This Year on Galileo
November 5, 2002 - September 20, 2003
DOY 2002/309 - 2003/263
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
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
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
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!