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OFJ97 Field Journal from Glenn Orton - 2/22/97

*Callisto-3 and a nasty surprise for our Europa-4 target:

Part of my work using ground-based telescopes to help with Galileo is to look for features in Jupiter's atmosphere that are relatively stable. We want to be able to predict when we will see, for example, a white oval, so that we'll know exactly how to time the spacecraft's cameras to see it. For our third orbit, we couldn't find any features that we could predict that would be stable over the 8-week period between planning for our next observations of Jupiter, and actually making them. So, the early November target in the C3 encounter was a region of Jupiter not associated with any discrete feature, but simply near the equator.

A group of us were using various telescopes in Hawaii to get both infrared and visible range images of Jupiter. There were also observations up in Jupiter's northern regions to observe auroral pheneomenon in a joint experiment with Galileo's particle and field observations. We were all, at best, "ambushing Jupiter" between clouds. Clouds surrounded us so much that we would try to grab glimpses of the planet between small holes in the clouds - so we'd take many images of something and then afterward see how many of them had viable images of Jupiter. This was a very frustrating time.

Our planning for the next orbit encounter, Europa-4, was causing some concern. Our target was a "5-micron hot spot," the same type of relatively cloudless feature into which the Galileo probe had descended a year earlier. The more observations we had of these hot spots, the better we could understand the probe data. The problem was that the hot spots could appear and then quickly disappear, so predicting their locations weeks in advance was, basically, guesswork.

This was the first time I had to figure out where a fairly ephemeral feature was going to be. Jose Luis Ortiz, a postdoc from Spain, had determined that the hot spot locations could be predicted in a limited sense. If you moved a longitude system at a particular "drift rate" with respect to the bulk of Jupiter's interior (whose rotation rate is known from the variations of the magnetic field and their influence on Jupiter's radio wave emission), then there were particular longitudes which were much more likely to have hot spots than others. Still something of a gamble, but the best we could do.

Our early C3-support observations--weeks before Galileo would make its own observations of the hot spot--showed that the selected feature was still strong. in early November, and the "service" observing that the staff of the IRTF did for the Galileo project to target atmospheric features and monitor the behavior of Io's volcanos helped fill in time. But, by the end of November, observations showed that major changes had taken place in the cloud structure of out targeted hot spot, and it was gone. Or it had moved--there was a much smaller hot spot at a different location!

I called all the Galileo remote sensing science coordinators who were in charge of the atmospheric observations and tried to pursuade them that, although we couldn't change *when* we did the observation,, we needed at least to change the pointing (that is, the location at which we aimed the cameras). Otherwise, we wouldn't be able to get any observations of a hot spot. This meant lots of work which no one really needed - but everyone understood the importance of this particular feature.

*Europa-6 planning and the final chapter on Europa-4, did we succeed?

(the next field journal will tell, stay tuned)....



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