This week, Galileo continues to return data from a historic encounter with Jupiter’s volcanic moon Io.
The third flyby since October 1999, the spacecraft flew within 198 kilometers (123) miles of Io’s
surface, the closest yet of the encounters. The data returned this week were stored on Galileo’s
onboard tape recorder during the flyby. Data playback proceeds uninterrupted during the week,
limited only by the amount of time allocated to Galileo on the Deep Space Network’s 70-m (230-ft)
diameter radio antennas.

During data playback, the spacecraft computer retrieves the data stored on the tape recorder,
then processes and packages the data, and subsequently transmits the data to Earth. Only one
observation is on the playback docket this week. The Fields and Particles instruments continue the
return of a 2-3/4 hour high resolution recording of the Io torus that was taken starting 3-1/2 hours
prior to the Io flyby. The Fields and Particles instruments are comprised of the Dust Detector,
Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave
instrument. The observation contains measurements of the plasma, dust, and electric and magnetic
fields within the Io torus, and will be used increase the knowledge of the structure and dynamics of
the torus region. The torus is a ring-shaped region of intense plasma and radiation activity with its
inner edge bounded by Io’s orbit. The torus is actively maintained by Jupiter’s strong electric and
magnetic fields and Io’s constant supply of volcanic particles.

On a final note, Galileo is operating normally this week after suffering a safing event late last week.
The spacecraft safing is believed to have been caused by an erroneous power reset signal, which
happens regularly during each encounter. You might remember that new software was installed on
Galileo to allow the spacecraft to recover autonomously from this type of anomaly. Unfortunately,
the new software must be disabled during tape recorder playback. Galileo suffered two other
power reset signals during last week’s encounter, and the new software allowed encounter
commands to continue executing unhindered, protecting the time-critical data gathering phase. This
means that the Galileo team expects that all planned observations of Io were completed successfully.

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: