October 10, 1999
Galileo Spacecraft Penetrates Deeply into Jupiter's Radiation
The excitement continues today on Galileo as the spacecraft passes closest approach to Jupiter
and Io, penetrating deeply into the intense radiation environment surrounding the gas giant.
During the day, the spacecraft has distant flybys of the other three Galilean moons, but the
observing plan is focused completely on the Io flyby. Two exceptions to this focus are minor
observations of Jupiter performed by the Photopolarimeter Radiometer, which are designed to
capture data on atmospheric waves in Jupiter's cloud cover.
The spacecraft flies past Callisto first at 1:10 am PDT-SCET (1:43 am PDT-ERT, see Note 1) at
a distance of 1.2 million kilometers (0.75 million miles). Approximately four hours later, the
spacecraft makes its closest approach to Ganymede at a distance of 923,000 kilometers (574,000
miles). Eight and a half hours after Ganymede, the spacecraft flies past Europa at a distance
of 221,000 kilometers (137,000 miles). Two and a half hours prior to Io closest approach, the
spacecraft flies past Jupiter at a distance of 5.5 Jupiter Radii (393,000 kilometers or 244,000
miles). Saving the best for last, the spacecraft flies past Io at 9:33 pm PDT-SCET (10:06 pm
PDT-ERT), skimming past the fiery moon's surface at a distance of 612 km (380 miles).
The Photopolarimeter Radiometer (PPR) performs the first remote sensing observation and the
first observation directly related to the Io flyby. The observation captures data covering the
entire globe of Io, with the objective of obtaining measurements for scientists to perform
thermal studies, including the determination of hot spot and passive background temperatures.
Just after 12:06 pm 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.
Six hours before Io, the Fields and Particles instruments begin a five hour, 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 sample the torus from
approximately 6.4 Jupiter radii (458,000 kilometers or 284,000 miles) through Jupiter closest
approach at 5.5 Jupiter radii and then back outward to Io at 5.9 Jupiter radii (422,000 kilometers
or 262,000 miles). This will be the deepest passage through the Io torus since the spacecraft's
arrival at Jupiter in December 1995. 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. They are also important for understanding the overall dynamics of the Jovian
The PPR resumes remote sensing with two more observations of Io's surface. PPR first targets
the Acala region and then the Loki region. Both regions are on Io's night side at the time,
allowing scientists to separate thermal radiation due to volcanism at Io's surface from thermal
radiation due to passive solar heating of the surface during Io daytime.
Starting 50 minutes prior to closest approach to Io, the Fields and Particles instruments
record their data to the tape recorder for 65 minutes. As they did during the Io torus recording,
the instruments acquire measurements describing the plasma, dust, and electric and magnetic
fields surrounding Io, including electromagnetic waves and radio signals. These data will assist
scientists with studies of the Io ionosphere and its interaction with the Jovian magnetosphere.
Measurements made by individual instruments can be combined to better understand processes such
as particle pickup by the magnetic field, and thermal and non-thermal plasma interactions near Io.
The PPR then takes another look at Io and the volcanic region Loki. Looking again at the moon's
night side, the observation provides more data that will allow scientists to separate thermal
radiation effects from passive heating due to sunlight. The Near-Infrared Mapping Spectrometer
(NIMS) also looks at Loki while the volcano is still on Io's night side. The observation is
designed to search for thermal emissions from the volcano's caldera.
The Near-Infrared Mapping Spectrometer then begins a series of observations performed in
conjunction with the Solid-State Imaging camera (SSI). Together, the pair of instruments
take turns looking at the same target on Io's surface. Their first target is the Pele volcanic
region, which is on Io's night side during the observation. The NIMS searches for thermal
emissions from the Pele caldera, while the SSI captures high-resolution images of the
region. The images are take in the dark wit h the hope of catching hot glowing lava near
Pele's volcanic vent. The instruments' next target is the Pillan volcanic region. The view
provided by the spacecraft is somewhat oblique, but provides good low-sun illumination. The
SSI obtains some high-resolution images of the region, while the NIMS continues to look for
thermal emissions. These are the final observations performed prior to the spacecraft's close
flyby of Io.
Observations of the Colchis Montes region are the first performed by the NIMS and SSI in full
sunlight. These dayside observations allow the NIMS to gather data describing the composition
of the surface, while the SSI continues to take high resolution images. High resolution imagery
and surface composition observations continue with a look at the Zamama volcanic vent, followed
by the Prometheus volcanic vent and associated lava flows. A comparison of clear and green filter
images of Prometheus are expected to reveal unresolved lava and allow scientists to determine
The instrument pair then returns to the Colchis Montes region with a wider, lower resolution
observation that should provide a context for the higher resolution data. Next on the schedule
is an observation of Tohil Mons, followed by a return to the Prometheus region. The SSI
observation at Prometheus is in color, and will be combined with the previous set of images
to provide stereo coverage. The NIMS and SSI pair then take another look at the Zamama volcanic
vent, providing coverage of a wider region as context for the higher resolution observations
performed earlier. The next target to present itself is Dorian Mons, which is characterized by
greenish colored deposits. The Dorian, Tohil and Colchis features are mountains, whose
geological structure, origin and history are presently unknown.
After Dorian Mons, the NIMS and SSI obtain moderate resolution images and spectrometer scans
of the Amirani, Skythia, and Gish Bar regions. The instrument pair then looks at a region of
Io's surface near the sun terminator (or line dividing night from day). The oblique lighting
provides conditions that are optimal for studying the topography of a region containing the
As the spacecraft recedes from the fiery moon, the PPR returns to the observation schedule
with a regional observation of the Amirani and Maui regions. The observation provides scientists
with the opportunity to study the stability of surface volatiles on the moon. The NIMS also
performs a regional observation of Io designed to study surface composition and detect thermal
In a final pairing for the day, the NIMS and SSI take a look at the Pillan plume. The
observation looks back at the Pillan hot spot as it sits on Io's limb. If Pillan is active,
its plume should be visible against the dark sky above the limb, providing scientists with
the best look to date at a plume's size, shape,
The last observation of the day is performed by the PPR. A regional observation that captures
the Bland, Prometheus, and Cullan regions in a North-South strip, it is also designed to study
the stability of the surface volatiles on the moon.
BUT WAIT! The excitement does not conclude with today's activity. Tune in tomorrow to find out
what else will happen during this thirteenth encounter of the Galileo Europa Mission!
Note 1. Pacific Daylight Time (PDT) is 7 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 33 minutes to travel between the
spacecraft and Earth.