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OFJ Field Journal from Steven Tyler - 12/11/95


It's Tuesday, December 12, and I'm driving to work. Last week sure was a good week! Probe relay worked. We already have some Probe data on the ground, and in about one more day, we will have all the Probe "symbol" data (much of our most important scientific data) returned. The Orbiter is in orbit around Jupiter.

I'm reflecting a little on how well everything went on Arrival Day. Everyone on the flight team spent a lot of time finding out everything there was to know about their part of the spacecraft. They gathered data, and rechecked calculations. But finally, everyone reached a point where they had to "make a call" about how thought things were really going to work out. There's always a little data missing, so nobody can ever be absolutely sure. There may be some uncertainty about your sensors and you need to make an informed guess. If you've done your job right, you have a very good idea about how things work, and the guesses will be the right decisions, and everything will work properly.

The AACS Team (Attitude and Articulation Control) sure came through. Among other things, they calibrated the accelerometers (devices that measure how much the spacecraft has accelerated, which tells us how much its velocity has changed) that stopped the Jupiter Orbit Insertion (JOI) burn at *just* the right change in velocity. It wasn't easy for them. For a variety of reasons, they couldn't get enough data about the accelerometer on board the spacecraft to do as good a job of calibrating it as they had hoped. In the end, they had to use data from an accelerometer here on the ground. This is like performing a tune-up on your car by measuring the timing on another car of the same model and year--they're similar, but not necessarily identical!

But as the big day approached, the AACS people were confident that they had made the right call. And they had! That burn was stopped within a small fraction of a per cent of the right velocity change. That was the call that nailed JOI for us.

The engine team (RPM, for Retro Propulsion Module) came through, too. In July, when we did our first main engine burns, we knew that there wouldn't be much time to figure out exactly how well the engine would do at JOI, based on that first main engine burn. And it wasn't an easy task! We hadn't expected to see differences between the fuel and oxidizer tank pressures (unlike on Earth, where you can just use the oxygen in the air to burn fuel, Galileo has to carry its own oxidizer along, a really nasty corrosive chemical called "dinitrogen tetraoxide"). We were puzzled by the fact that two sensors that should have been reading the same numbers were in fact different. But the RPM Team came through. They made a call on expected engine performance, and they were off by well under one per cent. As the big day approached, I had noticed that they seemed pretty confident about their engine...

...for our next burn, we will have a *very* good idea about engine performance.

And the navigators! Deprived of the pictures they expected to receive to help them navigate, they couldn't be quite so sure about where we were headed. Still, they made a call about a week before JOI: 937 kilometers from the surface of Io at closest approach (which was within 50 kilometers of being right, a very good call). That was our last chance to change the Io altitude. Three days later, they had guessed the final altitude to within a few kilometers of what it turned out to be. Their skill enabled us to skip all the scheduled maneuvers in the final weeks before JOI as well as the two cleanup maneuvers afterwards. Actually, we would have done one of the maneuvers, but the NAV team pointed out that we could avoid even that by arriving at Ganymede a week early (imagine that, Galileo getting anywhere early after so many years of delays).

Our final maneuver before JOI turned out to be a trajectory change maneuver done at the end of August! That's like a hole-in-one on a par 4! For a second, I try to remember...what if we had skipped that August maneuver...?

No. If we had skipped the August maneuver as well, we would have crashed into Io. That was a course correction we *had* to do. (There would have been an "impact" on the mission had we omitted it...). And had we also omitted the maneuver before that, ODM, we would have followed the Probe into Jupiter.

Yes, these people did a great job. A thought strikes me. What did I do to help? I helped create the Helium Loss Fault Protection algorithm, which guarded against leaks that might keep us from being able to use the engines (see my last journal). But (luckily) we haven't needed to use it. I've decided that if anyone asks me, I'll boast about the time I spent a year looking at the Command and Data Subsystem hardware (the spacecraft's main computer) to make sure that if there were a single failure, at least one of the two computer strings would survive. There were about 1000 chips on a total of 28 boards, so I had to trace the connections among more than 14,000 pins....

Well, I'm at work now. I'm ready to wake up from my reflections and face whatever is happening today.



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