There were two spacecraft, Voyager 1 and Voyager 2, launched about a month apart in 1977. They visited Jupiter, Saturn, Uranus and Neptune. For Jupiter, both encounters were in 1979. The spacecraft were spaced out by that time, so there was a Saturn encounter in 1980 for Voyager 1 and 1981 for Voyager 2.
At the Saturn encounter, Voyager 1 was targeted to go very close to Titan. The close approach to Titan required a trajectory that took it out of the ecliptic plane, so it wasn’t possible to retarget that for any more planets. But Voyager 2 didn’t have that restriction, based on its trajectory. So it was possible to keep it in the ecliptic plane after the Saturn encounter, which was the end of the nominal Voyager mission, and it continued on to Uranus in 1986 and Neptune in 1989.
With Voyager we made several discoveries along the way. We discovered the Io plasma torus, which occupied our time as we came in to Jupiter. We had a slit instrument with 128 detectors that were scanning across Jupiter, so if there was anything going on we might pick it up. What we found was the torus, and the torus is out at about five Jupiter radii. The source of the torus is Io, the satellite.
As we came in we saw this ultraviolet emission around the planet. If we hadn’t had an ultraviolet experiment we wouldn’t have a clue and wouldn’t know it was there today. So that was a big boon for our program.
We were looking across and we saw this thing and we modeled it as we approached. Now the particles and fields instruments are in situ instruments; you have to pass through the atmosphere in order to make the measurements. We were lucky on Voyager, saying, “Hey, you guys, there’s some high energy particles in orbit around Jupiter.” We went on telling them this as we came in, and finally I guess we had them convinced. We were flying close to Io, and a couple of the instruments were saturated and we had to turn down the gain, based on our measurements, because we told them what the density was and all this.
I, as the PI, had a dream one night. I told those guys that Io was volcanic. Unfortunately, we decided not to bring this up, because we thought it would be more trouble. There were a couple of theoreticians, and I think the problem was they couldn’t see how to get material off the satellite and into the atmosphere.
After we got by and I actually had come home, and was settling down here, I got a call from one of our team members at JPL. He said, “Guess what? Imaging has seen plumes, volcanic plumes, on Io.”
It took five or six days to fly by Jupiter because it’s such a big planet, so there was stuff going on every day. Every day there was a press conference, and each of the experimenters would present whatever they had seen the previous day. It was a lot of work. No sooner did we get through a press conference than we started having to figure out what to say the next day. Then we sent that off to the drafters and all that to get the artwork done. Yeah, it was a busy time.
There were two Voyager Ultraviolet Spectrometers, one on each spacecraft. By the time Lyle Broadfoot’s group joined LPL, the Voyager had passed Jupiter and Saturn, and we were headed for Uranus in 1985 and Neptune in 1989. It was a very, very busy time.
My involvement with the instrument was that I was the person who did the scheduling of the observations that were made, and was responsible for helping with people at the Jet Propulsion Lab to design the observations, making sure they went smoothly at the planets.
One of the principal things I did was I was responsible for using the instruments during the cruise, between Uranus and Neptune and so forth. In that respect I found that the instruments were extremely useful for ultraviolet astronomy. Since that was kind of my background, I used those instruments to make lots of observations of stars and the interstellar medium and so forth.
The instruments were designed and used for observations of the planets, but those observations occurred during a very brief period of time during the encounters, and you had all of this time in between. Those instruments were extremely valuable in helping to understand a part of the spectrum, the extreme ultraviolet and the far ultraviolet, that wasn’t being addressed by NASA at that time.
One of the biggest highs was when we were approaching Saturn, and they were taking all these pictures of the rings of Saturn and so forth. You could see all this structure in the rings. There were these papers that predicted the structure, or portions of the structure, has to do with the orbital resonances of the moons.
I was intrigued by this because I sat in a meeting and listened to these people talk about this. But no one really knew the scale of these pictures, so you couldn’t say, “Ah, that resonance there is due to that moon over there.” I knew nothing about planetary rings, but I had worked to get our instrument to watch a star go behind Saturn, and you could actually see the star through the rings. You see the light drop out and come back and drop out and come back. It’s called occultation.
That observation was the primary observation of another instrument on the spacecraft, but I realized that we could use our instrument just as well. So I got the observation designed so we were included in the observation. I got the data back and I was very intrigued by it—all this structure—so I just sat down with a pencil and piece of paper, and I knew what the trajectory was, and I worked out where everything should be. All of the sudden it all fell into place, because there were predictions of where these things should be. You could see just about every prediction lined up with one of these features.