The Stories Behind the Voyager Mission: Ed Stone

Edward C. Stone, an internationally renowned physicist, signed on as Project Scientist of the Voyager mission in 1972, responsible for coordinating the efforts of 11 teams of researchers.

After receiving his Master of Science degree and Ph.D. in physics from the University of Chicago, he joined the California Institute of Technology as a research fellow in physics in 1964, then went on to become a professor of physics. In addition, Stone served as director of the Jet Propulsion Laboratory (JPL) from January 1991 to April 2001.

Today, Stone continues in his roles as Voyager Project Scientist, and as professor and researcher at Caltech.

"Voyager is the journey of a lifetime. It has been an incredible flight of discovery. I can't imagine a mission with more discoveries than Voyager had -- it saw more new worlds for the first time than any mission and everything was different. And, it was happening in real-time.

My role as Project Scientist began in 1972 when Bud Schurmeier, who was the project manager, asked me to take the position. I was already part of a team that was hoping to get on the mission, so I was very interested in the science. We had a proposal that had been selected and today I'm the P.I. for that cosmic ray instrument. The only question about the position was exactly what I would be doing, because being a project scientist meant a considerable, required load of administrative paper work. I had to ask was whether or not it would be the best use of my time and interests.

While I was thrilled about the science, I was then (and am now) a professor at Caltech, who had my own research. I wanted to focus on the science and engineering issues. Bud understood that and worked with me to come up with a plan that would allow me to devote about 30% of my time in the role. Since no one had ever tried to do this before, this was an experiment. Up until then, project scientists had been fulltime and had done all of the administration, as well as the science.

We worked out an arrangement that brought in a fulltime science manager who worked with me - James Long initially, and then Charles Stembridge and Pete de Vries -- plus a group of experiment representatives. Eventually, there was also an assistant project scientist for Jupiter, Lonne Lane, and an assistant project scientist for the rest of the planets, Ellis Miner. The time I actually spent on the mission of course varied over the years - clearly during encounters it was 100%, but on the average it did command about 30% of my time over the years as planned. Ultimately, the key factors in my decision to accept the position were the mission itself, my desire to learn things, the support of both JPL and Caltech, and Bud Schurmeier.

In the end, when I look back on it say, 'Why did I decide to do it?' - well, Bud provided me with an opportunity to create a new kind of role. The creation of the position of a chief scientist who did the kinds of things I would do was not only a pioneering effort, it was a wonderful opportunity. You see, project scientists (or chief scientists) can have influence only to the extent that the project manager wants it and supports it, and I realized that Bud was going to be very supportive and that I was going to learn a lot from him and from the Voyager flight team. It was exactly the right decision. As I think back on it now, I cannot imagine it any other way. It has been a tremendous learning experience.

Big, unexpected surprises

There are so many discoveries that are highpoints and memorable moments, I'm not sure where to begin. I have 40 volumes (notebooks) of notes taken through all the meetings -- of all the things we thought we knew, what we didn't know, what we thought we knew that wasn't that way.

The surprises were not the things that we expected to see - those are nice of course and important -- but the most exciting things were the discoveries that were totally unexpected. Those were the biggest surprises. From a science point of view, when you see something that you had not expected at all - and which is not immediately obvious - like the volcanoes on Io or the fact that there are cracks on the ice crust on Europa, or the kinked F-ring at Saturn and the list goes on -- that's when you have the opportunity to learn something new.

My first space mission was in 1961 and I had flown instruments on missions in 1965, '67, '69, '72, and '73 - so I had lots of instruments in space, but they had all been on Earth-orbiters. The data would come in on a regular basis, always in a slow, steady stream, and if you made, say, one or two discoveries a year, you felt you had a wonderful program. Then - Voyager.

As we were cruising to Jupiter, we did have some of that same characteristic -- a few discoveries per year on the way there for about a year and a half. So basically not much happened scientifically from 1972 until 1979 - seven years. But with the encounter at Jupiter on March 5, 1979, the floodgates opened. I mean it just really was a flood. The data was coming in huge overwhelming deluges, faster than we could comprehend what we were seeing.

Each day, we would spend a few minutes looking at something and say - 'Now can I understand this' -- or not - and if not, we quickly moved on, because we knew the next day we were going to have even better data. It just got better everyday. It was a totally different kind of scientific experience than one normally has with the traditional sort of steady state, even science.

In 1980, another flood of data came from Saturn, and another one in 1981 at Saturn, and then five years later another flood at Uranus. There were so many more discoveries than any of us anticipated, because the solar system turned out to be much more diverse than any of us previously thought. To give you an idea -- when this mission was first conceived back in the early 1970s, the best ideas of what the moons of the outer planets would look like were that they would be heavily cratered, ancient objects, much like our own Moon. When we saw them, they were each different. They each have a geologic life and very few are ancient cratered objects. The discoveries were just so much more than we could have imagined, because there's so much more diversity. The moons are diverse . . . every single ring system is different . . . the weather systems were not what we expected . . . the magnetic fields - a few planets we found have their magnetic poles down near the equator. Nobody even imagined that.

Volcanic Io

Just five days after the initial encounter at Jupiter we had the fly-by of Io and the images changed our thinking about this Galilean satellite. The Io story is interesting, because it shows how narrow our mindset was. About one month before Stan Peale, who with Pat Cassen and R.T. Reynolds, proposed that heating from tidal flexing could melt the interior of a planet. He called to suggest that we might see a body that shows some evidence of this tidal heating effect. I sat down with Rudy Hanel, who was the P.I. for the infrared instrument, and said - 'Let's see if we can measure the increase of temperature of Io because of this extra heat.'

Well, while Peale and colleagues had calculated how much this tidal heating was going to provide, Rudy and I did our calculations for Io before the encounter and the numbers came out to be that the {tidal heating} would provide additional heat of five microwatts per square centimeter. Now the question was - 'How much heat does the Sunlight provide?' It turns out that it provides 1000 microwatts per square centimeter. So we said - 'There's no way we can measure that, it's just too small.'

Then came the first images of Io. We looked at this thing and we saw all these black splotches and people were saying - what is this? It doesn't look like anything we've seen before. Rudy came in with his spectra a couple of days before our closest approach and told the entire science group, some 100 people packed into one room -- that the temperature ought to be independent of the wavelength at which we measure it in the infrared. But he said: 'We're not seeing that. We're seeing this very wave-length dependent temperature.'

He offered three explanations. The first was that an unusual material or mineral on the surface had an absorption band that causes the apparent change of temperature over this particular wavelength range. He'd never seen anything like that before and it was hard to imagine what that material might be, so that didn't seem like a good explanation. The second suggestion was that there was a calibration problem with the instrument. But every other object he'd measured had rendered the correct temperatures at all these wavelengths, so it was hard to view it as a calibration problem. The third possibility was that there are different temperatures on the surface of Io. That didn't seem too reasonable either, because it's supposed to be quite cold. While we had known about the tidal heating and we knew about the extra energy and we saw all these black splotches -- nobody jumped up and said - 'Hot spots!'

A few days later, the infrared instrument got close enough to Io that they could look at one of these black lava lakes. It was room temperature, while the surface itself was about 125 degrees above absolute zero. The answer emerged -- all the heat is not coming out uniformly all over Io - it's coming out in these little black spots. And, if the heat is coming out in little tiny areas, it has to be much hotter. Ironically, this had been detected from Earth for years, but it was just so hard to break out of the mindset that this moon could not have volcanic activity. We just could not take the leap. Then - the plume, which Linda Morabito {now Kelly} first saw. Bingo!

Suddenly, it all clicked together -- we were seeing a world that had 100 times the amount of volcanic activity than Earth. We found ourselves saying: 'Okay, this is really different, and not just a little different - this is really different. Today of course we know that all these black spots are volcanic calderas - all 120 of them.

Europa: Making decisions and the heart of science

One of the key decisions that I had to make early on was in the selection of the trajectory of Voyager 2. Pioneer 10 told us the radiation environment at Jupiter was 1000 times worse than we expected, so we had to go back and redesign all the circuitry, and put shielding in and so on. That also meant we wanted to make sure that at least one of the spacecraft stayed far enough away from Jupiter to minimize risk. Remember, we had to get to Saturn - that was mission success. We wanted Voyager 1 to go by Io, because we knew Io had a large interaction with Jupiter's magnetic field. Io is at 6 Jupiter radii out, where the radiation environments are intense.

The trade-off was to take some risk with Voyager 1, because Voyager 2 which we would keep further out could always finish at Saturn if anything happened with Voyager 1. So we had to decide what to do with Voyager 2 at Jupiter -- do we go by Europa and look at it up close, or do we go by Ganymede again but go by behind it so we could look for a tenuous atmosphere by watching the sunset in the atmosphere, the solar occultation? There was some suggestion at the time from ground-based data that Ganymede might actually have an atmosphere.

It was a trade-off of one moon versus another, an atmosphere versus a surface. We would make discoveries either way. It was not only a question of what, but who. The imaging team would primarily make discoveries about Europa, and the ultraviolet team would primarily make discoveries about the atmosphere of Ganymede, so the outcome of the decision wouldn't involve the same people.

Such decisions -- determining what gets discovered and who gets to discover it - are at the heart of science. This is right at the core of what scientists do. So this was a key decision. I decided that we should go for Europa to complete the quartet of the four Galilean satellites. I had not really appreciated at the time how really interesting Europa would be, because this decision-making was all done in the early 1970s, long before launch. We knew Europa had ice on its surface, but we didn't have any idea about tidal heating at that time.

The Galilean moons and Jupiter form a kind of miniature solar system and the idea was that we could view the fourth one and therefore accept the fact that we could not look for the atmosphere on Ganymede. It was clearly, in retrospect, the right decision. But that's an example of the decisions I had to make without knowing until after the fact that we'd done the right thing.

As I moved into this activity, I recognized that we needed to have process so that everybody could accept whatever decision was made, even if it was not the one they would have made. I didn't feel that voting on things was the way. Somebody had to understand all the issues and that somebody was the chief scientist. I had to learn a lot in order to be able to make judgments ahead of time as to what things we would do -- we couldn't do it all. Fortunately, there were plenty of discoveries for everybody involved and that made it a little bit easier for people to accept not getting to do everything.

Amazing discoveries

Voyager started out as a four-year mission to Jupiter and Saturn, then turned into a 12-year mission to Uranus and Neptune, and with every new encounter there were startling surprises.

The kinked F-ring at Saturn was really a puzzle. Pioneer 11 had discovered the narrow F-ring and we also knew from recent theories that there needed to be some shepherding satellites. We found those and that was important -- physics said they had to be there and they were. But I don't think anybody realized that the rings would be kinked - I mean, planetary orbits are ellipses - that's what everybody knows - and yet here is this kinked, multi-stranded ring. This was a huge puzzle. Now there are models that indicate how this can happen from the shepherding effects, but that was a big surprise.

Then, there are the spokes on Saturn's rings. We had always thought rings were made up of particles that orbit in a plane, but here are these features that are radial, that come and go. We still don't understand, by the way, what they are. That's a real opportunity for Cassini, which will observe them for at least four years.

At Uranus, the magnetic field was mind-boggling - the fact that it was so tilted. Miranda was another major surprise. This little world that is 500 kilometers across, one-tenth the size of the Galilean satellites, and yet has this very complex surface - how did that happen? It's a world that should have formed, rapidly cooled off and froze, but that's not what it did. Maybe it was broken up and reformed . . . maybe it never quite melted the last time it formed to reach a new equilibrium . . . there are answers still to be found from the very complex surface of this little world.

The last object we visited was Triton. We knew it was going to be interesting, because models had suggested it's a captured object, a twin of Pluto in many ways. But we had no idea how interesting. Being captured into a retrograde orbit it was going to have a lot of tidal flexing as it circularized its orbit. And it's like our Moon today - with one side always facing Neptune, so it no longer has all that tidal heating, but while circularizing its orbit it was being violently heated. Sure enough, that surface was different than anything else we had seen before. We saw icy volcanic calderas, which are basically not rock but ice that is as hard as rock when it's only 38 degrees above absolute zero - and polar caps that are made-up of frozen nitrogen, not water. Even now - even at 38 degrees above absolute zero -- there are geysers erupting from the polar cap. So even that last object we saw surprised us.

The story is still a story

The fact that Voyager was a hit with the media and the public is important to note too. We had a story that built upon itself over a decade and longer and since this was a real-time mission we had the dedicated interest of the reporters. It took planetary exploration back into the mainstream.

Now, 25 years after the launches, we still have enough electrical power for another 20 years or so. I'm sure as we head off to interstellar space - when we find the termination shock and the heliopause, the outer boundary of the solar system, Voyager will reap even more surprises.

Looking back on this voyage now, if there is one thing we have learned along the way it is that nature is much more inventive than our imagination -- and the journey of a lifetime is still not over."