PART 1: Galileo fact of the day
PART 2: A lot of good calls contributed to success
PART 3: Explaining physics and unseen engineering work

Most of Galileo's actions are controlled by commands that have
been stored on board the spacecraft in advance, but there are still
many occasions when the spacecraft's commands have to be sent up
in "real time." How many times have we had to do this? As of
December 14, 1995, a total of 275,586 real-time commands have
been transmitted to Galileo since launch. In the past week alone,
169 real-time commands were transmitted. 

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.

December 13th, 1995
It's already been a week since the Galileo Orbiter and Probe 
performed their flawless activities at Jupiter. What an exciting and 
historical day. But most of my excitement has subsided.

During Arrival day, I was helping JPL's Public Information Office to 
provide info to the press and public about the technical aspects of 
what was happening. Most of the day I hung out in the lab's von 
Karman auditorium, the headquarters for the press. Many reporters 
simply wanted to know "what exactly is happening?". I was living in 
a teacher's paradise! (I have some teaching and speaking experience 
and I love doing those things, as well as engineering).

I don't know where the day went, but I remember that many people 
didn't quite understand the exact physics of what was about to 
happen, and what had already taken place. Some thought that the 
Probe was released from the Orbiter on December 7th, the same day 
that it would enter Jupiter's atmosphere (the Probe was actually 
released last July). Some didn't understand the difference between 
the Probe and the Orbiter (the Probe goes into the atmosphere, the 
Orbiter will orbit around Jupiter). Some didn't know that this is just 
the beginning of a two year tour for the Orbiter in orbit around the 
great planet, Jupiter! Some just needed to be educated. And 
educating is my passion!

Some things can be difficult to understand for people with no 
science background, too. For example, the effects on objects that are 
not subject to friction (a condition that exists in space) but only 
move when subject to external forces: gravity, thrusters, solar 
wind, etc., must be accounted for. And this can only be handled 
thoroughly with the appropriate mathematics of physical laws and a 
good computer. Many of the questions I received that day were 
explained with just a little knowledge of this physics.

One reporter asked me, "What (really) just happened?", referring to 
the event that marked the first communication from the Probe to the 
Orbiter after the Probe entered Jupiter's atmosphere. Each of the 
JPL/Galileo spokespersons used the term 'locking onto the Probe' to 
describe the event. Well, physically there was no 'locking' in the way 
that we usually think of locking something up with a key; it's just a 
way of saying that the Orbiter had established radio communications 
with the signal from the Probe. Needless to say, I helped the 
reporter to write a more accurate caption for his photo of project 
scientist Torrance Johnson jumping up and down with glee.

Throughout all of the day's activities, I sometimes felt a little sad. 
Possibly this was because I would be leaving the Galileo team soon. I 
have recently found a new position at JPL, building hardware for 
some new projects. I have been wanting to get back into the 
hardware business, actually building equipment that will fly into 
space someday, much like in the '80s when I was working on several 
devices used on Galileo. Somehow I will always be able to share in 
the excitement of Galileo's future discoveries (being that I am still 
at JPL), but not from the same vantage point.

Finally, the next big event of the day came - Jupiter Orbit Insertion. 
This event, however, I would spend with a group of friends and 
family of Galileo team members, including a few friends and family 
of my own. By the time the moment came to receive indication from 
the Orbiter that its main engine had started to burn and had burned 
for the proper length of time to put it into orbit, I had begun to lose 
my voice from all the previous excitement. My oldest son was there, 
but I think I was the bigger 'kid' at that moment. 

I had spent the last year and a half testing the Probe Relay and 
Jupiter Orbit Insertion sequence of computer commands that was 
eventually put into the Orbiter's onboard computer and run at this 
time. We had seen this sequence 'execute' (or run) in our test facility 
at least ten times - but now it was for real! And talk about perfect! 
The accolades from the press, the public, NASA, JPL and our own 
Project leaders have continued to pour in. Somehow, I just knew that 
everything would go fine - I really have to hand it to my co-workers 
who struggled over engineering this sequence to work just right. 
Especially for the efforts they made to think of ways to handle all 
the problems that could possibly occur - just in case!

This brings me to one last tidbit that I wanted to relate to everyone 
about engineering. Towards the end of the evening, after the burn had 
completed and the Project leaders had taken their bows, there were 
a group of reporters and TV news writers that asked many questions 
at the press conference. Bill O'Neil, the project manager, had just 
indicated that his team of Project engineers had to get going 
because they had a great deal of work to do. A little earlier, he had 
also stated that the next time we would receive data from the 
spacecraft would be on Sunday, December 10th -- 3 days later. One 
reporter spotted the inconsistency, and asked for an explanation: 
how is it that the engineers could be so busy that they have to leave 
now, when the next data doesn't even arrive until Sunday? 

Bill explained about some of the activities that will occur over the 
next few days. But it struck me that this is a typical 
misunderstanding by the public, the media and others not involved in 
science and engineering about how a space mission works -- that we 
only have to do work when the science data comes down. 

What really happens is that we can't gather all that science without 
first doing a LOT of engineering. There's a tremendous amount of 
work going on "behind the scenes." This is not to say that science is 
not important (the science people are working just as hard to get 
ready for their observations), or that science cannot be done without 
engineering. But, with a project like Galileo, or any other space 
science mission, the amount of engineering that is required to just 
*get* the science can be enormous. 

What Bill was saying is that he and his team of engineers were going 
to spend plenty of time in the next few days before the next science 
data were received, just making engineering decisions. The Project 
engineers must use the work from other engineers like me to make 
the final decisions. Nothing really happens until the Project 
engineers say so - as it should be. But this is still engineering and it 
is very important to the success of the mission. Without this 
process of careful review of the recommendations of the engineers 
at ground-zero, there may not be a Galileo Orbiter acting as a 
platform for the science instruments. Much, much more goes into 
delivering good science than simply the receipt, analysis and 
processing of the data. It is truly a team effort!

I want to say also that I have learned much from working with this 
team - and I cannot impress strongly enough the importance of team 
work. I wish the Galileo team that I am leaving, continued success 
and a willingness to continue to work together.

Gregory L. Klotz, GLL AACS System Test Team