Galileo deftly threads its way through the heart of the Jupiter system
today, diving deep into the intense radiation environment that surrounds
the solar system’s biggest planet. The spacecraft has distant flybys of
Jupiter and Ganymede, with a relatively close flyby of Europa, and an
extremely close flyby of Io. The geometries of both the Europa and Io
flybys are somewhat unique in that they provide good views of the polar
regions of these bodies. The onboard instruments spend the day performing
observations of the latter two Galilean satellites.

Eleven hours and 36 minutes prior to closest approach to Io, the spacecraft
passes closest approach to Europa at a distance of 8,642 kilometers (5,371
miles) from the moon’s surface. Nine hours and 40 minutes later, the
spacecraft flies past Jupiter at a distance of 5.7 Jupiter Radii (406,000
kilometers or 252,000 miles). The close flyby of Io is next on the flyby
schedule. The spacecraft passes 300 kilometers (186 miles) from the
surface of Io at 8:05 pm PST-SCET (8:40 pm PST-ERT). Three hours and nine
minutes later, the spacecraft flies past Ganymede at a distance of 611,000
kilometers (380,000 miles) from the moon’s center.

Observations of Europa occupy the first half of the day. In addition to
the views of Europa’s north pole, the flyby provides unique geometry in
that Galileo can see the hemisphere of Europa that faces Jupiter. Such
observations are difficult, if not impossible, to perform from Earth. The
Photopolarimeter Radiometer (PPR) makes the first observations of the day,
looking at Europa’s dark side. The observation will be compared to day
side observations of the same regions performed on previous orbits in hopes
of identifying heat sources or thermal anomalies originating from within
the icy moon. In the second observation, PPR takes polarimetry
measurements of Europa’s surface. The measurements will allow scientists
to study Europa’s surface texture and thermal properties.

The Solid-State Imaging camera (SSI) is next on the observation schedule,
and is followed closely by an observations performed by the Near-Infrared
Mapping Spectrometer (NIMS). Both of these instruments take a look at
Europa’s north pole. These observations will provided the highest
resolution views of the polar region to date. SSI follows these
observations with two more observations of features on Europa’s surface.
The first captures images of a pair of dark bands first detected in
September 1996. The region is believed to be the site of relatively recent
faulting and relative movement of blocks of Europa’s crust. The second
observation captures a view of mottled (or blotchy-looking) terrain
believed to be related to ice volcanic flows. NIMS also looks for evidence
of plate tectonics on Europa’s surface.

PPR returns to the observation schedule with a dayside thermal map of
Europa. The map captures a region that was also observed on Europa’s night
side in September 1998. The two sets of data covering this region will be
compared in hopes of determining if the anomalous temperatures that were
detected in the first observation are due to Europa’s heat retention
characteristics (or thermal inertia) or possibly due heat from volcanic
activity inside Europa.

Just after 10:40 am PDT-ERT, the radio science team begins to carefully
measure changes in the frequency of Galileo’s radio signal. The changes
are caused by Io’s gravitational pull on the spacecraft, and the resulting
Doppler shift in Galileo’s radio signal. The radio scientists will track
these changes for almost 20 hours, centered on the point of closest
approach to Io, and will use the measurements to refine models of Io’s
gravity field and internal structure.

SSI is next to observe Europa and does so by taking a global scale image of
the icy moon. PPR follows with another polarimetry map, followed by a
regional observation performed by NIMS. The latter observation will aid
scientists in constructing complete global maps of Europa. This
observation also brings to a close the Europa portion of the encounter’s
observing campaign.

The Fields and Particles instruments perform the first of the Io-related
observations. Seven hours and 17 minutes before closest approach to Io,
the instruments begin a six hour and 40 minute high resolution recording of
the Io torus. The Io torus is a region of intense plasma and radiation
activity, in which there are strong magnetic and electric fields. The
recording will gather data down through closest approach to Jupiter (5.7
Jupiter Radii), the third deepest Torus passage of Galileo’s primary and
Europa missions. The data acquired during this 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.

PPR is the first instrument to perform remote sensing of Io. Its first
observation of Io captures a global dark side map. The map will describe
night time thermal emissions on Io and will aid scientists in the
development of heat flow models. The second observation also captures Io’s
night side, but is regional in resolution and contains data on the southern
hemisphere hot spots of Babar Patera, Sengen Patera, and Ulgen Patera.
PPR’s final observation captures a high resolution view of Pele’s volcanic
vent. The observation should allow scientists to pinpoint the exact
location of the volcano’s vents.

Starting 18 minutes prior to closest approach to Io, the Fields and
Particles instruments record their data to the tape recorder for 49
minutes. As they did during the Io torus recording, the instruments
acquire measurements describing the plasma, dust, and electric and magnetic
fields surrounding Io. The primary purpose of this observation is to
determine if Io possesses its own internally-generated magnetic field,
similar to both the Earth and to another Galilean satellite, Ganymede.

Galileo’s flight path takes the spacecraft above Io’s south polar region
and was chosen in order to make this observation. In addition,
measurements made by individual instruments will be combined to better
understand processes such as particle pickup by the magnetic field, and
thermal and non-thermal plasma interactions near Io.

SSI and NIMS follow PPR’s remote sensing observations with a series of
observations designed to characterize Io’s surface at resolutions higher
than ever before. By collaborating, the instruments will provide a wealth
of useful information on the morphology, thermal state, and composition of
surface materials.

NIMS makes the first observation of the series by looking at a hot spot
called Tiermes. Both SSI and NIMS then take a look at Io’s south pole.
Io’s south polar region is relatively unknown, and these observations will
provide new information about this area. SSI follows next with a solo
observation of a feature that was seen to be “sapping” in an observation
that was made in June 1999. Sapping is the natural process of erosion
along the base of a cliff by which soft layers are worn away. The erosion
removes the support for the upper part of the cliff which then breaks off
in large blocks and falls from the cliff face. NIMS then takes a look at
Io while the Prometheus volcanic region is on the limb as seen from the
spacecraft. The observation will be used to examine atmospheric materials
at various heights above Io’s surface.

SSI and NIMS join again for the next several observations. In the first,
the instrument pair looks again at Io’s south pole. In the next couple of
observations, the pair looks at Emakong Patera in hopes of catching active
lava flowing from the hot spot. Next, SSI and NIMS look at Tupan Patera, a
caldera-like feature from which information about the form and distribution
of the hottest materials on Io’s surface is expected to be gleaned. In
another pairing, SSI and NIMS jointly look at the sapping feature detected
in June 1999.

SSI continues the observation campaign by looking a hot spot called Shamshu
Patera. This is followed by a solo observation from NIMS of the Tupan
Patera region. SSI and NIMS then join back up and make another observation
of Emakong Patera. Together with the previous images, scientists will be
able to construct stereo views of Emakong.

As the spacecraft recedes from Io, NIMS and SSI perform another joint
observation. In it, the two instruments look at two unnamed giant volcanic
calderas in Io’s northern hemisphere. This activity is followed by
observations of the Culann volcanic region. SSI contributes a color image
of the region to the growing Io data set. In the last joint observation,
the instrument pair look at a region of Io’s surface near the terminator
(or line dividing night from day). The last observation of the day is
performed by NIMS and contains a regional map of Io’s surface.

Come back tomorrow and learn of Galileo’s remaining observations and
initial plans to return all the data stored on board!

Note 1. Pacific Standard Time (PST) is 8 hours behind Greenwich Meridian
Time (GMT). The time when an event occurs at the spacecraft is known as
Spacecraft Event Time (SCET). The time at which radio signals reach Earth
indicating that an event has occured is known as Earth Received Time (ERT).
Currently, it takes Galileo’s radio signals 35 minutes to travel between
the spacecraft and Earth.

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: