This Week on Galileo
October 8-14, 2001
Galileo Is Preparing for the Next Io Flyby
In this final week before the next Io flyby, the pace of activities picks
up considerably. On Monday, four more optical navigation frames are
shuttered. These images frame the satellite Callisto and several stars. By
comparing the relative positions of Callisto and the stars, ground
navigators can help refine the position of the spacecraft, supplementing
the usual radiometric tracking data used for orbit determination. Two
additional such frames are shuttered on Wednesday.
On Tuesday, the Magnetometer instrument changes its configuration,
preparing for the more intense magnetic environment as the spacecraft nears
On Thursday, the sequence of commands that will govern Galileo's activities
during next week's flyby will be transmitted to the spacecraft. These
commands will be sent from the 70-meter diameter Deep Space Network
tracking station located at Robledo, near Madrid, Spain. There are two
other such antennas that support the Galileo mission, located at the
Goldstone tracking station in the Southern California desert, and also at
Tidbinbilla, near Canberra, Australia. For more information about these
communications complexes, please visit: http://deepspace.jpl.nasa.gov/dsn/ .
Friday sees the end of playback of the tape-recorded data acquired during
the last Io flyby on August 5. This week's data return consists of
collections of fragments of observations which were lost in transit during
previous playback attempts. Once playback has concluded, routine
maintenance of the tape recorder is performed on Saturday, to prepare for
the new recording.
Also on Saturday, the final targeting maneuver is executed. This motor burn
will fine-tune the trajectory of the spacecraft to reach the desired
aim-point 181 kilometers (112 miles) above the surface of Io next Monday.
Then at 7:41 p.m. PDT the commands sent to the spacecraft on Thursday take
charge, and the encounter is under way! First up is configuration of the
Fields and Particles instruments, and the start of twelve days of
continuous real-time data collection for those instruments.
The six instruments on Galileo which measure the particles and
electromagnetic fields that surround Jupiter and its satellites are the
Dust Detector Subsystem, the Energetic Particle Detector, the Heavy Ion
Counter, the Magnetometer, the Plasma Subsystem, and the Plasma Wave Subsystem.
On Sunday, the tape recorder is moved to the correct position to start
recording for the encounter. Also, the attitude control software is
configured to rely on sightings of a single bright star to determine the
spacecraft orientation. Ordinarily, three or more stars are used for this
determination. However, when Galileo approaches Jupiter, high levels of
radiation create electronic noise in the star detector, and only very
bright stars can be seen reliably. It then becomes a challenge to find a
star that is sufficiently bright and also falls within field of view of the
star scanner. During the 48 hours surrounding the closest approach to
Jupiter and Io, we will be viewing the star Achernar (Alpha Eridani), which
is the sixth brightest star in the catalog we maintain for use by Galileo.
Take a deep breath, we're about to get really busy!