An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

The time remaining to play back data stored on Galileo's tape recorder is steadily diminishing as only two weeks remain before the spacecraft returns to inner regions of the Jupiter system and its next satellite encounter. The data returned this week were stored on the onboard tape recorder during Galileo's previous two satellite encounters--Io in late November 1999, and Europa in January 2000. Data playback is interrupted once this week, on Monday, when the spacecraft performs a standard test that allows flight engineers to keep track of the performance of the attitude control system's gyroscopes.

Portions of six observations are returned to Earth this week. During data playback, the spacecraft computer retrieves data stored on the tape recorder, processes and packages the data, and subsequently transmits the data to Earth. This week's observations were made by the Solid-State Imaging camera (SSI), the Near-Infrared Mapping Spectrometer (NIMS), and the Fields and Particles instruments.

The first four observations of the playback schedule were acquired during Galileo's November 1999 encounter with the Jupiter system. SSI is first up with the return of a 12-frame global mosaic of icy Europa. Although the November encounter featured Io, the spacecraft's trajectory also offered a good view of Europa. NIMS follows on the schedule with the return of two spectral scans of Europa's surface. One of the scans captures an equatorial region of Europa, while the other is global in scale. The final observation from the November observation set is returned by the Fields and Particles instruments. They return parts of a 3-hour high resolution recording of the Io plasma torus. These measurements will be used to understand the structure and dynamics of plasma, dust, and electric and magnetic fields in the torus. The measurements contained in this recording will also be important for understanding the overall dynamics of the Jovian magnetosphere.

The remaining two observations of this week's playback schedule were acquired during Galileo's January 2000 encounter with the Jupiter system. The Fields and Particles instruments start off with the return of a portions of a 60-minute high resolution recording of the region surrounding the spacecraft's closest approach to Europa. The data contained in this recording will allow scientists to further refine and interpret estimates of Europa's induced magnetic field. The presence of this field was detected shortly after the Europa flyby in real time data received from Galileo during the actual flyby. The field signature is the best evidence yet that supports the existence of a conducting layer, possibly liquid water, below Europa's surface. SSI returns the final observation of this week. In it, the camera captured sharp-edged ridges, a multi-ring impact feature named Callanish, and blotchy-looking, or mottled, terrain on Europa.