This Week on Galileo
October 28 - November 3, 2002
On the Threshold
The week begins with the Galileo spacecraft at a distance of 80 Jupiter
radii (5.7 million kilometers or 3.5 million miles) from the giant planet.
Though this may seem close based upon recent history, there were five
previous orbits during Galileo's seven-year tour during which the
spacecraft never reached this far out from Jupiter. By week's end on
Sunday, Galileo will have closed that span to 25 Jupiter radii (1.8 million
kilometers or 1.1 million miles), and will be rapidly gaining speed.
The majority of the week is spent in collecting real-time fields and
particles science data with the Dust Detector, the Energetic Particle
Detector, the Heavy Ion Counter, the Magnetometer, the Plasma Subsystem,
and the Plasma Wave Subsystem instruments. During periodic gaps in our
ground tracking station coverage, the data are stored in a computer memory
buffer and copied to the on-board tape recorder for playback after the
coming encounter. This occurs six times during the week.
On Thursday, October 31, the last orbit trim maneuver of the mission was
scheduled to occur. This propulsive motor burn would be designed to
fine-tune the final position of the spacecraft flyby of the small inner
satellite Amalthea. The most recent tracking data have shown that the
current path of the spacecraft is close enough to the desired location that
the maneuver will not be necessary. Of the 109 maneuvers originally
scheduled to occur during Galileo's tour of the Jupiter system, 21 of them
have been cancelled as being unnecessary. The Navigation team has done a
truly remarkable job of steering the spacecraft to its desired location
over the years.
On Friday, November 1, a final conditioning of the tape recorder is
performed. This 16-hour-long activity prepares the tape for the recording
of the closest approach data to come.
On Saturday, November 2, at 2:44 a.m. [see Note 1], the sequence of
commands that will govern Galileo's activity during the flyby begins. The
sequence starts by configuring the fields and particles instruments for the
coming data collection. This configuration involves setting internal
voltages and biases, and selecting the appropriate measurement channels to
monitor for this portion of the Jupiter magnetosphere. At the moment, this
means that the instruments are configured to measure more sensitively the
relatively quiet outer portions of the system. As the spacecraft plunges
deeper in towards Jupiter, and the expected environment changes, these
configurations will be adjusted to provide the most accurate measurements
At 5:14 p.m., the tape recorder is moved to position the record heads to
begin storing the encounter data. When this positioning is complete, at
10:50 p.m., the memory buffer that has been collecting fields and particles
data is copied to the tape.
On Sunday, November 3, Galileo begins measuring a series of Plasma Sheet
crossings. Jupiter's Plasma Sheet is a flat portion of the magnetosphere
far from the planet at Jupiter's magnetic equator where plasmas, or charged
particles, accumulate. Since Jupiter's magnetic field is tilted 10 degrees
with respect to its rotation axis, the sheet waves up and down as the
planet rotates every 9.9 hours. This waving causes the sheet to move past
the spacecraft, and the instruments can measure the density and energy of
the particles. Galileo has been in a unique position in being able to
repeatedly measure this phenomenon over the past seven years. This
long-term study has shown how the structure has changed over time, and
helps scientists understand more about similar processes that are taking
place in Earth's magnetosphere, affecting the environment of our home
planet. The Plasma Sheet crossings are predicted to occur at 3:22 a.m.,
8:10 a.m., 1:21 p.m., 6:12 p.m., and 11:24 p.m.
At midnight Sunday, the spacecraft is less than 24 hours from the close
flyby of Amalthea, and the best is yet to come!