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This Week on Galileo?
Aug 28 - Sep 3, 2000
DOY 2000-241-247

Galileo Returns Observations of Io as it Continues to Orbit Jupiter

This week, Galileo returns observations of Io, and of Jupiter's atmosphere, rings, and magnetosphere as it continues to orbit the gas giant and its family of moons. The observations have been stored on the spacecraft's onboard tape recorder since they were recorded during Galileo's May passage through the heart of the Jupiter system.

Data playback is interrupted twice this week. On Tuesday, the Solid-State Imaging camera (SSI) takes four optical navigation images, one of Ganymede and three of Jupiter. On Thursday, the spacecraft performs a standard gyroscope performance test.

The optical navigation images will allow flight engineers to determine the health of SSI, after a recent unexpected increase in the amount of power being used by SSI and an increase in a voltage related to its detector. No change was expected in these values during the cruise period. Investigation suggested that a light source internal to the camera had unexpectedly turned on and was apparently the source of the anomalous measurements. This internal light source is used to completely eliminate possible "ghost" images on SSI's charge-coupled device (CCD) prior to shuttering a new image. The light source has since been disabled, and the camera voltage and power consumption have returned to normal. Tuesday's test of the camera will be performed, in particular to assess the degree to which any "ghost" images may need to be accounted for in the future.

Through the end of the week, data playback shares Galileo's transmission pipeline with the Dust Detector Subsystem. For the past three weeks, DDS has been providing real-time measurements of the dust environment surrounding the spacecraft. Periodic measurements made by the instrument in July showed thousands of impacts occurring on some days. The recent observation campaign will allow scientists to get a better understanding of the size, speed, and origin of these dust particles.

On the playback schedule, we find observations by the Photopolarimeter Radiometer (PPR), SSI, the Near-Infrared Mapping Spectrometer (NIMS) and the Fields and Particles instruments (F&P). PPR returns two polarimetry observations of Io, the most volcanically active body in our solar system. The observations were taken at different solar phase angles, and will provide scientists with data on the texture and small-scale properties of Io's surface.

SSI returns five observations this week. Two of the observations capture images of Jupiter's rings. The images were taken at high resolution, low solar phase angle, and relatively high tilt angle. They are expected to provide scientists with better determinations of the size distribution and light-scattering properties of ring particles, both in Jupiter's main ring and at its inner edge. The images may also allow detection of wave-like features in the main ring, and undulations in the ring's outer boundary, which could be important for understanding how the rings are maintained by Jupiter's small inner satellites. SSI's remaining three observations will provide scientists with high spatial and time resolution images of the Great Red Spot, which is over 400 years old. The Great Red Spot is so large that two Earths could fit across it! Similar observations were taken at the beginning of Galileo's orbital tour in June 1996, so scientists will be able to observe long-term changes in the characteristics of this long-lived storm.

NIMS returns five observations this week. Four are from a series of 10 spectral scans of Jupiter's north polar region. The series consists of 10-minute samples, each separated by 60 minutes, and will provide a unique view of auroral activity on Jupiter. The remaining NIMS observation is one from a series of three global observations of Jupiter. Once assembled, the three global observations will provide a near-complete spectral map of the planet.

Finally, continuing from previous weeks, the F&P instruments return portions of a month-long low-resolution survey of Jupiter's magnetosphere. The survey provides scientists with measurements of the plasma, dust, and electric and magnetic fields in the inner and outer regions of Jupiter's magnetosphere, and the transition out into the solar wind. In addition, the survey provides context for higher-resolution recordings also made by the F&P instruments during the encounter.

 
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