Editors’ note:
The decay of plutonium-238 is not the only factor that reduces the available power aboard the two Voyager probes. Their thermocouples—the devices that convert radioactive heat into electrical energy—also degrade over time. As of the beginning of 2015, the Voyagers were generating just 255 watts; they will have to shut off their last instruments around 2025. New Horizons, following a path similar to Voyager 2’s, could remain active into the 2030s, but because of its lower velocity it will not be able to explore as far from the Sun.