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U P D A T E # 3 5 Part 1: Getting
the tape recorder ready Greg LaBorde provided the following spacecraft status reports: At UTC 96-172/23:52:00 (Thu 20 Jun 4:52pm PDT) Playback of the Io Torus data finished yesterday morning. There is still about 10% more of the data that will be taken off the tape recorder later. A mini-sequence to perform "conditioning" of the tape was loaded aboard yesterday. It will execute tomorrow (Friday) at 11:40 UTC (5:15am PDT Earth-Received-Time) to run the tape from end-to-end to "repack" it and distribute any contamination over the tape (which, interestingly enough, is exactly what tapes and tape-recorders are designed to do...), following which it will position the tape at the starting point for the Ganymede Encounter #1 recording. Early this morning (PDT) the G1A Part 1 sequence was also loaded aboard. This sequence controls spacecraft activities from Sunday at 16:35 UTC (9:35am PDT, the official start of the encounter) until 46 minutes before Ganymede closest approach on Thursday at 07:04 UTC (12:04am PDT). At UTC 96-174/03:18:00 (8:20pm PDT Fri 21 June) The mini-sequence to perform "conditioning" of the tape executed this morning (PDT) without a hitch. The tape is now in position at the starting point for the Ganymede Encounter #1 recording. The G1A part 1 sequence is loaded aboard and is "active," but is currently waiting until Sunday at 16:35 UTC (9:35am PDT, the official start of the encounter) to execute any commands. G1A part 1 controls spacecraft activities from Sunday until 46 minutes before Ganymede closest approach on Thursday at 07:04 UTC (12:04am PDT), at which point G1A part 2 takes over. By Monday morning, Galileo will have completed an entire day of Fields and Particles measurements of the environment 3 million km from Jupiter. From that point on, an ever-increasing series of science observations will occur, rising to a crescendo on Wednesday and Thursday as the remote sensing instruments (the camera and near-infrared mapping spectrometer, and the photopolarimeter) collect close-up data from a mere 384km above Ganymede. It's SHOWTIME!!!! June 24, 1996 Now residing in orbit around Jupiter, NASA's Galileo spacecraft is primed for its first close flyby of Jupiter's largest moon, Ganymede, at 2:29 a.m. EDT on June 27, 1996. Equipped with 10 scientific instruments, Galileo will fly just 524 miles above Ganymede's surface to provide the most detailed images and other information ever obtained about the icy satellite. Galileo will be 70 times closer to Ganymede than Voyager 2 and 133 times closer than Voyager 1. Images and other data gathered by the spacecraft will be radioed back to Earth in the hours and months following the flyby. On June 23, Galileo's particle detectors and magnetic fields instruments began making nearly continuous measurements as the spacecraft approached Ganymede. Its optical instruments will shortly begin their periodic observations, including the first round of picture-taking (other than engineering images taken for navigation purposes) since months before the spacecraft entered orbit around Jupiter on Dec. 7, 1995. If spacecraft operations near Ganymede and the subsequent transmission to Earth of initial science data occur as planned, selected images of Ganymede taken by Galileo will be released in a televised news conference at the Jet Propulsion Laboratory, Pasadena, CA, tentatively scheduled for July 10. With a diameter of 3,269 miles, Ganymede is the largest moon in the Solar System -- bigger than Mercury and about three-quarters the size of Mars. It possesses a variety of familiar Earth-like geologic formations including craters and basins, grooves and mountains. The bulk of the satellite is believed to be about half water-ice and half rock. Portions of its surface are relatively bright, clean ice while the other regions are covered with darker "dirty" ice. The darker areas appear to be ancient and heavily cratered, while the lighter regions display evidence of tectonic activity that may have broken up the icy crust. A thin layer of ozone has been found in Ganymede's surface by Earth-based astronomers. Galileo will return high-resolution images showing features on Ganymede as small as 33 feet across. Instruments on board will assess Ganymede's surface chemistry and search for signs of an atmosphere around the big moon. Measurements will be made to characterize Ganymede's gravity field and to determine if it possesses a magnetic field. In the days just before and after the Ganymede flyby, Galileo's other studies will include a search for auroral activity on Jupiter's nightside and observations of other Jovian moons: Io, Europa and Callisto. The "Io torus," a hot, doughnut-shaped ring of charged particles swirling about Jupiter at Io's distance, will be another target of study, as will Jupiter's Great Red Spot. Galileo's Ganymede encounter marks the start of a steady stream of data to be returned to Earth by Galileo's instruments throughout the course of its two-year tour of the Jovian system, which continues through December 1997. Beginning in July, data return will include an average of two to three images per day. The remainder of Galileo's mission is to complete 11 orbits of Jupiter, conducting multiple close flybys of the moons Ganymede, Europa and Callisto, with numerous, more distant studies of the moon Io also scheduled throughout the tour. Studies of Jupiter itself are planned throughout the tour, and nearly continuous studies of Jupiter's enormous radiation and magnetic fields will be conducted. The fifth planet from the Sun is known primarily for the banded appearance of its upper atmosphere and its centuries-old Great Red Spot, a massive, hurricane-like storm as big as three Earths. Jupiter generates the biggest and most powerful planetary magnetic field, and it radiates more heat from internal sources than it receives from the Sun. Given its large size and its many natural satellites, Jupiter is often described as a miniature solar system. Jupiter has 318 times more mass and 1,400 times more volume than Earth, but is only 1/4th as dense, since it is composed primarily of hydrogen and helium. It is orbited by at least 16 moons (and Galileo -- its first artificial satellite). The 2-1/2-ton Galileo orbiter spacecraft was launched aboard Space Shuttle Atlantis on Oct. 18, 1989. It carries the most capable payload of scientific experiments ever sent to another planet. Communications to and from Galileo are conducted through NASA's Deep Space Network, using tracking stations in California, Spain and Australia. An innovative combination of new, specially developed software for Galileo's on-board computer and improvements to ground-based signal receiving hardware in the Deep Space Network will enable the spacecraft to accomplish at least 70 percent of its original mission science goals using only its small, low-gain antenna, despite the failure of its high-gain antenna to unfurl properly in April 1991. NASA's Jet Propulsion Laboratory built the Galileo orbiter spacecraft and manages the overall mission. Galileo's atmospheric probe, which plunged into the planet on Dec. 7, 1995, was managed by NASA's Ames Research Center, Mountain View, CA.
A PHOTO CONTEST held by PROJECT GALILEO
On June 27, 1996, the Galileo spacecraft will fly just 524 miles (844 km)
above the surface of Ganymede, a moon of Jupiter (and the largest moon in our
solar system). The imaging camera on board will take several close-up pictures
of the moon, at a higher resolution than any previous spacecraft. Images of two
selected regions on the moon, called Uruk Sulcus and Galileo Regio, will be
played back to Earth in early July and described at a press briefing on July
10, 1996.
What will we see in those images of the rutted and pock-marked surface of
Ganymede? This contest will help you consider the similarities between these
regions on Ganymede and places here on Earth!
- To enter the contest, first read more about Uruk Sulcus or Galileo
Regio below.
- Then, send us your photograph or slide of someplace on Earth which
you think will most closely represent one of the regions to be
revealed during the flyby, together with your reasons for choosing that
place.
- Your entry should be sent by regular mail and postmarked by Friday,
July 5 or by email by Monday, July 8. See below for more details on the
contest rules.
- Winners will be announced on the Galileo Web site on Wednesday,
July 10 at 5 p.m. Pacific Daylight Time, following the
"Galileo Mission Update" press briefing.
- If your entry is chosen as the winner for either region, the winning
photograph will appear on the Galileo Web Site, for the world to
see! In addition, we will send you a complete set of lithographs
collected by Galileo on this orbit around Jupiter's system, and a
certificate of recognition for your achievement!
Uruk Sulcus
See Voyager image at
http://www.jpl.nasa.gov/galileo/sepo/atjup/ganymede/GG1URSUL.html.
The Galileo image will have a resolution about 14 times
better than the Voyager image, and will be able to see features 70 meters
across or bigger. For the contest, focus your attention on the bright areas
of Uruk Sulcus (inside the big white square) where the grooves are. Given
that the total size of the white square is about 100 kilometers (60 miles),
you should be able to tell the approximate length and width of the grooves.
We know that water, mostly in the form of ice, exists on Ganymede. Do you
think these grooves could have been made from water or ice moving on the
surface? (Think about places like this on the Earth.) Can you think of other
reasons that grooves or trenches form on the Earth (even under the oceans at
great depths)?
Where on Earth might we find a similar looking feature to the grooves in
Uruk Sulcus? Send us your photograph!
Galileo Regio
See Voyager image at
http://www.jpl.nasa.gov/galileo/sepo/atjup/ganymede/GG1GALREG.html.
Galileo Regio, in this Voyager image of Ganymede, is located in the lower
right corner and marked by a white square with a double white cross (at
about 20 degrees North and 130 degrees West on Ganymede). The image mosaic
taken by the Galileo spacecraft will be about 100 kilometers (60 miles)
across, and will have a resolution about 14 times better than the one shown
from Voyager. Galileo will see features that are at least 70 meters across
(just smaller than a football field)! The image contains craters and wide
elongated features called furrows. How do you think craters on Ganymede are
similar to or different from craters on Earth? What do you notice about the
number of craters on the surface of Ganymede versus the number we know about
on Earth? Why do you think that the numbers are different?
Where on Earth might we find a feature similar to the craters or furrows in
Galileo Regio? Send us your photograph!
Galileo Photo Contest Rules
- Photos can be taken of a natural feature on Earth from any angle
(for example, from above in an airplane, while standing on the edge
of the feature and looking down at it, or a shot from inside looking
out).
- Public satellite images are not eligible. Entries must be a personal
photograph.
- The contest is open to those 6 years of age and older. One entry per
region of Ganymede per person.
- If you want to try to match both regions, send a separate entry for
each region Uruk Sulcus or Galileo Regio.
- You MUST include the name and location of the feature on Earth in
your entry, along with a description in 75 words or less, of why you
think this will look like the Uruk Sulcus or Galileo Regio areas on
Ganymede. Photos without such a description will not be eligible.
- Be sure to include your name, age (if under 21), and mailing
address, in case your photograph is selected as a winner. Entries
will not be returned.
- TO BE ENTERED IN THE CONTEST, ENTRIES MUST BE POSTMARKED:
- VIA SURFACE MAIL NO LATER THAN FRIDAY, JULY 5, 1996
- VIA EMAIL NO LATER THAN MONDAY, JULY 8, 1996
- JPL employees and contractors, and members of the Galileo Science
Team, and their families are not eligible.
- Entries will be judged by a geologist and members of the Galileo
Outreach Team at JPL.
Submission Addresses
Entries can be submitted in two ways:
Mail in hardcopy form (photo and text) to:
Galileo Public Outreach Office
Mail Stop 264-765
4800 Oak Grove Drive
Pasadena, California 91109-8099
Electronically by email with ASCII text and attached gif picture format to:
galileo-photo-contest@solstice.jpl.nasa.gov
Ron Baalke
http://www.jpl.nasa.gov/galileo/ganymede/fact.html
What Is Known About Ganymede SO FAR
Table of Contents
* Ganymede Summary
* Ganymede Quick-Look Statistics
* The Discovery of the Galilean Satellites
* Ganymede Images
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Ganymede Summary
Ganymede is the largest satellite in the solar system with a diameter of
5,262 km (3270 miles). It is larger than Mercury and Pluto, and
three-quarters the size of Mars. If Ganymede orbited the Sun instead of
orbiting Jupiter, it would easily be classified as a planet.
Since Ganymede has a low density of 1.94 grams/cubic centimeter (water's
density = 1.00), it is estimated that the satellite is half water ice with a
rocky core extending to half of the satellite's radius. The mantle is
composed of ice and silicates and a crust which is probably a thick layer of
water ice.
Voyager images of Ganymede has shown that the satellite has a complex
geological history. Ganymede's surface is a mixture of two types of terrain.
Forty percent of the surface of Ganymede is covered by highly cratered dark
regions, and the remaining sixty percent is covered by a light grooved
terrain which forms intricate patterns across Ganymede. The term "sulcus",
meaning a groove or burrow, is often used to describe the grooved features.
This grooved terrain is probably formed by tensional faulting or the release
of water from beneath the surface. Groove ridges as high as 700 meters (2000
feet) have been observed in the Voyager imagery and the grooves run for
thousands of kilometers across Ganymede's surface. The grooves have
relatively few craters and probably developed at the expense of the darker
crust. The dark regions on Ganymede are old and rough, and the dark,
cratered terrain is believed to be the original crust of the satellite.
Lighter regions are young and smooth (unlike the Moon). The largest area on
Ganymede is called Galileo Regio, and is one of the areas targeted by the
Galileo spacecraft.
The large craters on Ganymede have almost no vertical relief and are quite
flat. They lack central depressions common to craters often seen on the
rocky surface of the Moon. This is probably due to slow and gradual
adjustment to the soft icy surface. These large "phantom craters" are called
palimpsests, a term originally applied to reused ancient writing materials
on which older writing was still visible underneath newer writing.
Palimpsests range from 50 to 400 km in diameter. Both bright and dark rays
of ejecta exist around Ganymede's craters - rays tend to be bright from
craters in the grooved terrain and dark from the dark cratered terrain.
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Ganymede Quick-Look Statistics
Discovery: Jan 7, 1610 by Galileo Galilei
Diameter (km): 5,262
Mass (kg): 1.48e23 kg
Mass (Earth = 1) 0.0247
Surface Gravity (Earth = 1): 0.145
Mean Distance from Jupiter (km): 1,070,000
Mean Distance From Jupiter (Rj): 15.1
Mean Distance from Sun (AU): 5.203
Orbital period (days): 7.154553
Rotational period (days): 7.154553
Density (gm/cm^3) 1.94
Orbit Eccentricity: 0.002
Orbit Inclination (degrees): 0.183
Escape velocity (km/sec) 2.74
Visual Albedo: 0.43
Subsolar Temperature (K): 156
Equatorial Subsurface Temperature (K): 117
Surface Composition: Dirty Ice
----------------------------------------------------------------------------
The Discovery of the Galilean Satellites
Probably the most significant contribution that Galileo Galilei made to
science was the discovery of the four satellites around Jupiter that are now
named in his honor. Galileo first observed the moons of Jupiter on January
7, 1610 through a homemade telescope. He originally thought he saw three
stars near Jupiter, strung out in a line through the planet. The next
evening, these stars seemed to have moved the wrong way, which caught his
attention. Galileo continued to observe the stars and Jupiter for the next
week. On January 11, a fourth star (which would later turn out to be
Ganymede) appeared. After a week, Galileo had observed that the four stars
never left the vicinity of Jupiter and appeared to be carried along with the
planet, and that they changed their position with respect to each other and
Jupiter. Finally, Galileo determined that what he was observing were not
stars, but planetary bodies that were in orbit around Jupiter. This
discovery provided evidence in support of the Copernican system and showed
that everything did not revolve around the Earth.
Galileo published his observations in Sidereus Nuncius in March 1610:
"I should disclose and publish to the world the occasion of discovering
and observing four Planets, never seen from the beginning of the world
up to our own times, their positions, and the observations made during
the last two months about their movements and their changes of
magnitude; and I summon all astronomers to apply themselves to examine
and determine their periodic times, which it has not been permitted me
to achieve up to this day . . . On the 7th day of January in the
present year, 1610, in the first hour of the following night, when I
was viewing the constellations of the heavons through a telescope, the
planet Jupiter presented itself to my view, and as I had prepared for
myself a very excellent instrument, I noticed a circumstance which I
had never been able to notice before, namely that three little stars,
small but very bright, were near the planet; and although I believed
them to belong to a number of the fixed stars, yet they made me
somewhat wonder, because they seemed to be arranged exactly in a
straight line, parallel to the ecliptic, and to be brighter than the
rest of the stars, equal to them in magnitude . . .When on January 8th,
led by some fatality, I turned again to look at the same part of the
heavens, I found a very different state of things, for there were three
little stars all west of Jupiter, and nearer together than on the
previous night."
"I therefore concluded, and decided unhesitatingly, that there are
three stars in the heavens moving about Jupiter, as Venus and Mercury
around the Sun; which was at length established as clear as daylight by
numerous other subsequent observations. These observations also
established that there are not only three, but four, erratic sidereal
bodies performing their revolutions around Jupiter."
Simon Marius claimed to have observed Jupiter's moon as early as late
November 1609 (about five weeks prior to Galileo) and had begun recording
his observations in January 1610 at about the same time Galileo was first
making his observations. However, since Marius did not publish his
observations right away as Galileo had done, his claims were impossible to
verify. Since Galileo's work was more reliable and extensive, he is
generally given the credit for discovering the moons of Jupiter. In 1614,
Marius did provide the names of the Jupiter's moons that we are familiar
with today, based on a suggestion from Johannes Kepler:
"Jupiter is much blamed by the poets on account of his irregular loves.
Three maidens are especially mentioned as having been clandestinely
courted by Jupiter with success. Io, daughter of the River, Inachus,
Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the
handsome son of King Tros, whom Jupiter, having taken the form of an
eagle, transported to heaven on his back, as poets fabulously tell . .
. . I think, therefore, that I shall not have done amiss if the First
is called by me Io, the Second Europa, the Third, on account of its
majesty of light, Ganymede, the Fourth Callisto . . . ."
"This fancy, and the particular names given, were suggested to me by
Kepler, Imperial Astronomer, when we met at Ratisbon fair in October
1613. So if, as a jest, and in memory of our friendship then begun, I
hail him as joint father of these four stars, again I shall not be
doing wrong."
Galileo originally called the Jupiter's moons the "Medicean planets", after
the Medici family and referred to the individual moons numerically as I, II,
III and IV. Galileo's naming system would be used for a couple of centuries.
It wouldn't be until the mid-1800's that the names of the Galilean moons:
Io, Europa, Ganymede and Callisto, would be officially adopted, and only
after it became very apparent that naming moons by number would be very
confusing as new additional moons were being discovered.
Images of Ganymede are available on the Galileo home page:
http://www.jpl.nasa.gov/galileo
June 20, 1996 University of Colorado at Boulder faculty are readying for a new round of observations of Jupiter and its surrounding environment with NASA's Galileo spacecraft, including its bright aurora, charged-particle belt and moon, Ganymede. Researchers from the Laboratory for Atmospheric and Space Physics will use two ultraviolet spectrometers to look at the Jovian aurora and atmosphere beginning this week, said LASP researcher Charles Hord, chief investigator on both instruments. On Wednesday they began observing a doughnut-shaped ring of charged particles known as the Io torus that surrounds Jupiter in concert with other space- and ground-based instruments, including the Hubble Space Telescope. The research team also will point the CU instruments at Ganymede during a June 26 Galileo flyby that will come within about 600 miles of the Mercury-sized moon, said Hord. Spectrometer data from the Ganymede encounter, which will be stored on an on-board tape recorder, will not be available for about six weeks. The Galileo spacecraft also will pass by Europa, Callisto and Io, Jupiter's other three large moons, in the coming months. "All of these observations will be new, which is exciting," Hord said. In addition, the researchers will look at the bright aurora on Jupiter generated by the planet's powerful magnetic field, said LASP researcher Wayne Pryor. The team is particularly interested in the day-night differences of Jupiter's aurora that cannot be detected from Earth but can be by Galileo, which will make ultraviolet observations of the planet from the side, Pryor said. The two spectrometers -- a UV spectrometer built at CU and an extreme UV instrument from the University of Arizona modified by CU researchers for the mission -- are among 10 instruments on the Galileo orbiter. The instruments will take "fingerprints" of the gases in the Jovian atmosphere, including complex hydrocarbons like acetylene that are believed to be the building blocks of life. The Io torus observations this week will be synchronized with those of Hubble, the Extreme Ultraviolet Explorer satellite and telescopes at California's Mount Wilson and Lick observatories. The torus observing team includes CU-Boulder's Hord, Ian Stewart, Nicholas Schneider, Fran Bagenal and Dave Brain, as well as Melissa McGrath from the Space Telescope Science Institute, John Trauger from NASA's Jet Propulsion Laboratory, Doyle Hall from Johns Hopkins University and Mike Brown from the California Institute of Technology. Although the torus material is thought to originate from the volcanically active moon, Io, researchers are trying to understand how the torus obtains its energy, said Schneider, who will be at Mount Wilson this week for the observations. "The torus is over a million kilometers across, but its contents could be crammed into Folsom Field (the CU football stadium)," he said. The bright glow of the torus indicates it is being driven by the energy equal to all of Earth's electrical power generation. "We would like to know where all of this power is coming from," said Schneider, a research associate at LASP. "We know Jupiter is the generator, but we are not yet clear on how it works." Ultraviolet data taken on approach to the planet last October indicated the torus may be heating up, according to Stewart, a senior LASP research associate. Material for the torus is believed to originate from active volcanoes on Io that spew sulfur dioxide, which is broken into sulfur and oxygen ions that are captured by Jupiter's massive magnetic field, said Stewart. "It's not clear yet if the material making up the torus is shot directly from the volcanoes on Io or if it is sputtering slowly out of the atmosphere and into the torus," said Pryor. Other CU researchers on the Galileo science team include Charles Barth, Larry Esposito, Bill McClintock and Gary Thomas. The team also includes several scientists from the Jet Propulsion Laboratory and University of Arizona. A number of CU-Boulder students will participate in the Galileo data analysis, said Hord. Launched in 1989, the Galileo spacecraft arrived at Jupiter orbit in December 1995. Because Galileo's high-gain antenna failed to unfurl, the team reprogrammed the on-board computers to compress the data for speedier retrieval, said Pryor. If this is your first message from the updates-jup list, welcome! We are presently in a down mode where an update will be sent about once per month. We hope to reactivate the project more fully after a variety of science data begins streaming in. The likely timeframe for any such reactivation is early 1997. To catch up on back issues, please visit the following Internet URL: http://quest.arc.nasa.gov/galileo/journals
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