Sometime in the next 10 or so years, the massive antennas that comprise NASA’s Deep Space Network will pick up a faint, distant signal for the final time. When that day comes, humanity will say goodbye to Voyager 1, the first and to date only spacecraft to reach interstellar space. For scientists at NASA, Voyager’s death will be a moment long prepared for, and something they will have spent decades attempting to delay.

“It’s kind of like the death of a family member,” NASA scientist Suzanne Dodd told Vocativ. Dodd is the project manager of the Voyager Interstellar Mission at NASA’s Jet Propulsion Laboratory. “You know it’s coming, but it’s still sad to recognize it’s really over and you can’t communicate with the spacecraft anymore.”

No human-made object has ever traveled farther than Voyager 1, which has made it about 12.5 billion miles from Earth since its launch in 1977. That’s about 137 times the distance from Earth to the Sun, a measure known as an astronomical unit or AU. The probe’s twin, Voyager 2, is at about 112 AU, meaning it too should reach interstellar space sometime in the next few years. Given Voyager 1’s enormous head start and speed – it’s traveling at around 40,000 miles per hour – it’s certainly conceivable that nothing will overtake it as humanity’s most distant outpost in any of our lifetimes.

But there’s no getting around it: Their condition is terminal, and what Dodd and her team are engaged in is the equivalent of deep space hospice care. While the nearly 40-year-old spacecraft present many identical challenges like managing the fading power reserves and guarding against the incredible cold of outer space, each has developed its own particular chronic conditions. “You can think of them as twins, like twin sisters, but different ailments have affected the different spacecraft,” said Dodd. “Voyager 2 is kind of tone-deaf. You need to rack through several frequencies before it finds one that it can hear.”

To remain in contact with both Voyager 1 and its twin Voyager 2 nearly 40 years after launch, NASA scientists have had to slowly power down the scientific instruments until, sooner or later, nothing but the transmitter remains. According to Dodd, 2020 is when the shutdown will really begin in earnest in the final effort to squeeze as much time as possible out of the probes’ plutonium batteries.

“We will shut off the heater for a lot of the instruments, and that will save anywhere from two to four watts of power, so six months to a year of power,” she said. That will expose the instruments to the elements of outer space, where it’s only a handful of degrees above absolute zero. “When you shut off the heater, it could mean the instruments stop working because it gets too cold. It could mean that they will continue to work, we really don’t know.”

Here on Earth, Dodd and her team can use NASA’s most modern technology to stay in contact, but the Voyagers are effectively relics of the 1970s. “The computer systems and whatnot are vintage 1974, 1975, when the spacecraft was being built,” said Dodd. “Your iPhone has 100,000 times more memory than the Voyager spacecraft.” She compared the challenge of uploading new software to the spacecraft with playing a game of Tetris, with the team having to cram irregular blocks of data into tiny spaces.

Voyager 1 and 2 aren’t the only probes headed toward interstellar space and, eventually, out of the solar system. (Those aren’t exactly the same thing.) Their slightly slower predecessors in deep space exploration, Pioneer 10 and 11, lost contact with Earth in 2003 and 1995, respectively, meaning we can’t know precisely when in the next 15 or so years these spacecraft will cross the interstellar threshold. Then there’s New Horizons, which launched in 2006 to reach Pluto, the one planet left out of the Voyagers’ grand tour of the outer solar system – and arrived in 2015, just in time for Pluto to get demoted to dwarf planet status, much to the NASA team’s chagrin.

Talking with the leaders of these various missions, a common theme emerges: None of these probes were designed to last long enough to reach interstellar space, as that was never their goal. Voyager 1 and 2 completed their exploration of the outer planets in about the first dozen years of their missions, with Voyager 2 becoming the only spacecraft to visit Neptune in 1989 and Voyager 1 taking its famous “Family Portrait” photographs of the Sun and six of the planets on Valentine’s Day 1990. Every last one of the billion of miles traveled since then has exceeded official expectations. But even that has got nothing on how long the Pioneer missions lasted relative to the original plan.

“Both Pioneer 10 and Pioneer 11 were only designed to operate for 21 months,” former Pioneer project manager Lawrence Lasher told Vocativ. Pioneer 11, the shortest-lived of the four original deep-space probes, still managed to turn 21 months into more than 22 years. While its signal was working fine in 1995, its death came as it essentially lost control of its motor functions, with NASA unable to reposition the spacecraft to point its antenna back toward Earth.

Launched along with its twin in 1972, Pioneer 10’s 31-year lifespan ended up being longer than NASA’s balance sheets could quite account for. “The science mission was over on March 31, 1997. And the reason that had to stop at that time was budgetary considerations, and there were other missions that NASA felt would be delayed if we kept on funding Pioneer 10.” Lasher and a few colleagues managed to put together a significantly reduced new mission to keep in contact with Pioneer 10, so that they could take advantage of this unprecedented opportunity to study deep space communication and help pave the way for future interstellar probes.

His team took on managing Pioneer as an extra-hours special project, taking advantage of late nights and weekend hours when the Deep Space Network’s resources weren’t being used by higher-priority missions. After a few years, Pioneer could no longer transmit science data or even the basic telemetry of where it was in the solar system. The probe was slowly dying. “The last signal that we got was January 23, 2003,” said Lasher. “It was a very weak signal. We just weren’t able to get any more because of the fact that the power source had decayed, and it just didn’t have enough power to send additional transmissions.” A final effort was made to regain contact with the probe in 2006, but there would be no resurrection for Pioneer.

New Horizons is still in the middle of its science mission, having reached Pluto last year and now en route to a flyby with an object in the Kuiper Belt in 2019. “The spacecraft’s prime mission was nine and a half years, actually ten and a half counting the data downlink,” mission leader Alan Stern told Vocativ. “We’re there now. So now everything else is now gravy and bonus because we completed the prime mission.” He estimated New Horizons will have enough power to operate until around the mid-2030s, possibly even the early 2040s, with the rest of its mission after the next flyby likely being along the same lines as Voyager’s interstellar mission. The ship is still young, not yet having had to switch on any of its backup systems. The Voyagers have long since either gone to their backups or shut them down preemptively to conserve power.

Though New Horizons is three decades more advanced than Voyager 1, the nature of its mission means it will never reach the same speed as its predecessor. And, like Voyager and its tiny 1970s-bound memory, part of it will always be stuck in 2006. In this case, that means limitations on the lifespan of its plutonium battery. “Ours was not fully fueled at the time of launch because of difficulties at the place that produces plutonium called Los Alamos National Lab,” said Stern. New Horizons ended up going into space with 80 percent of a fully fueled battery. “For that reason, we have plenty of power to conduct our prime mission and our Kuiper Belt mission, but we’re not able to fly on as long as Voyager.”

Of course, the “we” there is meant in a general sense, as the scientists that direct New Horizons today likely won’t all be the same ones who oversee the eventual shutdown of New Horizons. Stern called this the challenge of longevity, or the question of keeping alive “the intimate knowledge of how to operate this complicated spacecraft decades from now after many of us have retired and a whole new generation of people who didn’t build it or fly it to Pluto.” Humanity is still a long way away from launching generation ships to the stars, but the task of maintaining these probes across the decades is a first approximation.

What the loss of these probes means varies depending on who you asked. Dodd compared it to the death of an elderly family member, while Lasher said he was philosophical about the loss of Pioneer 10 and 11, just satisfied that they had gotten so much extra data from their extended lifespans. How much you grieve for Pioneer 10 and 11, Voyager 1 and 2, and eventually New Horizons perhaps depends on how much you’re willing to anthropomorphize the robotic avatars of our most distant explorations.

Then again, maybe there’s another way of looking at the “deaths” of NASA’s deep space probes. After all, even after we lose contact with all five, they will still be speeding away from the solar system toward distant stars. In the vacuum of space, there’s nothing to corrode or degrade the spacecraft other than the occasional stray particle, meaning there’s every possibility that these probes will still be out there somewhere millions of years from now. By then, it’s entirely possible that the last humans in existence will be the couple carved onto the Pioneer plaque, those photographed for Voyager’s Golden Record, and Pluto discoverer Clyde Tombaugh, an ounce of whose ashes are aboard New Horizons.

“One of the motivations for the plaque was that maybe some sentient being with an advanced civilization around one of those stars might somehow come across it and they’d have that plaque,” said Lasher. When famed astronomer Carl Sagan spearheaded the effort behind that plaque more than 40 years ago, it was thought of as a greeting. By the time it’s finally found, it might well be our species’s epitaph.

This article is reproduced with permission from Vocativ (© Vocativ). The article was first published on October 23, 2016.