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OFJ Field Journal from Steven Tyler - 12/7/95

FINAL EXAM DAY

Final exam day! At least, that is how it feels. In a way, I've been waiting for this day for more than 17 years. I started working on Galileo in August of 1978. But the sense of waiting for this day increased after our first use of our big (90-pounds-of-thrust, 0-to- 60 mph in about 2 and a half minutes) engine back in July of this year.

Right now, I'm reflecting a little on Galileo's history. The 1981 mission. The 1986 mission and the Challenger disaster. The present mission...getting to Jupiter ten years after we originally planned to...

Galileo's mission has been interesting already. The earlier mission would have been at the wrong time to make even a limited observation of Shoemaker-Levy. We might not have discovered a moon about an asteroid. And we're arriving at Jupiter right after the discovery of a stellar object in the Pleidies that looks like Jupiter (well, it has methane in its atmosphere).

The Jupiter Orbit Insertion (JOI) burn is exciting for two reasons. First if it doesn't work, there's no second chance. Second, if something goes really wrong, and we lose the spacecraft, we lose the Probe data with it. That is the worst that can happen, because the Probe data is our highest priority. We're not planning to send any more probes into gas giants like Jupiter or Saturn (Cassini's probe will be into Titan).

In a way, it is strange, because the Orbiter mission seems more complex than the Probe mission. At one point in Galileo's design, the two missions were separated into an Orbiter and a Probe carrier/relay. Not that the Probe mission is easy. It isn't. The problems of Probe entry and survival (remember, this will move at over 100,000 miles an hour, the fastest man-made object in history), and of the relay link are significant. It's just that the Orbiter is an even bigger task.

I wonder what Galileo will discover. I suspect that the Probe will find something surprising. And what about the Orbiter satellite encounters? Will we be able to tell if Europa is truly ice-crusted with an underground ocean? (No, Europa does not resemble Europe...). We originally planned to take enough pictures of Europa to be able to answer this question. Now, without our high gain antenna, it will be harder to tell.

Am I nervous about what I just called "final exam" day? Oh yes. Will we hear anything useful from the Probe? Will the heat shield work, will the parachute work? Will the radio link work...?

Will our 400 Newton engine work? We are ready for any recoverable failure... but what if the filters clog or there is some other unexpected underperformance. Or even a surprise return to earlier engine performance expectations?

Also the radiation environment. The shielding. Will there be disruptions to the spacecraft's operations, events like PORs (Power Outage Resets) or SEUs (Single Event Upsets)? How many can we afford? How will they affect us? What will our telemetry link be like?

My guess is that there will be a small engine underperformance today at Jupiter Orbit Insertion (JOI). When we did our first 400 Newton engine burns in July of 1995, my guess was that there would be a small engine overperformance (I was wrong). There were plenty of experts on the engine to ask about it. And, back in July, I found a way to ask them! I put up a list of all possible burn durations (in seconds) and ran a pool, offering a beer to the winner! The engine experts, the accelerometer experts, the managers...all sorts of people guessed. I did too. But the winner was the System Fault Protection Engineer!

I am running the same pool for the length of the JOI burn. The burn should end when we have changed our velocity by the proper amount, so the engine performance should be decisive in determining the length of the burn. We'll see who guesses what! I've upped the award this time. The winner will get two CDs of classical music (Holst's "The Planets" and Mozart's "Jupiter Symphony"). As I said, this is the best way to tell what the engineers really think. Most of them seem pretty confident about the things they are responsible for.

I'm not particularly nervous about events under our control. If the burn is a little off, we're ready to correct it with an Orbit trim maneuver either on Saturday morning or on January 2. This evening, I will start coordinating the Orbiter Engineering Team effort to get ready for the first of these maneuvers. We'll have to start by figuring out how much the mass of the spacecraft changed at JOI. That needs to be done by midnight. Then from 5 AM to 10 AM Friday, we'll implement the navigation team's request for a trim maneuver (unless the maneuver is canceled). For some of us, today will be a long day. If we are going to do a maneuver on Saturday, tomorrow will be my longest day.

I've spent several years working on Galileo. Among other tasks, I designed a fault protection scheme and reviewed several others. I helped decide how serious some of the radiation threats to the spacecraft would be. I spent a year checking the computer hardware to make sure it was redundant (that is, that its critical functions had a backup). If Galileo is a success, I'm going to feel that this work has paid off.



 

 
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