Beyond Earth and its bubble of satellites; past Mars, where rovers explore; past Jupiter and its circling orbiter—outside the solar system entirely—two spacecraft are gliding across interstellar space. They have crossed over the invisible boundary that separates our solar system from everything else, into territory untouched by the influence of the sun. People have seen much deeper into the universe, thanks to powerful telescopes that catch the light of distant stars. But this is the farthest a human invention has ever traveled. These hunks of gleaming metal and circuitry—they are the furthermost tangible proof of our existence.
The twin Voyager spacecraft took off in 1977, carrying scientific instruments and golden records stuffed with information. Millions of miles away, they still communicate with Earth. They still collect data. But they are aging.
The spacecraft, traveling in slightly different directions, weaken every year. Their thrusters, which keep them steady, are degrading. Their power generators produce about 40 percent less electricity than they did at launch.
To keep the Voyagers going, engineers make some tough decisions from afar. They tell the Voyagers, in commands carried over radio waves, to shut down systems or switch to backups, rationing every single watt. They prepare for what may be the mission’s final years. “Someday we’re going to have to say goodbye,” says Candy Hansen, a scientist at the Planetary Science Institute who worked on the Voyager mission in the 1970s and 1980s.
But not yet. This summer, engineers instructed Voyager 2 to fire up a set of thrusters that the spacecraft hasn’t used since 1989. Thrusters help keep the spacecraft steady, and the ones Voyager 2 was using were deteriorating, which meant they had to fire more often, until the number of pulses became “untenable,” according to NASA. Without healthy thrusters, the spacecraft would lose its ability to keep its antenna pointed toward Earth, the point of communication between the spacecraft and its stewards back home. Voyager 1 made the same switch last year.
Engineers also shut off a heating component that keeps one of Voyager 2’s instruments warm enough to function in the frigid cold of space. Turning off a heater buys the mission four watts, the same amount it loses in a year. After months of deliberation, scientists decided that sacrificing this instrument, which last year helped confirm that the spacecraft had left the solar system, was worth it. Unlike the others, this instrument can point only in certain directions.
Some other instruments have, incredibly, tolerated the loss of their heaters, sometimes for years. According to NASA, the temperature of the cosmic-ray instrument, the most recent target of rationing, has dropped to –74 degrees Fahrenheit (–59 degrees Celsius), far lower than what it withstood during testing on Earth. But it’s still collecting data and beaming them home.
Heaters on instruments on both Voyagers will be next on the chopping block. Suzanne Dodd, the Voyager project manager at NASA’s Jet Propulsion Laboratory, says the team may someday be forced to turn off one of the engineering elements that help the spacecraft communicate with Earth. “If it does work, then we gain two more watts,” Dodd says. “If it doesn’t work, then we lose the mission.”
No one was thinking about interstellar space when the Voyager spacecraft were dispatched. Scientists and engineers had set their sights much closer to home: the other planets, arranged in a rare alignment that allowed the spacecraft to swing from one to the next. “The twin Voyagers, despite all the odds to the contrary, have been our accidental visitors to the beginning of the space between the stars themselves,” says Ralph McNutt, a scientist at Johns Hopkins Applied Physics Laboratory who still works on the mission.
Both spacecraft reached Jupiter first, capturing the planet’s swirling filigree of storms in unprecedented detail. Voyager 1 bopped from there to Saturn. Scientists were particularly interested in the planet’s largest moon, Titan, which would turn out to be one of the most intriguing spots in the solar system and a potential home of extraterrestrial life. If Voyager 1 didn’t collect enough good data before moving on, Voyager 2 would be redirected to try again. But the flyby worked, allowing Voyager 2 to swoop past Saturn and on to Uranus and Neptune.
Engineers took their first energy-saving step not long after that. After the planets, there was little to photograph aside from a handful of distant stars, so engineers turned off the Voyagers’ power-gobbling cameras. Hansen, who worked on the imaging team, says that if engineers resurrected the cameras now, “it would literally kill every other instrument on the spacecraft.”
Voyager 1 left the solar system in 2012, and Voyager 2 followed last year. They registered the shift; their scientific instruments detected significant changes in the cosmic environment around them. The touch of the solar wind, invisible particles that envelop the solar system in a protective bubble, had disappeared. Engineers had already shut down a few instruments years earlier. Those still operating today are designed to study the few detectable phenomena of interstellar space, like cosmic rays and magnetic fields.
NASA has asked scientists to study the possibility of a new interstellar probe, but no formal missions exist. For now, the Voyagers are it. And one day, engineers will come into work expecting to hear the faint pings from the spacecraft, messages that take hours—nearly a day, in Voyager 1’s case—to cross the expanse and reach ground-based antennas. They won’t hear anything, and they might not ever be able to decipher why.
It could be that the spacecraft, running without enough heaters, become so cold that the fuel lines freeze, cutting off the power that thrusters need to keep the antenna turned toward home. Or it could be that the transmitters, which send and receive signals, run out of power, a scenario Dodd predicts could happen in the 2020s. The invisible tether that has connected scientists and engineers to the spacecraft for more than four decades, always unspooling further, would run out at last. One or two instruments might still work. The Voyagers would keep chronicling their journey through the cosmos, making history with every mile, but they’d have no way of calling home.