A web exclusive story from Astronomy magazine

Voyager through the eyes of Project Scientist Ed Stone

Voyager Project Scientist Ed Stone receives the American Astronautical Society’s lifetime achievement award in July 2014. Ann-Marie VanTassell/AAS

By Francis Reddy

The twin Voyager spacecraft conducted the first-ever “Grand Tour” of the outer solar system, flying past Jupiter, Saturn, Uranus, and Neptune by taking advantage of a once-in-176-years alignment that would allow a spacecraft to visit all four. Rocketing into space in 1977 a few weeks apart, both spacecraft continue to return data from the greatest distances yet achieved by any human-made objects.

The Voyager spacecraft NASA

Voyager 1, 13.0 billion miles (20.9 billion kilometers) from the Sun as of September 1, 2017, has exited the heliosphere, a vast bubble of charged particles and magnetic fields flowing from the Sun, and is now moving through the interstellar environment. Voyager 2, now 10.7 billion miles (17.2 billion km) away, is cruising through the heliosheath, a zone where the pressure of interstellar gas begins slowing the outflow of solar particles.

Voyager mission Project Scientist Ed Stone, a Caltech physicist, has overseen the mission since its inception and coordinated its 11 science teams. As the Voyagers neared their 40th anniversary in space, I caught up with Stone to recall what we’ve learned from history’s longest-ever space mission.

The umbrella-shaped plume of Io’s volcano Pele rises some 185 miles (300km) above the moon’s surface. Voyager 1 captured this massive outburst in March 1979. NASA/JPL/USGS

Reddy: Let’s look back to just before the Jupiter encounters in 1979. What do you see as the high points?

Stone: About Io, there was a prediction that there could be tidal heating and volcanic activity. But no one expected there would be 10 times more volcanic activity on this small moon than we have here on Earth. Before Voyager, the only known active volcanoes were here on Earth, and then suddenly we have a little moon which has 10 times more volcanic activity. It was the amount of volcanic activity, the kind of volcanic activity — plumes that are 200 kilometers high. So they’re not like the volcanic activity here on Earth and could be powered by sulfur and other things besides silicate. About a ton per second of that material is picked up by the jovian magnetic field as Jupiter rotates, and that mass of stuff inflates the Jupiter magnetosphere to about twice the size it should be because of centrifugal effects.

Of course, we planned to fly through the Io flux tube [a conduit through which an electrical current flows], but we didn’t. We flew beside it because the flux tube was deflected by the presence of all these heavy ions. The net result was that the flux tubes were kind of like wakes behind Io and we just flew beside it, measured the magnetic field and the currents flowing — millions of amps.

Reddy: About 20 years later we started to see there were flux tubes from the other moons as well. They were leaving glowing footprints in the infrared and ultraviolet in Jupiter’s atmosphere just like Io was.

Stone: That’s right. In fact, the Galileo mission later discovered that the moon Ganymede has a magnetic field itself and has a miniature magnetosphere. So it’s kind of like nested dolls. Here is the heliosphere, a super-big magnetic structure, then inside that is the magnetosphere of Jupiter, which is the largest [planetary] structure in the solar system, and then there’s the magnetosphere of Ganymede inside of that magnetosphere. It is really quite an interesting connection.

I think another thing that was really important was Europa, which we knew had water ice on the surface from ground-based data. But when Voyager flew by, you didn’t see any evidence of any mountains or any valleys — the smoothest surface in the solar system. This suggested that the same tidal heating that created all the volcanoes on Io was much reduced but still enough to melt the ice such that there is a liquid water ocean beneath this icy crust.

And that’s what Galileo found out. There was indeed a magnetic field induced in Europa by the rotation of Jupiter’s magnetic field. And it reverses polarity with the rotation of Jupiter, telling us that it’s not a permanent magnetic field of Europa but one generated by current flows that are induced by the motion of Jupiter’s magnetic field.

So the system is highly coupled and connected, where the magnetic fields and the particles are all interacting with the moons, and the fact that the moons there are inside the magnetosphere, unlike our Earth’s Moon, which is almost always outside the magnetosphere.

Reddy: It’s a fascinating system. The tidal heating is a result of the orbital relationship between Io and Europa?

Stone: That’s right. The resonance between the two, such that every other orbit Io is on the same side and being pulled differently by Europa. Io is like our Moon, keeping one side facing Jupiter, but it doesn’t exactly work because it’s not in a circular orbit. So that means there is a slight tidal effect warping the crusts of Io and Europa, providing the heat sources needed for what we saw.

Jupiter’s icy moon Europa boasts many fine cracks that hint at an ocean of liquid water beneath its smooth surface. This Galileo spacecraft image shows the moon in high resolution. NASA/JPL-Caltech/SETI Institute

Reddy: I recall at the time people were saying that Europa was as smooth as a cue ball scaled down to that size.

Stone: There are just some small ridges where the cracks form, and presumably those cracks are still forming today. So there may be some very interesting places on Europa where there’s reasonably recent fluids from below the icy crust near the surface. Hopefully the next mission to Europa will map out in detail where this fracturing may be taking place.

So the same tidal heating which produced the volcanoes on Io melted the water ice that was on Europa. And of course the surface is frozen because it’s so cold out there, but below the crust of water ice is an ocean.

We know it’s there because of the magnetic field that’s induced by Jupiter’s magnetic field. How do you produce a magnetic field? You need a current flowing, and the easiest way to have a current flowing is to have an ocean of water containing some ionized materials. Salts and things like that.

Reddy: By the end of the flybys, was Europa seen as very likely to have a liquid interior?

Stone: Yes, that was certainly the suggestion of the Voyager data. The calculations all suggested that [tidal heating] could be enough to keep liquid water beneath the icy crust, and that seems to be the case from the Galileo mission.

Reddy: As you mentioned, NASA is developing a new mission, Europa Clipper, that will highlight flybys of Europa.

Stone: Yes, that’s right. Close flybys of Europa, mapping in detail the magnetic field, the ion flows around the moon, as well as detailed studies of its topography. There are a number of measurements that can be made when you can get in there close. The main challenge, that intense radiation environment, means you have to zip in there and zip back out.

It’s sort of like what is done with Cassini, where the spacecraft has done now 40 or 50 flybys of Titan. It’s the same idea.

Voyager 2 captured intricate details in and around Jupiter’s Great Red Spot, a huge storm larger than Earth.

Reddy: So what other jovian surprises come to mind?

Stone: The first surprise had to do with the atmosphere. We knew about the Great Red Spot, but we found it was just the largest of literally dozens of hurricane- or cyclone-like storms. The Great Red Spot is larger than Earth, but others were something like a half or third as big. These are massive storms which are generated by the jet streams in the atmosphere, and they merge and new storms are created. It is a very dynamic atmosphere.

Reddy: This is a planet that is putting out more energy than it receives from the Sun by almost a factor of two.

Stone: That’s right, exactly. Almost all the giant planets do that, except Uranus. Why isn’t Uranus that way? That’s one of the key questions, and I don’t know that anybody has an answer at this point.

The other thing that was a bit of a surprise was Ganymede. It now looks like Ganymede has a magnetic field. Callisto and Ganymede are about the same size, but Callisto’s surface is heavily impacted, lots of craters. Ganymede is about the same size object, and yet we find fault systems. So it was clear Ganymede had an active geologic life which Callisto did not have. And that’s presumably related to the fact that Ganymede also has an ionized medium somewhere deep inside that generates a magnetic field.

Reddy: Cassini discoveries at Saturn dramatically confirmed that moons with internal oceans are not so unusual after all. The little moon Enceladus is actively spewing its ocean into space.

Stone: We knew there was something special about Enceladus because it reflects almost all the sunlight striking it. It had markings indicating it had an active geologic life. Cassini found out the reason it’s so white — it’s snowing all the time due to geysers erupting from its south polar region. Future missions will look at the water coming out of those geysers to see if there’s any evidence that microbial life could have evolved.

Reddy: I’ve often wondered whether the best strategy is to go to Europa and drill down or hang around Enceladus and collect what’s there being spewed out.

Stone: Well, there are certainly suggestions that that’s what should be done at Saturn, so I hope someday it will be done. It’s not so easy with a flyby to scoop up molecules without damaging them, but those are the kinds interesting challenges people need to be working on.

Thick haze layers sit above the nitrogen-rich atmosphere of Saturn’s largest moon, Titan. The smoggy conditions prevented the Voyagers from seeing details on the surface. NASA/JPL

Reddy: What struck you after the Saturn flyby as the big highlights there?

Stone: Titan was clearly a big highlight. There is no doubt about that. Before that, the only known nitrogen atmosphere was here on Earth. But there, methane, one of the trace gases, is being converted by radiation into all kinds of complex molecules which then rain down on the surface and behave like water does on Earth. And that’s exactly the prediction that came from the Voyager data, which revealed an atmosphere much denser than Earth’s.

Reddy: The Huygens probe landed on Titan in January 2005, revealing an incredible frozen landscape that resembled terrain on Earth. After the Voyager flyby, did you think Titan might possess this kind of geology?

Stone: Oh, I think so. I think there was a prediction, in fact, that methane would form clouds. The clouds would rain methane — liquid natural gas — which would form rivers and lakes and evaporate, and then the whole cycle continues. So that was, I think, one of the predictions from the Voyager data, although we couldn’t see any of it because the smog in the atmosphere blocked any visible images. The probe got below the haze layers. More globally, the Cassini radar system has revealed that there indeed are lakes as large as the Great Lakes here in the United States on the polar regions of Titan. It really is a remarkable place. Given the fact the chemistry occurring there today in some ways resembles how it might’ve been on Earth before life evolved, it clearly is a place that we want to get back to.

Reddy: Is it a fair statement to say the closest alien planets are the moons of Saturn and Jupiter?

Stone: Well, they are certainly examples of what other planets could look like. They’re quite distinct and I think that the one thing we have learned is nature is remarkably diverse, and you don’t see replicas. Each body seems to have its own life history written on the surface and in its interior.

Saturn’s gossamer C ring appears blue in this false-color Voyager image, while the brighter B ring glows with a yellow hue. The Voyagers revealed that multiple ringlets make up the broader rings seen from Earth. NASA/JPL

Reddy: Voyager discovered the incredible complexity of Saturn’s rings. And with Cassini, they have become even more complex.

Stone: The Voyagers revealed the control that moons have on the rings. There does seem to be a very dynamic system of waves induced by moons inside the rings themselves, in gaps in the rings or even moons orbiting outside. Of course, we found the shepherding moons for the F ring that the Pioneer missions had discovered. It had been predicted there had to be two such moons shepherding material between them to keep the F ring from diffusing away. Of course, there is another set of moons [Epimetheus and Janus], which share the same orbit, as I recall, and yet do not collide — a very delicate gravitational dance. It’s really quite remarkable.

Reddy: So, it’s late 1980, and Voyager 1 has sailed past Saturn, its final planetary encounter, and Voyager 2 is on approach. When did you have approval to go on to Uranus with Voyager 2?

Stone: We got approval for that just weeks after our successful Voyager 1 encounter.

The story goes like this. When we launched the mission, we defined success as at least one spacecraft successfully encountering Saturn, Saturn’s rings, and the moon Titan. Those are the primary targets, and because the equatorial plane of the planet was inclined a fairly good degree at the time we flew by, it meant that Voyager 1, which had the flyby with Titan in the ring plane and flew behind the rings, ended up going upward and out of the ecliptic plane, with no planetary encounters possible after. Having succeeded with Voyager 1 in November 1980, we were given permission to leave Voyager 2’s flyby in the ecliptic plane as it flew by Saturn, therefore not flying close to Titan or flying nicely behind the rings for radio occultation.

 If Voyager 1 had failed, Voyager 2 would’ve gone exactly the same way Voyager 1 did. Fortunately Voyager 1 worked, so that meant we could leave Voyager 2 in the ecliptic plane heading toward Uranus.

Reddy: Voyager 2 reached Uranus in 1986. What were the highlights of that encounter?

Stone: One big surprise was a magnetic field. Earth’s magnetic field has its pole near Earth’s rotational pole, and that’s great for compasses. Saturn’s the same way. Jupiter’s the same way. The small magnetic field on Mercury is the same way. So there was a model that said a magnetic field comes from the rotation of the ionized material in the planet. This was reasonable.

Yet when we flew by Uranus, we found a magnetic pole down closer to the equator than to the rotation pole. The same thing turned out to be true at Neptune as well. So the global current system generating such an offset tilted field was quite distinctly different from what we had believed to be the case for planets in general. We found we had to rethink the geometry of these fluid flows inside planets because we had evidence that they are not always nicely aligned with the rotation of the planet itself.

Reddy: The other big surprise: Miranda?

Uranus’ small moon Miranda appears as a hodgepodge of different parts stitched together in this Voyager 2 image. NASA/JPL/USGS

Stone: Little Miranda, a 500-km moon. Very tiny world. It had some highly cratered areas, but it had tremendous landforms, some scarps which were kilometers high. Really quite remarkable. So this is a moon that had a very dynamic life at one point. There is no evidence as far as I can recall that it is currently active. So this is again probably due to tidal heating as the moon ended up in its final circular orbit.

Reddy: I was fortunate enough to be present at JPL during the Neptune flyby. Each flyby was a very exciting and intense time, but do the Voyager missions represent an end of an era in space exploration?

Stone: Yes. Now missions will go back to go into orbit, and it’s not like there’s a particular time period like there was during encounters, where we seriously started looking at the planet a few months out and already could make discoveries. But it was really just an increasing rate of discovery, which peaked over the five to 10 days around the closest approach to each planet. Quite intense. Just every day seeing things no one had seen before. Seeing things that you initially couldn’t understand, why they looked the way they did, and the magnetic fields being different than you expected. It was just one surprise after the other every day for a period of a week or two. Yes, it was very special, and it had to do with the fact these were flybys — first-time looks at a whole suite of bodies which turned out to be remarkably diverse.

Reddy: The Neptune flyby occurred in 1989. Your highlights?

Voyager 2 discovered an Earth-sized Great Dark Spot in Neptune’s massive atmosphere. Although the feature was the largest the spacecraft saw, it has since disappeared. NASA/JPL

Stone: We knew we would be surprised, we just didn’t know how.

Neptune had a Great Dark Spot nobody expected, which has since disappeared. And as I said, the magnetic field is also tipped.

Neptune gave us another big surprise. That was the moon Triton, which is about the same size as Pluto. It probably started out like a Pluto but it was captured by Neptune, presumably into an elliptical orbit. And then as that orbit circularized, all of that orbital energy ended up melting this moon, which has a lot of water ice. We saw a surface unlike any we had seen. Even at very cold temperatures, we saw active geysers erupting from that polar cap of frozen nitrogen.

Of course, now the New Horizons mission has seen even Pluto, while very cold, is not frozen solid. They really do have a geologic life because they have more than just water ice. Nitrogen ice or methane ice or other substances we think of as gases can be solids, and they have these changes of state at much lower temperatures, so you can have geologic activity even it is very, very cold.

Neptune’s largest moon, Triton, shows a unique “cantaloupe terrain” in the top half of this image as well as dark streaks associated with geysers in the bottom half. NASA/JPL/USGS

Reddy: I recall noting dark streaks that looked as if some kind of wind was blowing dark particles in a specific direction. And you caught a geyser in the act.

Stone: Yes. We actually saw two in the act. Two geysers erupting, and they sent their material up to higher altitude, where the wind at that altitude would carry it away. The particles gradually precipitate to the surface and make these dark streaks.

Reddy: When do you expect we’ll receive the last data from the Voyagers?

Stone: Around 2030. We have to start turning off power from various heaters and then instruments, so the last instrument will have to be turned off around 2030.

Reddy: Which would that be?

Stone: Well, it will depend on what’s still working. [Laughs] But if everything’s working, it will be the one which requires the least amount of power. That’s really the magnetometer or the plasma wave instrument, both of which use very low power.

Voyager 1 currently lies some 13 billion miles (21 billion km) from the Sun and is outside the heliosphere where the solar wind dominates. Voyager 2 is following close on its heels. Astronomy: Roen Kelly

Reddy: Where are the spacecraft now? And what are they seeing?

Stone: Voyager 1 has been, since August 2012, in the local interstellar medium, looking at the region where the interstellar wind is flowing around the outside of the heliosphere and carrying along with it the galactic magnetic field, which is wrapped around the outside of the heliospheric bubble the Sun makes around itself.

Reddy: The sign of that passage was due to some solar activity, I recall.

Stone: The solar activity was actually in March 2012. It got to Voyager 1 about a year later. That’s when we had the first measure of what the density of the plasma was. The plasma inside the heliosphere is hot and less dense, and outside it’s about a hundred times denser but much colder. That’s where the pressure balance comes from. Our other instruments, the ones measuring energetic particles, indicated we left the heliosphere in August 2012, but we had not been able to measure the plasma density on Voyager 1 because the plasma instrument ceased working back in about 1980.

Reddy: Can you describe the signal that provided the information Voyager 1 has actually made the crossing?

Stone: It’s a plasma wave instrument. It was not sensing any signals at the time we crossed. The signals at the time we crossed were from the two energetic particle instruments, which revealed that the low-energy particles accelerated in the region just inside the heliopause, in the termination shock and heliosheath — those particles disappeared on August 25, 2012, and haven’t come back. And at the same time, the galactic cosmic-ray intensity jumped up to its highest value and has stayed at that value. Those were signatures we obviously left behind what was inside the heliosphere and were seeing particles on the outside.

But we didn’t get the plasma measurement until that solar event finally reached Voyager 1 and caused the plasma there to oscillate. The frequency of the oscillation depends upon the density; from that, one can detect what the density was, and it was a higher density than we had seen inside. So we knew that we were indeed outside the solar bubble. Inside the bubble, the wind comes from the Sun and the magnetic field is carried out by that wind. Outside the bubble, the material is coming from a supernova that exploded [several] million years ago, and that plasma is carrying the galactic magnetic field and wrapping it around the outside of the heliosphere.

Wisps of interstellar gas appear behind Voyager 1 in this artist’s impression that shows the spacecraft’s observations of space beyond the Sun’s magnetic influence. NASA/JPL-Caltech

Reddy: What do we hope to learn about this cold dense plasma and the local interstellar medium?

Stone: We hope to continue to see how the density changes as we move farther away because there is a kind of a bow wave, if you like, in the interstellar medium created by the flow. The ions that are in the interstellar wind cannot directly enter the heliosphere because of the magnetic field, so they flow around the outside of the heliosphere. We don’t know whether there was a shock upstream or if it is just a bow wave without a shock. In any case, we expect the density in the field direction we’re seeing now is being distorted by this bow wave of the interstellar material sort of piling up just ahead of the heliosphere.

Reddy: When I think about how long these spacecraft have lasted, what they’ve accomplished and far they’ve gone, I am simply amazed. Looking back on it all, what impresses you the most?

Stone: It’s been a remarkable journey. We keep discovering new things, discovering things nobody knew we were going to discover. And I think from that standpoint it’s really been a wonderful journey as a scientist to fly by the planets and now have a spacecraft in interstellar space and one still inside. Hopefully, Voyager 2 will be leaving the heliosphere in the next few years. It’s in a different place and is seeing different things inside so we would expect there may be some differences outside as well.

Reddy: T. S. Eliot wrote that “the end of all our exploring will be to arrive where we started and know the place for the first time.” Do you think this applies to the Voyager mission in particular and to planetary exploration as a whole?

Stone: I think that’s really what this is all about. It helps us understand what’s out there and that, in turn, helps us better understand what’s here. Because the Earth has its own particular special history, but its processes are similar to the processes that shape all the other bodies in the solar system.

I think the general interest in this is shown by the fact that recently there was a big press release about TRAPPIST-1, a star which has seven Earth-like planets. And it was worldwide news because people really want to understand our place, and the one key part of that is actually understanding what’s out there and what it looks like.

Francis Reddy is the senior science writer for the Astrophysics Science Division at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.