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OFJ Field Journal from Lou D'Amario - 11/21/95

A WEEK OF ACTION

As I write this journal, there are 10 days to go to Jupiter arrival. The big day is a week from this Thursday. I can feel the intensity and emotion of the Galileo Flight Team building. It is now 6 years since launch and 18 years since I started working on Galileo.

What follows are highlights (as best as I can remember) of the two weeks since my last journal entry:

Monday, November 13

In the middle of all the work happening on the Navigation Team, I had to interview a job applicant today. We have an opening in the Trajectory Analysis Group for this coming January. It's hard to concentrate fully on a job interview at this time. My mind is elsewhere.

Tuesday, November 14

I interviewed another job applicant today. This is probably the last one (at least until after Jupiter arrival). There was also a Section Staff Meeting in the afternoon that lasted over two hours. (A Section is one of the basic organizational units at JPL. I am in the Navigation and Flight Mechanics Section.)

Thursday, November 16

This morning there was a meeting at which the strategy for OTM-1 (Orbit Trim Maneuver #1) was discussed with Project Management. OTM-1 is the first correction maneuver after the Jupiter Orbit Insertion (JOI) burn. The entire development of OTM-1, starting with the Navigation Team's design of the maneuver and ending with sending the maneuver commands to the spacecraft, is done in only about 24 hours starting a few hours after JOI is completed, so it's important that we are ready to go. OTM-1 is scheduled to run soon after JOI because the longer we wait, the more propellant we will have to spend for the maneuver. This particular maneuver will allow us to compensate in case the gravity assist from the Io flyby is too high or too low, or if Galileo's main engine "overburns" or "underburns" during JOI.

At this meeting, Dennis Byrnes (a trajectory analyst on the Navigation Team) talked about a new strategy for OTM-1 that would allow us to move the date of the first satellite encounter in the orbital tour (Ganymede 1). The advantage of this that the size of the correction that's needed at OTM-1 becomes smaller. This saves propellant which is generally in short supply. However, changing the date of the Ganymede 1 flyby might mean that some or all of the science observations at Ganymede 1 would be lost. This might be the preferred strategy, however, if OTM-1 required such a large velocity change to get to Ganymede 1 at the currently scheduled date that there would not be enough propellant to finish the orbital tour. In other words, it might be better to lose the first satellite encounter in the orbital tour instead of the last three or four satellite encounters. This is a major decision, and choosing one way or the other was postponed to a later date.

Friday, November 17

Today is the approval meeting for the final versions of the spacecraft sequences that run from a few days before Jupiter arrival through the end of December. Events in this time period include the JOI Tweak (discussed in two of my earlier journal entries), the Io flyby, Probe Relay, JOI, OTM-1, solar conjunction, and OTM-2. Solar conjunction is the several-week time period when the Sun lies nearly between the Earth and Galileo, and we cannot reliably send commands to Galileo because of interference from the Sun. It was a long meeting, lasting several hours. Today is 20 days before Jupiter arrival, the day that we would have performed TCM-27, which was canceled. (I discussed the story of TCM-27 in my last journal entry). So in some ways, today's workload is a little lighter than I might have predicted it would be.

Saturday, November 18

You might ask: what am I doing at work on Saturday? TCM-28, the next scheduled maneuver, would be performed (if approved by the Project) on November 27 (or 10 days before the Io flyby). The schedule for TCM-28 calls for the Navigation Team to finish designing the velocity change for the maneuver 9 days before the maneuver is performed. Nine days before November 27 is November 18, which happens to be a Saturday. Orbital mechanics doesn't recognize weekends. We completed the maneuver design by about 4:00 PM.

The situation for TCM-28 is very similar to that for TCM-27. If we don't do TCM- 28, the spacecraft is predicted to pass by Io at an altitude of 1080 kilometers, or 80 kilometers above the desired 1000 kilometer altitude. This is a significant difference. However, the predicted flyby altitude could be in error by several hundred kilometers, because the tracking data has not been as accurate as we had expected. Nevertheless, because there is only one more scheduled maneuver after TCM-28, the Navigation Team will recommend that we do perform TCM-28.

Sunday, November 19

The Project decided to go ahead with the next stage of the maneuver development for TCM-28 (this is the work done by the Orbit Engineering Team -- figuring out how to achieve the desired velocity change with the spacecraft's thrusters). However, the actual decision about whether to perform TCM-28 won't be made until next Tuesday at the Maneuver Design Approval Meeting.

Tuesday, November 21

The TCM-28 Maneuver Design Approval Meeting was taken up entirely with a discussion of whether or not to continue development of TCM- 28 and perform the maneuver. The arguments for performing the maneuver were: (1) the spacecraft trajectory would be moved closer to the "aimpoint", (2) it would protect against us getting into a situation where we needed to change the spacecraft's trajectory but, for whatever reason, the one remaining TCM (TCM-28A at 5 days before Io) could not be performed, and (3) it would give us early warning if there were a problem with the propulsion system that might affect TCM-28A.

On the other hand: (1) the uncertainty in the Io flyby altitude (several hundred kilometers) is much larger than the correction that would be made (80 kilometers), so it's not clear that performing the maneuver actually helps, (2) performing TCM-28 does not reduce the likelihood of having to do TCM-28A, and (3) members of the Flight Team would have to come in to work during the Thanksgiving weekend (meaning a greater workload for the Project, and increased stress for the flight team).

In the end, the Project decided not to proceed with TCM-28 and to focus instead on ensuring a successful TCM-28A. This is part of working on a complex mission like Galileo: in order for the mission to succeed, we have to explore many possible paths thoroughly, even if we don't journey down them. (By the way, the day that I am writing this journal entry is the day that we would have performed TCM-28. Oh well.)

In addition to the events described above, I had to deal with a number of questions coming from the mission planners. Since these questions dealt with events happening a long time after Jupiter arrival, I found them to be distracting. I find that my mind wants to focus only on the Jupiter arrival events now.

As I write this, the major Jupiter arrival activities remaining for the Navigation Team are:

  • Generating daily orbit determination solutions for the trajectory.
  • The design and execution of TCM-28A (Friday and Saturday, Dec 1 - 2).
  • The JOI Tweak (Monday and Tuesday, December 4 - 5).
  • Monitoring the radio tracking (Doppler) data during the Io flyby and JOI on December 7.
  • The design and execution of OTM-1 (Friday and Saturday, Dec 8 - 9).

So there is still a large amount of work ahead of us

 
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