Dec 18, 1996

Galileo Updates

The current encounter with Jupiter will occur on 19 Dec 1996, 03:22 UTC at a range of 9.2 R(J). This will be followed by the 5th orbit which will pass through perijove on Jan 19 during the time when Jupiter is in the solar conjunction zone (Jan 10-28), therefore no observations of Jupiter will be attempted and data from E4 will be sent back starting in the last half of December through early February.

In case you have heard rumors concerning the operation of Galileo the following may clarify the current state.

Subject: Galileo Tape Recorder Anomaly Resolved!
From: William J O'Neil
Date: 12/16/96 6:47 PM

Galileo Tape Recorder Anomaly Resolved

The Sunday (12/15) Galileo Tape Recorder lockout occurred because the Beginning Of Track (BOTR) marker on Track 1 extends up to three tics beyond the BOTR marker on Track 3. Friday night's tape recorder conditioning completed properly with the Track 2/3 turnaround (6DTRN) stopping the tape at Ready Track 3 just off the marker. Sunday's positioning slew command properly switched the recorder from Track 3 to Track 1 for the Playback positioning slew to tinc 1750 for the start of recording in the Europa encounter sequence. Immediately, the slew read the "extended" marker on Track 1 and interpreted it as the Track 1 End Of Track marker and the Fault Protection properly stopped and locked-out the recorder 50 tics later. The reason this hadn't happened before is that the auto-unstick from Ready at BOTR ignored the marker so the slew got past the marker without detecting it. The CDS Flight Software Patch on December 4th removed the auto-unsticks on the forward tracks (1 & 3) because they are unneeded and undesirable. This was the first occurrence of starting the slew at BOTR without the auto-unstick.

Following the Tape Recorder recovery and positioning today, the Europa encounter sequence is not susceptible to the above described fault. The start of E4 recording is expected to proceed nominally this evening. All recovery actions were developed, commanded, and executed in time.

Subject: Status of DMS Recovery Commanding
From: James K Erickson
Date: 12/16/96 3:54 PM

The Monday a.m. commanding to unlock the tape recorder and reset the error condition was sent with an incorrect ordering of the commands. This relocked out the DMS from further commanding. The subsequently sent DTRN and Slew to tinc 1750 was then ignored by the S/C.

Try number 2 is underway, with results expected to be known by 7:00 p.m. tonight. Further results will be disseminated when known.

-Jim Erickson-

Formal news of tape status is not available but the following message from Glenn Orton indicates recovery.

Preliminary processing of a file of mediocre seeing at 4.78 microns shows that the hot spot has accelerated in System III and, while it had disintegrated and moved to as westward a location as 349 degrees (working in a system rotating 100.3 m/s with respect to System III and taken at the E4 epoch of 1996 Dec. 17 @ 7:00 UT) - placing it out of our fields of view, it is now between 339 and 343 degrees as of 1996 Dec. 17 @0:33 UT.

The cylindrical map projection of its history and including the current data set from yesterday (although astigmatic and still uncorrected for a double image) is on in pub/opag/outgoing as showfirste4hs.gif.

That, plus the tape recorder recovery, todays' clear skies and VERY low humidity are making my pre-Christmas quite jolly (if fatigued! :)

Glenn Orton

Orton also submitted the following timetable for observations to support the E4 observations

E4 support observation times

Critical times during the Europa-4 encounter, during which atmospheric observations will be taken are as follows:

96/12/1610:00 - 13:06UVS auroral maps
96/12/173:22 - 3:36NIMS global mosaic
5:02 - 5:16NIMS global mosaic
6:04 - 6:18NIMS global mosaic
6:19 - 7:08feature track @ 65 deg. phase angle
8:41 - 8:55NIMS global mosaic
12:00 -12:14NIMS global mosaic
15:38 - 18:24feature track obs. #2 (55 deg phase)
96/12/181:38 - 4:23feature track obs. #3 (31-41 deg phase)
11:46 - 15:39feature track obs. #4 (16 deg. phase)
96/12/1910:55 - 11:57feature track obs. #5 (95 deg. phase)
22:45 - 23:42PPR feature track RAD obs.
96/12/206:52 - 7:59feature track @ 148 deg. phase, NIMS 3-/5-micron maps
16:23 - 18:06feature track @ 160 deg. phase

(All times are spacecraft event times: Add ~50 min light time for UTC)

Galileo Status Report

December 9 - 15, 1996

This week Galileo returns the last bit of data from the Callisto encounter and starts its encounter with Europa. The encounter with Europa marks the last "first" close encounter of the Galileo orbital tour. After Europa, with the exception of Io, the spacecraft will return to each Galilean satellite two more times before the end of the primary mission in Dec 1997.

Playback plans for this week continue to include observations that had been previously previewed. Some very unique data is being returned in this final week of data return. Included in this set of observations is the highest resolution observation of Io performed by the NIMS instrument during the orbital tour. This data set is even more unique because it will include all of the observable frequencies available to NIMS. Also included in this set of data are observations of some unique terrain on Europa and some unique views of Jupiter's rings. These particular ring observations are also taken from the closest distance Galileo will come to the rings for the rest of the orbital tour.

The only non-previewed observation returned during this week is one of Jupiter's atmosphere obtained by the Ultraviolet spectrometer. This particular observation was taken of Jupiter's darkside while the spacecraft was blocked from the Sun by Jupiter. This view is not possible from Earth and will provide unique information on the activity of the upper atmosphere in the absence of sunlight.

Encounter science starts with remote observations of the Io torus conducted on approach to the Jupiter system. Optical navigation images continue to be taken this week in preparation for the last pre-encounter orbit trim maneuver. This maneuver is sent to the spacecraft toward the end of the week for execution within the first couple of days of the start of the encounter sequence. The encounter sequence of commands is also sent to the spacecraft during this week and begins to execute on December 14 at 4 pm PST.

For more information on the Galileo spacecraft and its mission to Jupiter, see the Galileo home page.

Formal NASA Press Release

Mary Beth Murrill
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)

RELEASE: 96-255


Jupiter's big moon Ganymede is not only the size of a planet -- it sounds like one too, as heard in audio recordings made from data returned by NASA's Galileo spacecraft released today.

Characterized by a soaring whistle and hissing static, Ganymede's song reveals that the Solar System's largest moon is also the only one known to possess a planet-like, self- generated magnetic cocoon called a magnetosphere, which shields the moon from the magnetic influence of its giant parent body, Jupiter.

Published in the scientific journal Nature this week, these new Galileo findings and other measurements from several Galileo sensors were presented today in a news briefing held at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. Taken together, scientists say the new Ganymede findings are painting a portrait of a body that from the inside out closely resembles a planet like Earth rather than other moons in the Solar System.

"The data we get back is in the form of a spectrogram, and reading it is kind of like looking at a musical score," said Dr. Donald Gurnett, University of Iowa physicist and principal investigator on Galileo's plasma wave instrument. It was his experiment that first detected the telltale signals of a magnetosphere during the spacecraft's close flybys of that moon on June 27 and Sept. 6.

Gurnett said the unique pattern of frequencies his instrument detected is characteristic of a magnetosphere and closely matches his previous studies of the magnetospheres of Earth, Saturn and Jupiter. "The instant I saw the spectrogram, I could tell we had passed through a magnetosphere at Ganymede," Gurnett said.

Describing the electromagnetic wave activity that his experiment detected at Ganymede, Gurnett said the approach to the large moon was relatively quiet, "until all of a sudden, there's a big burst of noise that signals the entry into Ganymede's magnetosphere. Then, for about 50 minutes, we detected the kinds of noises that are typical of a passage through a magnetosphere. As we exited the magnetosphere, there was another big burst of noise."

Gurnett checked with Dr. Margaret Kivelson of the University of California at Los Angeles (UCLA), principal investigator for the magnetometer experiment on the spacecraft. Kivelson confirmed the detection of a large increase in magnetic field strength near Ganymede. Related data from the two close flybys have confirmed that Ganymede has a magnetic field of its own.

Using extremely precise data from tracking the spacecraft, investigators on Galileo's celestial mechanics team also have been able to confirm that Ganymede's interior is differentiated, probably having a three-layer structure. "These data show clearly that Ganymede has differentiated into a core and mantle, which is in turn enclosed by an ice shell," said JPL planetary scientist Dr. John Anderson, team leader on the Galileo radio science experiment.

"Combined with the discovery of an intrinsic magnetic field, our gravity results indicate that Ganymede has a metallic core about 250 to 800 miles in," said Anderson. "This is surrounded by a rocky silicate mantle, which is in turn enclosed by an ice shell about 500 miles thick. Depending on whether the core is pure iron or an alloy of iron and iron sulfide, it could account for as little as 1.4 percent or as much as one-third of the total mass of Ganymede."

This differentiated structure is the most likely cause of Ganymede's newly discovered magnetic field, which in turn gives rise to the magnetosphere, reported UCLA geophysicist Dr. Gerald Schubert, an interdisciplinary investigator on Galileo's science team. Scientists suspect Ganymede's magnetic field is generated the same way as Earth's, through the dynamo action of the fluid mantle rotating above a metallic core. The only other solid bodies in the Solar System known to have magnetic fields are Mercury, Earth and possibly Jupiter's volcanic moon Io.

"At Ganymede, the magnetic field is strong enough to carve out a magnetosphere with clearly defined boundaries within Jupiter's magnetosphere, making it the only 'magnetosphere within a magnetosphere' known in the Solar System," Kivelson said.

The strong magnetic characteristics of Ganymede, in combination with its residence within Jupiter's powerful magnetosphere, make the Jovian environment even more intriguing, said Galileo project scientists Dr. Torrence V. Johnson. "The physics taking place within Jupiter's magnetosphere are of great interest to scientists attempting to understand the complex interplay of magnetic forces and matter throughout the universe," he said.

Data from the plasma wave instrument can be heard and seen on the Galileo home page.

Newly received Galileo images of Jupiter's moon Callisto and one of Europa also were released at the briefing. Scientists were surprised by the lack of small craters visible in the images. Some small craters appear to have been softened or modified by downslope movement of debris, revealing ice-rich surfaces.

The Europa image received from the spacecraft earlier this week shows the cracked surface of this moon in greater detail than it has been seen before. The new image shows an area 150 miles by 140 miles that has been highly disrupted by fractures and ridges.

Arizona State University planetary scientist Dr. Kelly Bender of Galileo's imaging team said that symmetric ridges in the dark bands suggest that Europa's surface crust was separated and filled with darker material, somewhat analogous to spreading centers in the ocean basins of Earth. Although some impact craters are visible, their general absence indicates a youthful surface, she said.

The youngest ridges, such as the two features that cross the center of the picture, have central fractures, aligned knobs, and irregular dark patches. These and other features could indicate cryovolcanism, or processes related to eruption of ice and gases.

Galileo was launched in 1989 and entered orbit around Jupiter Dec. 7, 1995. The Galileo mission is managed by JPL for NASA's Office of Space Science, Washington, DC.