OFJ Field Journal from Lou D'Amario - 12/1/95

But first, some background. The new strategy for designing OTM-1 allows us to move the Ganymede 1 flyby date earlier (in one week increments). This allows us to minimize the OTM-1 velocity change (and propellant consumption) in some situations -- namely, when the time it takes for Galileo to orbit around Jupiter after JOI is longer, or higher, than expected. So, if the spacecraft has a higher "orbit period" than was initially planned, we have a straightforward way in which to save considerable propellant.

Does this work the other way? If the orbit period is shorter than expected, or "low," can we move the Ganymede 1 flyby date later to conserve propellant? Unfortunately, no, since that would require flying closer to Ganymede than is safe. So we want to avoid errors that will result in a low period, but we can deal effectively with high period errors. Now back to the TCM-28A meeting.

The situation was as follows: If we don't do TCM-28A, that leaves the trajectory with a slightly high period (by about 3.5 days), a result of the predicted Io flyby altitude now being about 60 kilometers low. If we do TCM-28A, then we will get rid of this period error, and move the trajectory toward the "low period" cases -- precisely the situation we want to avoid. The Navigation Team recommended that TCM-28A be canceled so that the trajectory would have a slight "bias" toward a low period. Of course, the orbit determination results can (and will) change as we get closer to Io, and the JOI burn may underperform (moving the period lower) or overperform (moving the period higher). After considerable discussion, the Project agreed with our recommendation, and TCM-28A was canceled. (The new OTM-1 strategy was made official today; the Project Manager issued a memo declaring that the Ganymede 1 date should be moved to minimize the propellant required for OTM-1.)

With the cancellation of TCM-28A, that makes three maneuvers in a row that have been canceled. In many jobs, having an upcoming work activity get canceled would be regarded as a failure. For us, it means that we're doing our jobs right. It has been possible to cancel these maneuvers because of a combination of factors: excellent navigation (smart, hard working navigators :-), the new strategy for OTM-1 (more navigation ingenuity -- what can I say, I work with a group of smart people), the removal of any science requirements on the Io flyby (since we don't have to worry about camera pointing angles, this allows us to leave the ever- present "too far South" error uncorrected), and, finally, a little bit of luck.

The next big event is the JOI "Tweak" which starts next Monday afternoon (three days before the Io flyby). Even though TCM-28A was canceled (it would have been done on Saturday), the Navigation Team will be working this weekend to update the trajectory predictions as more tracking data comes in and to practice the design of OTM-1.