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They are now in interstellar space, far beyond the orbits of Neptune and Pluto. Voyager 1 is more than 15 billion miles from Earth and Voyager 2 nearly 13 billion miles. Both have passed the heliopause, where the “solar wind� of particles streaming from the sun terminates.

“They’re going someplace where we have nothing, we have no information,� NASA Deputy Administrator Pam Melroy said. “We don’t know anything about the interstellar medium. Is it a highly charged environment? Are there a lot of dust particles out there?�

Even as the Voyagers continue their journeys, engineers and scientists at the NASA Jet Propulsion Laboratory in Pasadena, California, are mourning the loss of Ed Stone, the scientist who guided the mission from 1972 until his retirement in 2022. Stone, a former director of JPL, died June 9 at the age of 88.

“It’s great. This is exploration. This is wonderful,� Stone told The Washington Post in 2013 when he and his colleagues determined that Voyager 1 had reached interstellar space.

Voyager 1 has four scientific instruments still operational in this extended phase of its mission, but it suddenly ceased sending intelligible data on Nov. 14. A “tiger team� of engineers at JPL spent the ensuing months identifying the problem — a malfunctioning computer chip — and restoring communication.

That laborious process is nearly complete. Data is coming from all four instruments, project scientist Linda Spilker said, though engineers are still checking to see whether data from two of the instruments is fully usable.

What no one can change, though, is the mortality of a spacecraft with a limited power supply. Voyager 1 is running on fumes, or, more precisely, on the dwindling power from the radioactive decay of plutonium.

The Voyagers have traveled so far from the sun they can’t rely on solar power and instead use a radioisotope thermoelectric generator. But such generators don’t last forever. Voyager 1 and Voyager 2 will eventually go silent as they continue to cruise the galaxy. NASA scientists and engineers are hoping Voyager 1 can keep sending data until at least Sept. 5, 2027, the 50th anniversary of its launch.

“At some point, we’ll have to start turning off the science instruments one by one,� Spilker said. “Once we’re out of power, then we can no longer keep the spacecraft pointed at the Earth. And so [the Voyagers] will then continue on as what I like to think of as our silent ambassadors.�

In a sense, this is all a bonus because the primary mission for the two Voyagers was the exploration of the outer planets. Both visited Jupiter and Saturn, and Voyager 2 went on to Uranus and Neptune in what was known as the “Grand Tour� of the outer solar system, enabled by a rare orbital arrangement of the planets. The Voyagers delivered spectacular close-up images of the outer planets, and the mission ranks among NASA’s greatest achievements.

The gravitational slingshot from the planetary encounters sent Voyager 1 out of the elliptical plane of the solar system and did the same to Voyager 2 in a different direction.

About four years ago, Voyager 1 encountered something unexpected — a phenomenon scientists have dubbed a pressure front. Jamie Rankin, deputy project scientist, said the instruments on the spacecraft picked up a sudden change in the magnetic field of the interstellar environment, as well as a sudden increase in the density of particles.

What exactly caused this change remains unknown. But NASA scientists are eager to get all the data flowing normally again to see whether the pressure front is still detectable.

“Is the pressure front still there; what is happening with it?� Melroy said.

Voyager 1 is heading toward the constellation Ophiuchus, according to NASA, and in about 38,000 years, it will come within 1.7 light-years of an unremarkable star near the Little Dipper. But although it will have long gone silent, it does carry the equivalent of a message in a bottle: the “Golden Record.�

The record was curated by a committee led by astronomer Carl Sagan and includes greetings in 55 languages, sounds of surf, wind and thunderstorms, a whale song and music ranging from Beethoven to Chuck Berry to a Navajo chant. The Golden Record is accompanied by instructions for playing it, should the spacecraft someday come into the hands of an intelligent species interested in finding out about life on Earth.

“The spacecraft will be encountered and the record played only if there are advanced spacefaring civilizations in interstellar space,� Sagan said.

But that advanced spacefaring civilization might not be an alien one, NASA scientists point out. It’s conceivable that the cosmic message in a bottle could be picked up someday by a human deep-space mission eager to examine a vintage spaceship.