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Twin Voyager probes nearing 45th birthdays and hard decisions

On August 20, 1977, NASA’s Voyager 2 spacecraft launched to space. Its twin, Voyager 1, launched 16 days later. Today, they are not only the most distant man-made objects — at 12 billion and 14.5 billion miles (19.3 billion and 23.3 billion kilometers) away from Earth, respectively — but also NASA’s longest-operating mission, continuing to send back data from their interstellar journeys toward the edge of the solar system as they approach their 45th birthdays.

But each Voyager spacecraft is powered by a finite nuclear energy source, and both sources are dwindling to dangerously low levels. Each spacecraft carries a store of the radioactive isotope plutonium-238; as the isotope decays, it releases energy that is converted into electricity by three radioisotope thermoelectric generators (RTGs). At launch, the RTGs provided each spacecraft with 450 watts of power. Now, they’re producing less than half that amount and their electrical output is decaying by four watts each year.


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