Powered by the radioactive decay of plutonium-238 pellets housed inside its radioisotope thermoelectric generator (RTG), Voyager 1 and 2 are still operational, albeit on a dwindling electricity supply. RTGs are used for deep space missions where the light from our sun is too weak for solar arrays to be practical. But RTGs don't last forever.
In an interview with Space.com, Stone estimated that both spacecraft only have about 10 years' supply of power before the plutonium heat output dwindles to levels that won't sustain any spacecraft instrumentation or critical subsystems. Though the Voyagers will eventually die, they continue to take data, and Voyager 1 is taking measurements of a very alien region – the interstellar medium.
On Aug. 25, 2012, NASA confirmed that Voyager 1 had officially left the heliosphere, speeding into interstellar space (Voyager 2 is traveling in a different direction and hasn't encountered interstellar space, yet). The mysterious outermost regions of the solar system have now been probed, and Voyager 1 was able to take measurements of the magnetic field and particle energies – important measurements that have allowed scientists a very privileged view of how our sun's magnetic field and solar wind particles interact with the space between the stars.
It took nearly four decades to reach the interstellar shores – a fact that underlines the multigenerational effort the Voyager program has become.
"I will never forget my first day on the job – walking into a room full of world-class scientists engaged in lively discussion about humankind's first in-situ measurements of interstellar space," remembered Caltech graduate student Jamie S. Rankin in her personal mission story. Rankin joined the Voyager team only six days after Voyager 1 entered interstellar space: "Indeed, it was a historical moment, and there I was, a kid in her early 20s wearing sandals, shorts, and a comical t-shirt with a black-hole Pac-Man eating a bunch of planets."
Rankin – who's adviser is Stone – is using Voyager data to explain "how the intensities of galactic cosmic rays change through their interactions with the heliosphere," she said.
So, after four decades of exploration, two spacecraft built from 1970s technology are still exploring, and a new generation of scientists are using them to carry out cutting-edge research in a region of space that no other robot that we know of has ever experienced.
Even after their power supplies dwindle and the Voyagers lose communication with Earth, they will be silent interstellar emissaries for humanity, carrying the Golden Records as shrines of the civilization that built these incredible machines, should an extraterrestrial intelligence stumble upon them in the eons to come.