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This Week on Galileo?
October 23 - October 29, 2000
DOY 2000/297-303

Galileo Finishes Playing Back Science Data from May's Ganymede Flyby

This week Galileo finishes playing back science data gathered during its May flyby of Ganymede and then embarks on a campaign by its fields and particles instruments to collect a continuous survey of the Jovian magnetosphere. Galileo's schedule this week also includes other engineering and navigation activities. On Thursday, the spacecraft performs standard maintenance on its onboard tape recorder. On Friday, Galileo executes a small flight path adjustment. And on Saturday, the spacecraft performs standard maintenance on its propulsion systems.

On Thursday, the spacecraft will also perform a test to determine the status of the Ultraviolet Spectrometer (UVS) instrument. UVS has been turned off for several months now. Electronics which control the position of the instrument's grating appear to have been damaged by some combination of Jupiter's severe radiation environment and over 10 years spent in the harsh space environment. Engineers have been hoping that time will allow the damaged electronics to anneal, restoring the instrument to a working state. Annealing is the process in which defects in a material (in this case, radiation damage to a semiconductor) migrate toward the edges of the affected component, thus restoring the material to something closer to its original, undamaged state.

The fields and particles campaign initiated this week is designed to obtain a 100-day survey starting from the solar wind, into the Jovian magnetosphere, and back out again. Galileo has been mapping portions of the Jovian magnetosphere since arrival at Jupiter in December 1995, but this particular portion of the survey is part of a joint investigation with the Cassini spacecraft, which is now approaching Jupiter and will pass by in December enroute to arrival at Saturn in 2004. The dual-spacecraft observations will allow scientists to observe both the solar wind and the interior of the magnetosphere at the same time. With these unique simultaneous measurements, they can begin to see, for the first time, how changes in the solar wind can affect the interior of the Jovian magnetosphere. In addition, the Galileo effort marks the second of three planned transits through a poorly-mapped region of the magnetosphere. The Fields and Particles instruments are the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.

But why, you might be asking, is it necessary to halt data playback to perform the survey? Hasn't the spacecraft always been able to share playback with the fields and particles survey? This was true for Galileo's Prime Mission (i.e., its first 11 orbits) at Jupiter. But Galileo's survey data are collected at low rate and stored in onboard data buffer, which holds only about seven hours worth of data. The data are then transmitted to Earth whenever Deep Space Network (DSN) antennas are scheduled for Galileo's use. Now that Galileo is on its second mission extension, and given the large number of other spacecraft that are currently in flight, Galileo does not always receive its choice of DSN antenna time. When DSN antennas can't be scheduled frequently enough, Galileo's data buffer can't be emptied. It then overflows, and the survey data are lost.

The solution to this data loss problem is to record the contents of the data buffer to the tape recorder. The data then remain stored on the tape recorder until more DSN time can be made available. It is very complicated to juggle between data playback and data recording, so data playback is halted to implement this plan. Galileo will perform this operation for the next month or so. A similar strategy was used after Galileo's May encounter with Ganymede. Some of those data are being played back during the first half of this week.

Speaking of playback, this week's playback plans are part of a third pass through science observations stored on the spacecraft's tape recorder during Galileo's May encounter with Ganymede, Jupiter's largest moon. Multiple passes through the tape recorder allow playback planners to replay data lost during previous transmissions. In addition, new, additional data can be returned, or data can be processed using different parameters and re-transmitted.

The Near-Infrared Mapping Spectrometer (NIMS) returns six observations this week. The first is a spectral scan of Ganymede's limb, which should provide insight into Ganymede's tenuous atmosphere. Next is a high-resolution spectral map of Ganymede's entire disk. The next three observations are part of series of ten scans of Jupiter's north polar region designed to capture auroral activity on Jupiter. The last NIMS observation is one of three observations designed to map a substantial proportion of Jupiter's atmosphere.

Next, the Fields and Particles instruments return portions of a month-long, low-resolution survey of Jupiter's magnetosphere. The lengthy survey contains information on the inner and outer regions of Jupiter's magnetosphere and its transition out into the solar wind. These data were recorded using the same method Galileo will be using in the next few weeks to ensure survey data continuity.

The Solid-State Imaging camera (SSI) returns the remaining observations on this week's playback schedule. First, a portion of an image of Jupiter's main ring is returned. Next, SSI returns parts of seven images contained in three observations of Jupiter's Great Red Spot. Finally, SSI returns part of an observation consisting of a set of global images through different color filters that captured Europa while eclipsed from the Sun by Jupiter.

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