Space Topics: Voyager
The Stories Behind the Mission: Ed Stone
As Told to A. J. S. Rayl in 2002
on the occasion of Voyager's 25th anniversary
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."
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