Galileo continues to orbit Jupiter with a bounty of science information and
images stored safely on its onboard tape recorder. The science data were
acquired during the spacecraft’s latest venture, an extremely close flyby
of Jupiter’s fiery moon Io performed on Thanksgiving. During the flyby,
the spacecraft passed within 300 kilometers (186 miles) of Io’s south polar
regions. That is about the same altitude at which the Space Shuttle orbits
around the Earth!

The Thanksgiving flyby also provided a unique view of Jupiter’s icy moon
Europa. The flyby geometry was such that the spacecraft was able to see
the hemisphere of Europa that faces Jupiter. Given Jupiter’s size and
brightness, observations of this hemisphere are difficult, if not
impossible, to perform from Earth. Or by Galileo, until now.

Throughout December, the science information on the tape recorder will be
retrieved, processed, and packaged, and then transmitted to Earth. This
process is known as data playback, and is interrupted once this week so the
spacecraft can perform a standard gyroscope performance test.

This week’s playback schedule returns data acquired by nine of Galileo’s
instruments: the Solid-State Imaging camera (SSI), the Photopolarimeter
Radiometer (PPR), the Near-Infrared Mapping Spectrometer (NIMS), and the
suite of Fields and Particles instruments – the Dust Detector, Energetic
Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and
Plasma Wave instrument.

First on the playback schedule is a portion of an observation returned by
SSI. The observation consists of a 12-image global mosaic of Europa. The
second observation is returned by PPR and contains a polarimetry map of
Europa’s surface. These polarimetry measurements will allow scientists to
study Europa’s surface texture and thermal properties, and in particular,
to look for evidence supporting the existence of liquid water. NIMS
follows on the playback schedule with the return of two spectral scans of
Europa’s surface. One captures an equatorial region of Europa, while the
other is global in scale.

The Fields and Particles instruments round out this week’s data return by
returning portions of a 3-hour high resolution recording of the Io plasma
torus. Originally planned to be 6 hours and 40 minutes long, the recording
was truncated when the spacecraft entered safe mode during the observation.
The flight team members here on Earth were able to bring the spacecraft out
of safing just four minutes after the closest approach to Io, allowing
Galileo to complete more than half of its planned observations.

The Io torus is a region of intense plasma and radiation activity. The
recording gathered data from 6 Jupiter radii (429,000 kilometers or 267,000
miles) above Jupiter’s cloud tops down to an altitude of 5 Jupiter radii
(357,000 kilometers or 222,000 miles), making it the third deepest
recording of Galileo’s entire mission to date. The data acquired during
the recording will be used to understand the structure and dynamics of
plasma, dust, and electric and magnetic fields in the torus region. The
data will also be important for understanding the overall dynamics of the
Jovian magnetosphere.

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: