Ed Stone, Voyager project scientist, discusses the journey, including its flirtation with the outer edges of the solar system. (NASA/JPL-Caltech)

Ed Stone, 76, is project scientist for the Voyager planetary mission, launched by NASA in 1977. The program includes the twin spacecraft Voyager 1 and Voyager 2, which have made an unprecedented tour of the outer solar system. The spacecraft took the first color photographs of Jupiter, Saturn, Uranus and Neptune, and discovered dozens of moons of these worlds.

In recent years, the spacecraft headed to unknown territory. The Voyagers are now flirting with the edge of our solar system, defined by the magnetic bubble, or heliosphere, created by the sun. This summer, new data showed Voyager 1 had slipped into an unexpected “magnetic highway” that connects the bubble to interstellar space — another in a long line of Voyager discoveries.

In an interview at the American Geophysical Union’s fall meeting in San Francisco, he discussed his long involvement with the mission.

It seems the Voyager mission has surprised us once again.

Absolutely. This is another exciting milestone in Voyager’s mission of discovery. We’ve discovered a magnetic highway that connects us to the outside. The particles inside [the solar system] escape along this highway, and at the same time, the cosmic ray particles that are outside can stream in along this magnetic highway.

Does this mean Voyager 1 has left the solar system?

Not yet. The heliosphere is a huge bubble the sun creates around itself. Outside the bubble is material that was created by the explosions of supernovae millions of years ago. Three indicators will tell us when we’re outside this bubble. The first is an increase in the intensity of galactic cosmic rays from outside. These are particles created by the supernovae explosions. The second is a decrease in the slower particles that are created inside our solar system. And a third is a shift in the direction of the magnetic field. The field created by the sun goes east-west. Right now, there has been no measurable change in magnetic field direction. So we have no evidence that we have actually gone outside, only that we’re in a compressed region where the magnetic field is piled up and has become more intense. We call that the highway.

How did you discover this region?

We became very excited on July 28th, when suddenly the particles from inside the solar system dropped to half. They had never been that low. But in five days, they came back. Then, on Aug. 13, another rapid decrease. We were in that for about a week. And then the boundary moved out again. And, finally, on Aug. 25, the intensity dropped to 10 percent of what we had inside and has since been decaying away. At the same time, the cosmic rays from outside were jumping up. With this last change, the galactic cosmic rays jumped up and have been at a steady intensity since then. We may now, in fact, be seeing for the first time the intensity of the cosmic rays from outside, even though we’re not outside yet.

What does this discovery mean?

It really is a new area. It’s a surprise, like so many things with Voyager. When we manage to get there, we find out our ideas were somewhat right but almost always incomplete. This is one of those incompletes. Now what’s ahead, of course, is finding how long is this highway and how far do we have to go to actually get to interstellar space. Based on past experience, I think it would be two or three more years before we actually cross the highway. We are just learning what it’s like in the very outer reaches of our solar bubble, on our way to interstellar space.

In the 35 years you’ve been working this mission, the two spacecraft have traveled more than 100 times the distance from the Earth to the sun. Which of the many memorable milestones really stick with you?

The Jupiter flyby in 1979. That was the first for Voyager 1 and 2 both. First of all, we saw the great red spot was this huge storm system. Then we suddenly realized it was just the largest of dozens of huge storms. Another big discovery at Jupiter was the volcanic activity on Io. Before Voyager, the only known active volcanoes in the solar system were here on Earth. And here was a moon with 10 times the volcanic activity of the entire Earth.

How do you explain the durability of the two spacecraft?

Three things. The first is the power supply, which uses radioactive decay with an 88-year half-life. The other key was Jupiter, which has strong radiation. We had to [shield] the electronics. Radiation is a form of rapid aging. So having made the system survive that rapid aging, it can survive the much slower aging going on normally. The third thing is, we had redundant systems for most of the spacecrafts’ systems. We had two transmitters, two computers, so as things wore out, we could switch to the second system.

What are you most proud of with Voyager?

So many things. People often tell me the missions inspired them in their own technical work. And there is no other mission making fundamental discoveries after 35 years.