OFJ97 Field Journal from Jim FH Taylor - 2/27/97
DID JUPITER CAUSE A GALILEO DATA LOSS - - AND REFLECTING ON CHANGEThis has not been exactly the day I thought it would be when I went to bed last night. For example the sound from unexpected rain that woke me this morning. And, at work, I followed a path of thought from a report of Galileo data loss to a speculation about the shape of Jupiter.
The first thing I do as I enter my office is to jiggle the mouse on my engineering workstation. That brings up a display of the most current measurements about "my" spacecraft hardware. Those of us who have been on the flight team for long times are very possessive about the health of this object that is now orbiting Jupiter. Just as a person might be concerned about their health from a slight fever, so I'm concerned if any of the measurements aren't quite right. Before I leave for home tonight, I'll jiggle that mouse one last time to make sure all's well.
The other day I explained this to Sue Kientz who is developing a CD-ROM on the Galileo spacecraft and mission. She e-mailed me back:
"Just read your article in Jupiter Online about the Coronal Mass Ejection. Very interesting and informative. Now that I'm involved w/Galileo I know what you mean about getting that feeling in the pit of your stomach when you think something may be wrong w/the spacecraft. I'm getting so *attached* to it, following it and reading its history, that I find myself actually worrying about it up there in space! It's amazing it just keeps on working and giving us all the wonderful information!"
This morning I looked over an e-mail report from Glen Elliott, the Galileo Mission Control Team analyst who keeps track of each "frame" of data sent back from Galileo. The tracking station decodes the data it receives in chunks that are called "frames"). It is part of my job to try to understand why "lost" frames were lost. The communications link could be one such reason, if the amount of noise should overwhelm the signal and prevent the earth receiving station from decoding the data. I wrote about one kind of such noise in my journal page, about the coronal mass ejection.
The e-mail report about yesterday's frame losses said,
"One TF [transfer frame] lost due to early LOS [loss of signal] entering Earth Occultation. The 'Cause' is listed as 'None' because there was not any ground station problem..."
This report struck me as strange. The word "occultation" refers to when one object (in this case, the spacecraft) is occulted or hidden from another (in this case, the earth). An earth occultation of Galileo occurs when Jupiter or one of its satellites gets in the direct path the radio signal takes from the spacecraft to the ground station. Last week we had an occultation caused by Jupiter's satellite Europa, and I was involved in the detailed planning of how to control the radio uplink and the radio downlink. We wanted to give radio science as much information about the occultation as possible. The planning paid off last week, and the occultation started and ended within seconds of the expected times. I learned a couple of new words, "ingress" for enter and "egress" for exit, from that work. But the important thing I remembered was that our ability to predict the path of Galileo and the satellites is so good that we were able to forecast ingress and egress to within seconds!
Losing that data frame means that we missed calculating something by several *minutes*! I knew that yesterday's major event was an earth occultation by Jupiter itself. I looked at the log kept by the Mission Controller as he monitored the loss of radio signal at the start of the occultation. The time was nearly 6 minutes earlier than was predicted. I e-mailed our radio science coordinator, Randy Herrera, to ask how this could be. Randy wrote back that this was the first time the spacecraft had passed behind Jupiter (as seen from the earth) near to Jupiter's equator. He speculated on two reasons that "ingress" on an equatorial occultation might occur earlier than predicted. Can you think about simple properties of Jupiter or its atmosphere that might be the cause? (The answer is at the bottom of this journal.)
Oh, by the way, it turned out the lost frame occurred as part of the planned preparation for the occultation. I figured out this much less interesting reason simply by comparing the loss time with the published time for each part of the preparation sequence. But, even though the result ended up routine, I had relished the chance to learn about something a little outside my ordinary work experience.
With the resolution of the data loss issue, and a few other technical problems like it, my mind turned to another matter. The main Galileo mission is nearly over. What I mean by this is that we are relatively much closer to the end of the mission in December of this year than to launch in October of 1989. I saw an electronically posted newspaper article this morning that an "extended" mission has been approved for Galileo. That means the spacecraft, its continued good health willing, has been budgeted by NASA to continue to return valuable scientific data for two years beyond the main mission, to December 7, 1999.
Why, with this good news, would some members of the flight team wear worried expressions on their faces or feel anxious for the future? The budget for conducting the extended mission means that we have to fly this spacecraft with just 50 people, down seven-fold from the current staff. Some are anxious because it's clear that not everyone who wants to stay faithful to Galileo will be able to. To them, it may feel like leaving their family. Others, their positions assured, worry about how to conduct a mission just as challenging as it is right now, but with a relatively tiny staff! In telecom, as in the rest of the flight team, we'll automate our essential processes to a greater extent, and we'll have to choose carefully the problems that we look at in detail.
A telecom colleague, Wade Mayo, went with me this afternoon to listen to the JPL Director talk about "Operations Plan 2000", of which the changes in Galileo flight operations is one small part. We reflected on "change" as we left. Wade is an evening class teacher. He said in the schools, the common concept now is that job security, even career security, is a thing of the past. That's very different from my 35-year experience, of being in one kind of career (telecommunications analysis), and with only two employers the whole time, AND on a single project for half that time!
Nonetheless, I felt invigorated from what I heard our Director Ed Stone tell us about the JPL vision for the future. These are not his exact words, but the vision is we will *develop* the systems we need to, in order to *conduct* our voyages of exploration, and *continue* to "push the envelope" in space exploration. It reminded me, I remarked to Wade, of what I remember about the Chinese word for "crisis". As I recall, the word is made up of the forms for both "danger" and "opportunity." I am really looking forward to meeting the challenges of planning to fly Galileo safely through the next 2-3/4 years and continuing to be involved in the design of efficient deep space telecommunications systems.
P.S. - I promised you a couple of possible reasons (courtesy of Randy Herrera of the Galileo Radio Science team) why the occultation by Jupiter might occur at a different time than predicted. Most engineering and scientific predictions are based on mathematical models. (1) The occultation prediction assumes that Jupiter has a atmospheric thickness, below which radio signals can't penetrate. (2) It also assumes that Jupiter has a particular shape, that of a slightly flattened sphere. Randy reminded me that this occultation was the first one to "test" the Jupiter atmospheric thickness and planet shape models near the equator.
P.P.S. - my wife Barbara and I did go see the movie "Hamlet". We knew the movie would be four hours long plus an intermission. I only fell asleep once. 8-) And my Galileo beeper didn't go off.