This Week on Galileo

November 29 – December 5, 1999

This week Galileo continues with the return of images and other science information acquired during its amazing flyby of
Io last week. Last week’s flyby was the closest yet of three performed during Galileo’s mission at Jupiter. The first
flyby was performed in December 1995 during Galileo’s arrival to the Jupiter system but was limited in science scope
as a result of an anomaly in the spacecraft’s onboard tape recorder, detected on approach to the gas giant. A workaround
to the anomaly was implemented shortly after arrival, allowing Galileo to perform its mission at Jupiter. The second
flyby occurred on October 10.

Despite a safing event that halted spacecraft activity a mere four hours prior to last week’s closest approach to Io, quick
action by flight team members here on Earth brought the spacecraft out of safing four minutes after the closest approach
to Io. This was in time to allow Galileo to complete more than half of its planned observations of Io and its plasma torus
(a region of intense plasma and radiation activity, in which there are strong magnetic and electric fields), and all of the
planned observations of Europa. Among the lost observations was a Fields and Particles recording scheduled to be made as
the spacecraft flew closest to Io. The data from this observation would have allowed scientists to determine if Io
possessed its own internally-generated magnetic field, similar to both the Earth and to another Galilean moon, Ganymede.

This week’s playback includes observations performed by the Solid-State Imaging (SSI) camera, Near-Infrared Mapping
Spectrometer (NIMS) and Photopolarimeter Radiometer (PPR). During data playback, the spacecraft computer
retrieves the stored data from the tape recorder, then processes and packages the data, and subsequently transmits the
data to Earth. Here on Earth, the large antennas (70 meters in diameter) of the Deep Space Network capture Galileo’s
radio transmissions and send them to JPL for ground processing and, ultimately, delivery to the science community.
Data playback is interrupted three times this week. On Monday, the spacecraft will execute a small flight path
correction. On Wednesday, the spacecraft performs standard maintenance on its propulsion systems. Also on Wednesday,
NIMS performs an instrument calibration.

SSI and NIMS return the first couple of observations of this week. In a joint venture, the instrument pair looked at a
region of Io’s surface near the terminator (or line dividing night from day). This observation is followed by the return
of a NIMS observation containing a regional map of Io’s surface. SSI then returns an observation of Amalthea, one of
Jupiter’s minor moons. The observation will provide the best resolution ever of the moon at 3.7 kilometers (2.3 miles)
per picture element.

Playback focus then turns to the observations of Europa that were taken while Galileo was inbound to the Jupiter system
last week. PPR returns three observations from the Europa observing campaign. In the first, PPR looked at Europa’s
dark side. The observation will be compared to day side observations of the same regions performed on previous orbits.
The comparison is expected to help identify heat sources or thermal anomalies originating from within the icy moon.
PPR’s second observation contains polarimetry measurements of Europa’s surface. These measurements will allow
scientists to study Europa’s surface texture and thermal properties. PPR then returns a dayside thermal map of Europa.
Similiar to PPR’s first observation, the map captures a region of Europa’s night side that was observed in September
1998. The two sets of data covering this region will be compared in hopes of determining whether the anomalous
temperatures detected in the first observation are due to Europa’s heat retention characteristics (or thermal inertia),
or possibly due to heat from volcanic activity inside Europa.

SSI and NIMS also return observations of Europa. NIMS returns an observation that captured measurements of Europa’s
north pole. The observation is expected to provide the highest resolution views of the polar region to date. NIMS thens
returns a spectral map looking for evidence of plate tectonics on Europa’s surface. In the last observation of this week,
SSI returns a global scale image of the icy moon.

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:

http://galileo.jpl.nasa.gov
http://www.jpl.nasa.gov/galileo
http://galileo.ivv.nasa.gov