Voyager 2 shut down an instrument heater, but is still going strong, more than forty years after launch. (Credit: NASA/ESA/G. Bacon/STScI)

Launched in 1977, Voyagers 1 and 2 are the longest-running spacecraft, still operating at more than 11 billion miles from home, decades after the end of their nominal goal of exploring the outer solar system planets. They still get their power from the same three radioisotope thermoelectric generators, or RTGs, that have served them for years. But with th