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For the Pioneer encounters, these fly-bys of Jupiter, we had a series of maybe 20 thousand commands that we had to write and figure out where to point the telescope and how to make the pictures. The spacecraft was a spinning spacecraft, so there was a little one-inch telescope where the angle from the axis of the spacecraft could be varied—could be stepped—in half-millimeter steps. As the spacecraft rotated, it would sweep out one line of photometry across Jupiter, and then you would step the telescope half a millimeter, and then scan out a second line and scan out a third line, and this way build up an image over a period of an hour or two, with scan lines one after another.
Not only that, but the round-trip light time to Jupiter is like an hour in a half, so you have to transmit the command 45 minutes before you want it to take effect, and then 45 minutes later you can check off that it actually got there and took effect and the telescope is doing what you told it. The other thing about it was that Jupiter has powerful radiation belts, trapped energetic particles, and these would whack the instrument. Commanding the instrument was kind of like telling an old, senile guy to go to the store and get a quart of milk. You tell him, “Go to the store and get a quart of milk,” and he starts marching off, moving in the direction you want. He knows he’s going to the store and he’s getting his quart of milk. About halfway through the images: “Where am I going? What do I want to do?” It would reset the registers when the energetic particles hit the instrument, so the instrument would start moving the way you wanted, and then halfway through, it would just forget its mind. It would just go off doing something else completely different.
We had a crew of guys around the clock, three shifts of guys, checking off this list 20 thousand commands as each one got transmitted, and the other guy with a downlink book, checking off that, yeah, the spacecraft did that, it took effect. Then finding: No, that command didn’t take effect, and in fact, that instrument is off doing something else. But that’s what it was doing 45 minutes ago, and in the meantime you sent an hour and a half of commands to the instrument to do something different, so it’s now going to be starting at the wrong place, and those are going to be bad. So now you have to figure out: What commands do we have to insert in the sequence to get back on the track and put the instrument back in and reconfigure it to do what we want it to do?
It was actually a major effort. One of my first jobs here was to figure out how to generate this command sequence, and then train the guys who were going to monitor the uplink and the downlink, and make sure we actually got some pictures from Jupiter in the process. It was really a heck of a challenge, and a very different flavor than today. Today you write a command sequence and it gets loaded into the computer and it goes zip, and it’s up there, and inserted into the machine, and it does what it’s supposed to do. It was quite a different flavor.
The first flyby of Jupiter had an imaging device on it. The instrument did spin-scan imaging. But of course all the while that it’s building up an image, it’s getting closer and closer to Jupiter. So the image is not like a snapshot that you take, but it’s accumulated over an hour and a half, and it’s got ferocious geometrical distortion.
To make it look like a picture, there was a whole other set of processing that had to be done. There was no Internet, so this data was collected at Ames Research Center, and put on tapes. The tapes were flown back to the University of Arizona, and they went through some complicated geometrical rectification step, then flown back to California, and then they could be displayed and people could see the pictures.
They were beautiful, and some of them were really quite spectacular. But you have to understand that Jupiter is a fairly bright object. The limitation of the kind of imaging you get with a ground-based telescope is the scintillation of the atmosphere. The atmosphere has turbulence and refracts the image, and doesn’t make a very sharp image. But if you take thousands of images, you can find instances when the atmosphere is fairly still, and you can get occasionally very good images of a bright planet like Jupiter from a ground-based telescope.
The images we got were better than the best ground-based images, but not by such an enormous knock-your-socks off factor. The main advantage was that we flew around Jupiter, and Jupiter’s far away from the Sun compared to the Earth, so we only see Jupiter essentially illuminated face-on. But the spacecraft flew around and could look at Jupiter as a crescent, at ninety degrees phase, and we could observe Jupiter in all these different new geometries that we’d never seen before, and we could learn something about the nature of the cloud particles by flying around and looking at it from other directions.
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