The two Voyager spacecraft launched in 1977 are now the farthest man-made objects from Earth, at more than 19.5 billion and 16 billion kilometers away, respectively. In 2012 Voyager 1 became the first spacecraft to leave the solar system and enter interstellar space—and its sister spacecraft is not far behind.

Both probes were the first to visit the solar system’s giants, Jupiter and Saturn, and Voyager 2 flew by the other outer planets; they changed our understanding of those worlds profoundly. The Voyagers also carry “golden records” packed with recordings of sounds, songs and pictures to communicate a sense of life on Earth to any extraterrestrials that might encounter them.

Arizona State University planetary scientist Jim Bell recounts the story of the Voyagers in his new book The Interstellar Age: Inside the Forty-Year Voyager Mission (Dutton, 2015). Bell spoke to Scientific American about the impact of the spacecraft and how scientists decided what human facts and achievements to put on the golden records.

[An edited transcript of the conversation follows.]

These days we’ve become almost jaded about sending spacecraft to the solar system, but what was the mood among the scientists when they were getting ready to launch the Voyagers?
They had this real sense of history, of the potential for this mission to be historic. We knew so little about the giant planets and their moons. We still didn’t know what these places would turn out to be like. We had just done some of the first flybys and orbits of Mars. It was clear that to get to know a place you’ve got to go there.

When the Voyagers reached the outer planets, what effect did their images and data have?
They were revolutionary. The Voyagers discovered many moons around the planets we never knew were there. And even the ones we knew were there were literally just points of light in telescopes before that. All of sudden they changed to geologic objects, to worlds that had weather and volcanoes and tectonics. It was just night and day.

And then for the giant planets themselves, the real revelation was that there are two kinds of giant planets in our solar system, and it turns out now in other systems as well. There are the large gas giants, and then the Uranuses and Neptunes, which are smaller, dominated by volatiles—a different kind of beast. We didn’t know that at the time.

How long can the Voyagers keep running?
They have a plutonium power supply that is down to about a half from when they were launched, so the prediction is that the spacecraft could continue to have enough power to transmit until the early to mid-2020s. If they go into the mode of shutting down some of the more power-hungry systems as the power drops, they might be able to stretch that.

There’s nothing to photograph out there. But the squiggly-line science—the fields and particles, cosmic rays—those instruments are lower power, simple to operate. It’s conceivable that it could easily be another decade of having these outposts of human civilization out there.

As they continue on into interstellar space, what will they encounter?
Both of them will serendipitously pass by some nearby stars in 30,000, 100,000 or 200,000 years. There’s nothing that’s going to slow them down or stop them. They’re not going to change direction unless they happen to pass by something—a star we don’t know about or a rogue planet.

In the absence of sun or wind or anything that’s going to wear them down they could easily outlast us—our entire civilization, outlast our planet. The Earth will eventually be swallowed by the sun and the Voyagers could still be out there. That’s what’s so exciting about having the golden records on them.

How did the golden records come about?
This was really driven by Carl Sagan. He was a visionary, big-picture thinker about space, and when they realized that the mission was being developed, that they would be able to accelerate the spacecraft beyond the sphere of the sun, I think Sagan realized this opportunity.

They made a conscious effort to be positive and uplifting, to show pictures and music and readings that were the best part of humanity. They debated a little bit about whether to talk about war, to include a mushroom cloud, and Sagan was very adamant that these represent some of the best parts of humanity. We don’t want it to be seen as threatening. I agree with that philosophy. Science and technology are the best parts of us.

And you put a message in a bottle not for the person on the other side but you do it for yourself, reminding ourselves how far we’ve come as a species and the amazing things we can do for good and for knowledge. It’s a good thing for all of us.

Did anyone think aliens would actually ever find these?
I think in general not. They are so small, there are such vast distances between stars, and they’re not going to fly close to any stars or planets. The odds are ridiculously small that they would be found by alien civilization.

I speculate that it’s really going to be us that finds them—as our technology advances, as we travel out into the solar system and the universe we could easily catch up to the Voyagers and it will be us who reads these.

How active is the Voyager science team today?
There are people like Ed Stone [Voyager project scientist at California Institute of Technology] still working on the project, and his students, like Jamie Sue Rankin, who was born after the Voyagers launched. She’s a young researcher getting her PhD with Ed using the data. And there are others out there as well. It’s not as large of a community as it used to be, but it’s still a fairly sizable commodity.

I make the point in the book that there’s this misconception among the general public that there are these robots exploring the solar system. But it’s really human exploration, because it’s people controlling these machines, people who wish they were there instead of the machine and who try to experience the place, visually and through chemical instruments, as much as possible. So really Voyager is a voyage of human exploration.