OFJ Field Journal from Greg Harrison - 12/8/95

For years, the Galileo Project has been concerned about the harsh radiation environment around Jupiter - and it was an important part of the spacecraft design. However, in the last two years, we in attitude control have taken further steps to figure out how to deal with potential weaknesses in the face of this radiation threat.

One big concern was how the star scanner would operate in the radiation environment. The star scanner is one of the ways in which the spacecraft orients itself: it basically compares the stars that it sees with a star map. In a high radiation environment, though, a cosmic ray can hit the star scanner and make the scanner think that there's a star where there's really nothing. Or, a cosmic ray could end up enhancing the brightness of a real star, which means that the star scanner won't properly identify it.

Long ago, we put together plans so that we wouldn't depend solely on the star scanner (gyros were the primary attitude source and the use of the acquisition (sun) sensor was incorporated). Nevertheless, we hoped that the star scanner would work okay. It turns out that the star scanner worked remarkably well. We did "lose lock" on one star (Canopus), which caused some problems, but that didn't occur until we were at a fairly sizeable radiation level. And, after flailing for about 30 minutes, we locked back onto stars - even though the radiation level had not significantly subsided. One factor that helped was that we had made some software patches to the star scanner parameters so that it would initialize at a much higher background noise level (in case we lost celestial reference in high rad environment). I was ecstatic that we only lost stars for about 30 minutes. I fully expect that we'll lock back onto stars after we spin down today.(the main engine burns are performed while the spacecraft is at high spin - 10.5 rpm, rather than our nominal spin rate ~3rpm.)

Another amazing point: our approach trajectory was so good (well, good enough) that we cancelled 3 planned maneuvers that were supposed to take place before the Jupiter Orbit Insertion (JOI) burn. These correction maneuvers are frequently small (they change the spacecraft's velocity by less than 1 meter/ second), and gently nudge the spacecraft to where we want it to be. The JOI burn, which changes the spacecraft velocity by approximately 645 meters per second, is a HUGE maneuver. Yesterday, it executed with UNBELIEVABLE accuracy - the NAV team reported it was only off by one tenth of one percent!!!! This is so accurate that we have cancelled (*completely* cancelled) the clean up maneuver Orbit Trim Maneuver-1 (OTM-1), and we will most likely cancel the second clean up maneuver (OTM-2) also. Just think - here was a maneuver so large that we had planned on TWO clean-up maneuvers to correct for errors and we end up not needing either!

It really amazes me. Everything went better than predicted. And we had prepared for MUCH worse than predicted. We had so many scenarios of possible problems and backup plans that no one person can even comprehend all of it. Yet, all these plans remained in our "back pockets", keeping the engineers sane during the nail biting time. I guess it really says a lot about the original designers - this spacecraft is healthy and performed great.

Anyway, my excitement and awe is slightly tempered by some tiredness, and some surprise, and a sense of free-floating. After all, this was our main goal ever since I started working at JPL - perform probe relay and get into orbit. And now that we've done it...I guess I'd just like to bask in this glow and then start thinking about what to focus on next. Certainly there is no shortage of work on the spacecraft team... I guess for me, I now have to get used to the passing of the torch - from an engineering mission to a science mission. We've done our part, and now they get a chance to do theirs. I suppose the first sign of that happens over the weekend, when we'll get the first glimpse of the probe data. It will be the first information back from the atmospher of jupiter. A place that has never been visited before. And we were there. Yesterday.