This Last Year on Galileo - November 5, 2002 - September 20, 2003

From: Amara Graps (amara@amara.com)
Date: Fri Feb 28 2003 - 02:44:29 MST

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    The Galileo tape-recorder is retired today. Then only realtime
    (mostly infrequent) data.

    Saturday, September 20, 2003 is the day to tune in to Galileo
    and its last nineteen hours of 'life' before impact into Jupiter.

    This Year on Galileo
    November 5, 2002 - September 20, 2003

    The Story So Far...

    As usual for the combination of an aging spacecraft and an intensely
    energetic environment, Jupiter dealt Galileo a temporarily crippling blow
    during our flyby of Amalthea on Tuesday, November 5, 2002. Approximately 17
    minutes after zipping by the tiny satellite at over 18 kilometers per
    second (41,000 miles per hour), as the spacecraft neared its closest
    approach to the giant planet, the intensity of the radiation caused a
    failure in computer circuitry that handles timing of the events on the
    spacecraft. This caused the computer to switch to a set of backup
    circuitry, which is a serious enough change to warrant the computer to
    declare an emergency, shut down operations, and phone home for help. Even
    in this relatively quiet state for the spacecraft, the radiation
    environment was still raging, and several additional faults triggered
    repeated software requests to place (or in this case, keep) the spacecraft
    computers in safe mode.

    Once Galileo had cleared the depths of Jupiter's radiation field, engineers
    could start wading through the flood of error messages received to
    determine which ones represented temporary conditions, now passed, and
    which might represent permanent failures in spacecraft systems.
    Surprisingly, with the exception of the switched timing circuit, there
    appeared to be no hard failures! Indications were that we had successfully
    captured two full tracks of recorded science data, including the orbiter
    instruments' first taste of the environment well inside Io's orbit. It
    appeared that the spacecraft systems, though showing expected additional
    wear and tear due to the radiation exposure, were all still in operating
    condition! Computer processing, telecommunications, and attitude control
    were still providing the heartbeats of a working spacecraft.

    Then it was time to begin to retrieve the digital gold stored on the tape
    recorder, and that was when the headaches returned. On Friday, November 8,
    2002, test commands were sent to the recorder to attempt a short movement
    of the tape. Engineering measurements indicated that the tape did not move,
    and the signs pointed to a different problem than the sticky tape that has
    affected operations in the past. Over the next month diagnostic tests were
    run both on the spacecraft and on the ground that convinced engineers that
    the problem was most likely a radiation-induced failure in
    light-emitting-diode circuitry in the motor control of the recorder.
    Radiation experts suggested that time spent away from the radiation
    environment, as well as running current through the circuits without moving
    the tape, might help anneal the circuitry and allow a return to operations.

    More tests were performed on the spacecraft, during which over 111 hours of
    current was applied to the circuits, and eventually limited motion was
    restored! (Imagine a doctor trying to diagnose an illness when the patient
    is a half a billion miles away, and he has to wait at least an hour and a
    half between asking a question and receiving an answer! This is the nature
    of deep-space operations.) Though not fully operational, enough tape motion
    was possible to allow the playback process to return data. Finally, on
    Thursday, December 12, 2002, with careful control of the amount of motion
    allowed, playback of the recorded science data began.

    Throughout this tape diagnostic period, standard maintenance of other
    on-board systems continued. On November 15, December 6 and December 27, the
    propulsion system was exercised to keep the lines cleared. On November 14
    and January 7, tests of the gyroscopes were performed. These activities
    were in support of the final attitude change in the mission, on January 15,
    2003. This turn was also the largest that Galileo has performed in three
    years, changing the pointing of the spacecraft by 18 degrees. At this final
    attitude, the communications antenna is now pointed in a direction that
    will be only 2.5 degrees away from Earth in mid-September 2003, when
    Galileo next and finally encounters Jupiter.

    As part of final cleanup from the safing activities of early November, on
    December 16 and 18 the Plasma Subsystem and Energetic Particle Detector
    were turned on and configured for science data acquisition. These two
    instruments were turned off in response to the problems encountered in the
    intense radiation bombardment.

    Since very few communications passes are scheduled with the giant antennas
    of NASA's Deep Space Network between now and September, on January 15, the
    spacecraft was instructed not to worry if it doesn't hear from ground
    controllers.

    The Story Yet To Come...

    Playback of the recorded Amalthea and Jupiter radiation environment data
    will continue until Friday, February 28. At that time, the playback process
    is stopped, and the tape recorder, that workhorse of data collection and
    return for Galileo over the past seven years, is consigned to a
    well-deserved retirement.

    With playback completed, the Fields and Particles instruments (Dust
    Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer,
    Plasma Subsystem, and Plasma Wave Subsystem) are configured to send their
    data directly to Earth in real time, and the spacecraft continues on its
    long, slow loop away from Jupiter before returning for our terminal
    encounter with the giant planet in September.

    At this time, the high level of spacecraft monitoring via the Deep Space
    Network antennas that has characterized the past thirteen years drops to
    one contact per week, just enough to verify the health and status of the
    craft, and to verify that it is still on the correct trajectory. With the
    exception of a few flight controllers, the flight team, which once numbered
    in the hundreds, has moved on to other projects, other jobs, other lives.

    The distant orbital loop takes the spacecraft farther from Jupiter than it
    has been since before entering orbit in December 1995. On April 14, Galileo
    reaches 370 Jupiter radii (26.4 million kilometers or 16.4 million miles)
    from the planet. This is about 1/6 the distance from Earth to the Sun, and
    light takes nearly a minute and a half to travel from Jupiter to the
    spacecraft!

    During the summer months, as the Earth proceeds in its own orbit about the
    Sun, Jupiter, with Galileo in tow, appears to pass behind the Sun, an event
    known as Solar Conjunction. This limits our ability to hear from the
    spacecraft, due to interference from the Sun's turbulent atmosphere.
    Between Monday, July 28 and Monday, September 15, the radio signal from
    Galileo changes to put more power into the carrier signal, giving the
    ground antennas a better chance to receive the signal.

    Between Tuesday, August 11, and Monday, September 1, the spacecraft is
    within 7 degrees of the Sun as seen from Earth, and communications of any
    sort are not expected. The spacecraft appears to be closest to the Sun on
    Friday, August 22, when the separation between the two is only 0.83 degrees.

    On Thursday, September 18, Galileo is again streaking in towards Jupiter,
    and reaches 50 Jupiter radii (3.6 million kilometers or 2.2 million miles)
    from the planet. Finally on Saturday, September 20, just before 6 p.m.
    Pacific Daylight Time, Galileo is just over 18 Jupiter radii out, and a
    scant 19 hours before impact with the clouds.

    For the conclusion of Galileo's trek through the solar system, tune in
    again in early September!

    For more information on the Galileo spacecraft and its mission to Jupiter,
    please visit the Galileo home page at one of the following URL's:

    http://galileo.jpl.nasa.gov
    http://www.jpl.nasa.gov/galileo

    -- 
    ********************************************************************
    Amara Graps, PhD          email: amara@amara.com
    Computational Physics     vita:  ftp://ftp.amara.com/pub/resume.txt
    Multiplex Answers         URL:   http://www.amara.com/
    ********************************************************************
    "Be glad for life because it gives you the chance to love and to work
    and to look up at the stars."   -- Henry Van Dyke
    


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