PART 1: Galileo WebChat tonight
PART 2: New on Galileo's web site: instrument info and color Io image
PART 3: Preparing for the Ganymede encounter
PART 4: Jubilant engineers and scientists
PART 5: A great team delivers precious jewels
PART 6: Galileo spacecraft makes new discoveries at Ganymede
PART 7: Physics news update: Io may generate a magnetic field of its own

Got a question about Galileo's first science results from Ganymede?

Key members of Galileo's engineering and science teams will be
available for an interactive on-line question-and-answer period on
Thursday, July 18, from 4:00-8:00PM PDT (PDT = UTC - 5 hours).

The discussion will take place at a special WebChat area made just for
this purpose. The URL is

To participate on that night, you need only to have a forms-capable Web
browser.


Please join us with some well thought-out questions.

Visit the Galileo Home Page at http://www.jpl.nasa.gov/galileo/

for the following information:

Detailed descriptions of each of the 12 science experiments carried on the
Galileo orbiter is now available on the Galileo Home Page, including

  - Photograph of the instrument
  - Mission objectives
  - Instrument summary and description including diagram
  - Design details
  - Instrument parameters

Galileo's First Io Color Image (P-47109 color)

The mottled face of Jupiter's volcanically active moon Io [pronounced
"EYE-oh" or "EE-OH"], viewed by the camera onboard NASA's Galileo
spacecraft, shows dramatic changes since it was seen 17 years ago by
the exploratory NASA spacecraft Voyagers 1 and 2. This Galileo image,
taken on June 25, 1996 at a range of 2.24 million kilometers (1.4
million miles), is centered on the Media Regio area and shows details of
the volcanic regions and colored deposits that characterize Io. North is
at the top of the picture and the Sun illuminates the surface from the
east (right). The smallest features that can be discerned here are
approximately 23 kilometers (14 miles) in size, a resolution comparable
to the best Voyager images of this face of Io.

Io's surface is covered with volcanic deposits that are thought to contain
ordinary silicate rock, along with various sulfur-rich compounds that give
the satellite its distinctive color. In the brighter areas the surface is 
coated with frosts of sulfur dioxide. Dark areas are regions of current or 
recent volcanic activity. Planetary scientists say many changes are evident in 
the surface markings since this region of Io was imaged 17 years ago by the
Voyager spacecraft. The bright regions near the eastern limb of the moon are
much more prominent in the Galileo images than they were previously. Surface
details have also changed dramatically in the vicinity of the eruptive volcano
Masubi (the large, predominantly white feature seen near the 6 o'clock
position in this view).  Masubi was discovered as an active volcano during the
Voyager encounters of Io in 1979.

NASA's Galileo spacecraft proceeded toward its first close flyby of
Jupiter's big moon, Ganymede, scheduled to occur at 6:29 a.m. Universal
Time on June 27, 1996 (11:29 p.m. on June 26 Pacific Daylight Time).
One-way light time from the spacecraft to Earth at that time will be 35
minutes, so the spacecraft's signal showing that the closest approach
has occurred will be received on Earth at 12:04 a.m. PDT June 27.

Initial observations of the Io plasma tours by the ultraviolet experiment
are complete and the first remote observations of Io by the camera
were done today. Tomorrow, Galileo's instruments will be looking at
both Ganymede and Jupiter's Great Red Spot.

Yesterday afternoon, Galileo's energetic particle detector (EPD) was
autonomously turned off by the spacecraft and placed in a "safe" mode.
This is a standard safety feature built into the particle detector's
operating software and is triggered if the instrument's own computer
detects that any one of a number of readings are above or below 
predetermined limits. The automatic turn-off allows EPD engineers to
search for the cause of the anomaly and determine whether the
instrument can safely be turned on.

To avoid interfering with the Ganymede encounter sequence now
being executed by the spacecraft, Galileo engineers have decided to
leave the EPD instrument off until at least a day or two after the
Ganymede flyby is completed. Meanwhile, engineering data being
received from the spacecraft may point toward the problem that
initiated the instrument's automatic shut-off. No other scientific
instruments are affected and all of Galileo's other observations are
proceeding as planned.

The EPD is one of several instruments on Galileo that measure Jupiter's
magnetic fields and particles. Systematic measurements of the Jovian
magnetic environment and particle population began on Sunday. The
instruments will continuously send data back to Earth during Galileo's
close passes of Jupiter, the moons and from other specially chosen
locations within the planet's magnetic environment.

Today Galileo is 1.3 million kilometers (862,000 million miles) from
Ganymede and 627 million kilometers (389 million miles) from Earth.
One-way communication time is about 35 minutes. Galileo's is
approaching Ganymede at a speed of 16 kilometers per second (30,900
miles per hour).

Engineers and scientists on NASA's Galileo mission were
jubilant early this morning as the spacecraft completed the first
targeted encounter of its orbital tour at Jupiter, a close flyby
of the giant planet's moon Ganymede.

Galileo flew by the icy moon at 06:29 Universal Time today
(11:29 p.m. Pacific Daylight Time on June 26), passing within 832
kilometers (517 miles) of Ganymede at a relative speed of 7.8
kilometers per second (about 17,400 miles per hour). One-way
light time from the spacecraft to Earth at that time was 35
minutes, so the spacecraft's signal showing that the closest
approach has occurred was received on Earth at 12:04 a.m. PDT
today.

"The data tell us that we had an excellent flyby," said
Galileo Project Manager William J. O'Neil. Ground controllers
detected changes in the frequency Galileo's radio signal as it
swung by Ganymede due to the Doppler effect, confirming that the
flyby took place as planned. In addition, configuration changes
executed by the spacecraft shortly after the flyby confirmed that
it was executing its command sequence as planned.

Team members expect to receive additional, detailed telemetry from
Galileo today with more information on performance of various
spacecraft systems during the flyby.

The first images and other scientific data from the flyby
will be sent to Earth during the following days. If all goes
well, the first images will be released at a news briefing
tentatively scheduled for July 10.

June 28, 1996

"Jubilant" is a good word describing how *I* felt last night. Prior to
January of this year, my involvement with Galileo has revolved around
the engineering challenges of the delivery of the Probe to Jupiter and
orbit insertion. Since then I have worked more with the Science Team in
testing the software and strategies that comprise "Phase 2A", and have
come to respect this very dedicated group of people. I have been, am, and
will be very excited for them to at last see the collection of data for
which they have been working for more than a decade, rising to
overcome many obstacles along the way. I am proud that our test team
has and continues to support this effort. The encounter is filling a
treasure-chest with an astonishing variety of precious jewels. When
playback starts Monday, we will start taking those jewels out 
one-by-one with plenty of time to "oooh.." and "aaaah..." as we examine 
each one
of them. It would have been NICE to be able to dump them all out on the
floor at once, but they are still beautiful jewels...

Project personnel have been monitoring the progress of the spacecraft, and
collecting the small amount of real-time (live) science data being
transmitted to the ground. The G1B sequence, which will control the start
of the playback of collected data, has been loaded aboard the spacecraft.
Commands were transmitted last night to adjust the tape recorder position
so that data recording did not jeopardize the recorded "markers" that
define the end of the usable tape area. (A personal note here, despite the
fact that IMHO this encounter is the real "maiden voyage" of the new flight
software, and I expected we would find some shoals along the way, this is
the only such adjustment I know of so far, indicating that the Flight,
Science, and Sequence Teams have done an excellent job of understanding
how to use it.)

Final preparations for OTM-7 are underway in preparation for loading its
controlling sequence tomorrow afternoon (PDT) and for execution just after
midnight Saturday night (PDT). Commands slated for uplink tomorrow morning
will modify operation of the "Playback Manager" to avoid some problems that
were discovered in system testing in the Galileo Testbed. In the meantime,
the SSI camera will image volcano plumes and then join the Near-Infrared
Mapping Spectrometer and Photopolarimeter to observe the Io eclipse.
Finally tomorrow evening around 7:30pm (Saturday PDT, 0230 Sunday UTC) F&P
instruments will record the plasma sheet crossing, and the encounter will
be over. Playback should begin around 9:30pm Sunday night (PDT). I can't
wait...

NASA's Galileo spacecraft has returned stunning 
close-
ups of Jupiter's moon Ganymede revealing that the face of the
huge satellite has been extensively bombed by comets and
asteroids and dramatically wrinkled and torn by the same
forces that make mountains and move continents on Earth.

"These images have exceeded our wildest
expectations," said Dr. Michael Belton of the National Optical
Astronomy Observatories, who leads Galileo's imaging team.

At the same time, scientists studying data from space
physics instruments on the spacecraft have made the major
discovery that planet-size Ganymede possesses its own
magnetosphere -- a bubble-shaped region of charged particles
that surrounds many of the planets but has never been found
to exist around a moon. The finding indicates that Ganymede,
which is three-quarters the size of Mars, very likely creates
its own magnetic field. Possible sources of a magnetic field
include a molten iron core or even a thin layer of conducting
salty water underneath its icy crust. 

"What we've found is a magnetosphere within a
magnetosphere," said Galileo Project Scientist Dr. Torrence
V. Johnson at NASA's Jet Propulsion Laboratory, Pasadena, CA.
"While we expected some degree of interaction between
Ganymede and Jupiter's magnetic environment, the size and the
effect at Ganymede were completely unexpected," he said.

The crisp new images and magnetospheric findings were
revealed in data returned by Galileo in the days since its
first flyby of Ganymede on June 27, when the spacecraft came
within just 519 miles of the big moon. Ganymede is the
largest moon in the solar system. It is made of about equal
proportions of rock and water ice. It is one of Jupiter's
four large satellites that will be repeatedly visited by the Galileo
spacecraft over the course of its two-year mission in orbit
around the giant planet. Galileo entered orbit around
Jupiter on December 7 last year. The spacecraft was launched
from Earth on October 18, 1989.

The discoveries announced today are based on just a
small portion of the data gathered and returned from the
Ganymede flyby and mark the start of a steady stream of
images and other information to be returned from Galileo over
the next 18 months. The data were returned using new
software radioed to the spacecraft earlier this year that
allows Galileo to send back its scientific findings in
shorthand form. This helps compensate for the loss of the
use of Galileo's high-gain antenna and allows Galileo to
return its findings via the smaller low-gain antenna also on
the spacecraft.

These first images show two of the regions selected
for close photographic study on Galileo's first pass of
Ganymede yielded surprising new information about its
geological past. The areas, called Galileo Regio and Uruk
Sulcus, both show ancient cratered ice fields adjacent to or
overlain by younger ice volcanic plains, ridged ice
mountains, deep furrows and smooth broad basins that are
products of tectonic forces. About half of Ganymede's older
cratered surface appears to have been resurfaced by younger
volcanic and tectonic activity. 

"These images reveal fundamental details about how
features seen by Voyager formed and show us age relationships
and sequences that turn our previous thinking upside down,"
said Imaging team member Dr. James Head of Brown University.

The discovery of Ganymede's magnetosphere was made by
space physicists using data from Galileo's plasma wave
spectrometer, which measures variations in electromagnetic
waves in Jupiter's environment and from the magnetometer,
which measures the strength and direction of magnetic fields.
Both instruments were sending data to Earth during the
Ganymede flyby while recording even more detailed information
to be returned later this month.

The plasma wave spectrometer also showed that the
densities of charged particles around Ganymede increased by a
factor of more than 100 near Galileo's closest approach. 
"This indicates that Ganymede is surrounded by a thin
ionosphere," said Dr. Donald A. Gurnett of the University of
Iowa and principal investigator on the plasma wave
spectrometer experiment. "The existence of an ionosphere
suggests that Ganymede also probably has a tenuous
atmosphere," he said.

As the spacecraft approached Ganymede, the
magnetometer found the measured field was as expected at that
position in Jupiter's powerful field -- fairly uniform and
pointed in a southerly direction. But as the spacecraft
crossed into the region where the plasma wave spectrometer
sensed signals characteristic of a magnetosphere, the field
increased in strength by a factor of nearly five and abruptly
changed direction to "point" at Ganymede itself, said Dr. Margaret
Kivelson of the University of California at Los Angeles, principal
investigator of the magnetometer experiment.

Taken together, these two measurements strongly
suggest that Ganymede is the first known moon with its own
magnetosphere and the first example ever seen of a
"magnetosphere within a magnetosphere."

"We knew Ganymede was an interesting place," said
Johnson. "What we have just found makes it even more exciting."

The new discoveries will be quickly followed up by
other data to be returned by the spacecraft this summer. All
the experiments on Galileo that measure magnetic fields and
particles recorded detailed data during the close approach,
and these data will be played back from the tape recorder in
the next two months. "With all the data in hand, we will
gain better insight into what is causing the strange
environment around this moon," said Johnson. The Galileo
science and engineering teams are planning three more close
flybys of Ganymede over the next 18 months, which will take
the spacecraft to different regions of the big moon's
magnetosphere and allow close study of other regions of its surface.

The Galileo spacecraft recently measured the magnetic field in the
vicinity of Jupiter's moon Io and found the field strength to be
approximately 38% lower than the 1860 nanotesla expected if only
the field originating at Jupiter itself were present. Researchers have
previously speculated that additional fields may be generated near
Io by the presence of accelerating ions in the moon's neighborhood.
But at the May meeting of the American Geophysical Union in
Baltimore, Margaret Kivelson of UCLA suggested that even the
most charitable estimates on the numbers of ions encountered by
Galileo during the measurements could not account for this sharp
dip in the magnetic field. The most likely way to explain the
results, Kivelson said, would be if Io's core (known to be heavy and
currently believed to consist of iron or an iron-iron sulfide mixture)
generates a magnetic field, perhaps through the sloshing of molten
fluid in the core; this is essentially what happens inside Earth and
Mercury. If this hypothesis holds up to more detailed analyses of
Galileo's ion flux measurements, Io would be the first moon known
to produce its own magnetic field. (Upcoming article in Physics
Today, July 1996).

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