U P D A T E # 2 3 PART 1: Galileo
fact of the day (see http://www.jpl.nasa.gov/galileo/fact for a complete list) Galileo's cameras will capture pictures that can detect objects as small as 12 meters (39 feet). That's an improvement on Galileo Galilei's original telescopic observations by factors up to 100,000 to 1,000,000. Steve Collins and Greg LaBorde recently shared this in response to a continuing question barrage of the type "when are we going to get pictures": There are no new photos to get. The last image was taken on Oct 11, the day the tape recorder suffered its anomaly. Images are very tape-recorder-intensive, since they cannot be downlinked directly. To preserve the tape recorder, it was moved only a few times to "exercise" it, remaining essentially where it was after the anomaly to capture the all-important arrival day science data (now much scaled down from the originally planned imaging and high-rate spectroscopy data that would have burned up a lot of tape), especially the Probe Relay data. Now that the science/Probe data record is finished, the tape recorder will just sit until we have implemented certain software protections to allow us to move it in the reverse direction safely (that is the problem direction associated with the sticky tape head). In the meantime we have solar conjunction, and the readout of the Probe data stored in the memory of the central computer. This is important, because tape playback uses the memory where the Probe "symbols" are stored (tape playback involves dumping the tape into memory and then reading that memory out over many hours) so we have to be done with the readouts to everyone's satisfaction before we start dumping the tape. Symbol readouts are expected to be over by mid-January. The tape recorder protection algorithms should be aboard after that, and tape playback will commence around Jan 29. It should take about a month. Only after we are completely satisfied that there is no more left to get will the tape recorder then be free to go through "conditioning" to "work it in" to increase its reliability for Orbital operations. That brings us to March, and the Perijove Raise maneuver to raise the low point of Galileo's orbit out of the heavy radiation belts it traversed on arrival. It would probably not survive a second dip into the Io torus. Finally in April we could actually think about doing some science again, including possibly images. However, towards the middle of April we will be loading the new flight software which will control orbital operations, so there is not much of a window. Finally the flight software should be loaded by mid-May, in time for two Orbital Trim Maneuvers to fine-tune the Ganymede encounter and flyby. Then we can start data-collection in earnest, pictures and all. I would not expect, therefore, any pictures to be taken before mid-May, to be downlinked at the end of May and into the Summer. ------------------------------------------------------------- For those who cannot wait for pictures, Ron Baalke writes: Jupiter images taken by NASA's Infrared Telescope Facility (IRTF) as the Galileo probe entered Jupiter's atmosphere are now available on the Galileo home page: http://www.jpl.nasa.gov/galileo/ The caption file to the IRTF images is appended below. MIRAC2 Images of Jupiter at the Time of Galileo Probe Entry These images of the heat radiation from Jupiter were obtained at the NASA Infrared Telescope Facility (IRTF) during the period Dec. 7 UT 22:06 to 23:18 covering the Galileo probe entry time of 22:57. The 4.8 micron image shows the cloud structure with holes in the clouds appearing bright because of increasing temperature with depth in the Jupiter atmosphere. The image at 10.74 microns is sensitive to ammonia gas abundance and those at 17.4 microns and 20.6 microns to tropospheric temperature. The images all show Jupiter's bands. All but the 20.6 micron image show the longitudinal wave structure. On the 4.8 micron image the probe site is at the lower edge of the upper (North) equatorial band 2/3 of the way from the center to the right (West) limb. These observations were part of a special daytime observing program carried out Dec 1-10 with the IRTF's NSFCAM at 5 microns and the guest instrument MIRAC2 at 4.8, 7.95, 8.57, 10.74, 13.3, 17.0, 17.4, and 20.6 microns. The telescope primary mirror was covered with a sheet of white polypropylene to protect it from direct radiation from the sun, which was 8 degrees from Jupiter during this period. This resulted in a reduction of signal-to-noise of a factor of 20 at 4.8 microns and 4 at 20 microns. The observing was done by Jim Friedson, Jay Goguen, Glenn Orton, Sarah Stewart, and Padma Yanamandra-Fisher (JPL), John Spencer (Lowell Observatory), Bill Hoffmann (Steward Observatory), and Bill Golisch (telescope operator, IRTF), with support from Joseph Hora at the Institute for Astronomy and Joe Spitale and Jose Ortiz at JPL. MIRAC2 was built by the Steward Observatory, University of Arizona, and Smithsonian Astrophysical Observatory, with support from NASA, NSF, Steward Observatory, and Smithsonian Astrophysical Observatory. It is operated by Bill Hoffmann, Giovanni Fazio, Joseph Hora, and Lynne Deutsch (FCAD University of Massachusetts). Steve Tyler December 11, 1995 "What Project do you work on?" I've heard this question around JPL a lot in the past 17 years. The people who said "Voyager" received looks of respect. Those who said "Galileo" got looks of pity. Until December 7. We passed two big tests, and now we're JPL's flagship Project. I always knew we had a good team. Now everyone knows. The most dramatic moment was getting the first data showing a communications relay link to the Probe. Everyone was nervous and looked anxious. Then the data finally arrived. The spontaneous smiles were something to behold. The Jupiter Orbit Insertion (JOI) burn was different. There was little doubt in my mind that the burn would start. There was little doubt that it would end if it made it to the finish. But what would it do in the meantime? The first few minutes told the tale: this burn would be a success. It was very satisfying (it would have been so frustrating if JOI hadn't worked and the Orbiter had flown past Jupiter...). There was a triumphant press conference at 6:45 PM, right after the successful JOI burn. After about an hour, the conference ended, with the participants explaining that they had to get back to work! Work? How do you work effectively after your day starts with what looks like a successful Probe Relay and JOI? Do you wonder what we did? First we had to get some tracking data (specifically, two-way Doppler data) to find out what kind of orbit we were in. If our orbit was too long or too short, we might change it with our first Orbit Trim Maneuver (OTM-1). Several of us were asked to say "Go" or "No Go" on getting 2-way data. After a short discussion, everyone said "Go", and we prepared to get data that would pin down our orbit. I was waiting to see what would go wrong on December 7. So far, everything had been perfect. But, sure enough, there was an error...just about the most minor error I could think of (but one I could have prevented). We were considering sending a command at the wrong time. Sending it at the right time would now be a nuisance. Now, what was this command? The command was to change the "limit on helium loss fault protection." This limit was a "trip point," meaning that when you reach that point, you essentially trip an alarm, and something happens to respond to the alarm (this is what is meant by fault protection). In this case, once the helium pressure dropped below the trip point, it meant that helium was leaking out of the helium tanks and into space, presumably through what's known as the "pilot valve vent." Something would have to be done to prevent more helium from leaking out, or there would not be enough left to push fuel and oxidizer into the engine for our next big maneuver and our entire Tour would be jeopardized. I had designed a computer program--a piece of fault protection-- that responded to "tripping the alarm" by automatically sealing up the pilot valve vent. We needed this program on board because there might not be enough time to give the commands from the ground after we found out about a leak--all of the helium could leak out during that time. We couldn't simply seal off the vent as protection against a leak, because then we couldn't use the pilot valve at all. We wouldn't want to use the engine in a "no pilot valve" setup if we didn't have to, and we would now lose everything if our latch valves ever leaked. So we were stuck with having the spacecraft fix itself automatically in the event of a helium leak. Now that we had this fault protection on board, we had to be careful to make sure that the trip point was set correctly. A lot of helium was used at JOI, and the helium pressure in the tanks was now lower, so the trip point that would warn us about low helium pressure had to be reduced at the end of the burn. It was. But (this is a tricky point) after the end of the burn the limit would have to be increased a little. This was because the helium would warm up after the burn, increasing the helium tank pressure. The limit had to be raised or there would not be enough protection to save the Tour if the trip point were reached. We were planning to raise the limit. I didn't care about whether we changed the limit on December 7 or December 8, as long as we did it *sometime* in the next few days. After December 10, the craft would be behind the sun (or close to it) for about three weeks. We wouldn't be able to communicate with the craft during most of those three weeks, so I wanted to be sure that we were protected against a slow helium leak during that time. There was one catch. If we sent up the command too soon.... (...you might be thinking that if we sent up the command too soon, we would then have a high trip point when we were supposed to still have a low one...but that wasn't the problem....) ...the spacecraft would still be in "critical mode" and for very complex reasons, we did not want to make this particular change during critical mode. It wasn't especially dangerous, but it was just a tiny bit riskier than sending the command at the right time. Just enough so I would have to say "No Go" to the sending of the command at the wrong time. Just enough so that I was surprised that we were considering making this mistake. We could have changed the command to make it safer to send at this earlier time, but it was too late to do that safely any more. I felt annoyed. I should have noticed this change before. We had indeed made a very small mistake. *I* had made a mistake. Luckily, it was not very important. We had two reasonable choices. We could send the command early or postpone it. Well, we decided to postpone the helium loss trip point update to the following day. That was quite reasonable. Normally, a decision on such a matter would be made pretty quickly. But everyone was tired after such an eventful day, so it took us all much longer than usual. Next was a big issue, the "spin-down." The spacecraft's spin rate, usually around one revolution every 20 seconds, had been "spun up" to about one revolution every 6 seconds for the JOI burn. Originally, we had planned to spin down automatically, right after the JOI burn. But then we realized that the spin-down might be unsafe or even undesirable in some fault scenarios. So we decided to spin down only if all systems were "Go." It was about 9:30 PM, and already the word "go" no longer had a unique meaning. Most of us simply wanted to "go" home and get some rest! We decided it was safe to spin down. For some of us, there was still no rest. As I said, it wasn't all that late, but it had been a very long day. Now we had to decide whether or not to perform the first Orbit Trim Maneuver (or OTM for short). Maybe by midnight, we would have proof that we no longer needed to do it. Shortly before midnight, we looked at the evidence. Our orbit about Jupiter was almost exactly a week shorter than intended, just perfect for a June 27 encounter with Ganymede. That would mean no trim maneuver Saturday! But what if we were wrong? We decided to wait until morning for the final decision. The Navigation Team would stay up all night to get more data. The survivors would show up at 9 AM to report on it. The rest of us would be awake enough to make an intelligent decision. Best of all, if we had to implement some maneuver, our team would have had enough sleep to do it right. As I drove home, I felt happy not only about the Probe and JOI, but about OTM-1. Of course, OTM-1 would be cancelled. We already knew early on Thursday that after a perfect JOI, OTM-1 would not be needed. And JOI had been just about perfect! The one-way Doppler data confirmed it. The more accurate 2-way Doppler data did too. Not only would Saturday's maneuver be cancelled, but the next one, scheduled for January 2, would be unnecessary as well. Sure enough, there were no navigational surprises the following morning. The NAV team, speaking with inspiring clarity after their long night, explained why they recommended no OTM-1. Yesterday was a big day for Galileans. We are on our way to Ganymede. Bob Gounley 7 December 1995 (Part 1) A billion kilometers from Earth, the Galileo Orbiter and Probe began executing their programmed instructions to collect important scientific information from Jupiter. On a quiet street in Pasadena, California, I stumbled about my home, alternately dressing and looking for a mislaid lapel pin. Galileo was preparing to encounter intense radiation and heat. I would meet the Press. For the previous six years, most of Galileo's big events, including flybys of two asteroids and the Earth, put me in Galileo's Mission Support Area (MSA). That's where we "watch" the spacecraft. From the computer displays, cryptic numbers and symbols tell minute details of how the spacecraft is working. Sometimes, hidden in all that information, we can hear the spacecraft calling, "Pasadena, we have a problem." Everyone on this assignment spends long uneventful hours hoping there would be no such message on their watch. Today was different. Other engineers were staffing the MSA, each one having spent months and years preparing for the big day. This was my opportunity to help out in a different capacity. I volunteered for "Guest Ops" -- escorting and speaking to the many visitors on Lab that day and, possibly, giving interviews to journalists visiting and phoning JPL. Driving to work I listened to the radio give news reports from JPL. At the Lab's entrance, I saw a row of large TV broadcast vans, just like there were when Magellan and the Voyagers were the center of attention. The parking lot was full and, for the first time in 12 years, I used an illegal space, hoping the security guards would be lenient. Descending the area known as "Cardiac Hill", I made my way to Von Karman Auditorium where most of the day's visitors would be. Trudging the opposite way was Dan Carlock, a member of the Probe team. He greeted me with news that the Probe should have turned on and started preparations for Jovian entry. Of course, he couldn't say for sure since the Probe, preserving its precious batteries, wouldn't turn on its transmitter until reaching the atmosphere. All night long, Dan watched a computer display to observe the radio receivers on the Orbiter turn on and stabilize. Nothing much else was expected to happen that night and Dan made sure nothing much did. Bleary-eyed, he excused himself to crawl into a camper for a short nap. Nearing Von Karman Auditorium, two men in suits passed me by. One of them was Dan Goldin, NASA's Administrator. He seemed cheerful and animated, chatting briefly with guards he seemed to know on a first name basis. At the auditorium, other men in suits promptly escorted him in. I showed my badge and walked in alone. Inside, the rooms were filled with people and electronics. TV cameras were being hauled into formation in a line facing the stage while technicians checked the microphones for bad connections. Elsewhere, reporters alternated between their tape recorders and laptop computers. Walking through this maze, I had to be careful not stand in the way of cameramen televising reports "Live from JPL". Past this hubbub, I greeted members of the Public Information Office, letting them know I was here and ready to be of any service. No, there wasn't anyone needing someone to interview at the moment. Perhaps later. I found a quiet area and waited. My first request came an hour later. I would give a telephone interview sometime after noon for a radio station in Toledo, Ohio. That sounded like a good beginning. Work out any problems with nervousness speaking to a small geographic area, then things would be comfortable while talking to a larger audience. It didn't quite turn out that way. Before the Dayton interview, I was asked to give a live telephone interview for the BBC World Television ten minutes from now. A director at the other end of the telephone line advised me on what to expect. To be sure he could reach me when needed, I should stay on the line for about five minutes and he would tell me when my time was getting close. In the background, I could hear a news report about events in Indonesia. Then, the announcer began introducing a story with, "Scientists and NASA's Jet Propulsion Laboratory ...". ONE MINUTE. After a brief description of the Galileo mission, I was introduced and the interview began. There followed several brief, innocuous questions. "What is the mood like?" ("Tense, but filled with anticipation", or something like that.) How soon could we expect to receive Probe data back? (I started to say next Monday, but began thinking it might really be Sunday. "Sunday or Monday, depending on your time zone", was my improvised reply.) Then came the tough one. "Aren't there concerns that the Probe may not survive its fiery entry?" This is a difficult area to explain to someone who hasn't spent much of their life making something on the other side of the solar system perform complex gymnastics on command. From the beginning, everybody on the Galileo project has been dedicated to searching for the hidden flaw. Every assumption gets questioned. What can't be tested gets analyzed by experts. Yet, we know this is a risky business. Space exploration is inherently intolerant of human error. Of course something _could_ go wrong. It just isn't likely. Taking the easy route, I said something to the effect that we were confident in the integrity of the Probe, thanks to extensive analyses and tests. Soon we would know for sure. The announcer thanked me and I was off the air. Thus ended my first interview. |
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