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

Galileo becomes selective in its playback schedule this week, returning bits and pieces from different observations stored on its onboard tape recorder. Data return is uninterrupted as the Deep Space Network antennas receive parts of three observations taken in February during an Io flyby, and three observations taken in May during a Ganymede flyby. The Io flyby was the lowest-altitude flyby of Io ever, with the spacecraft passing only 198 kilometers (123 miles) above the surface. By comparison, the space shuttle typically flies at an altitude of 320 kilometers (200 miles) above the Earth's surface. The Ganymede flyby was Galileo's fifth flyby of Ganymede, and its second closest. The spacecraft passed within 808 kilometers (502 miles) of Ganymede's surface.

First on the playback schedule are the Io observations. Two of these observations are returned by the Solid-State Imaging camera (SSI) and one by the Near-Infrared Mapping Spectrometer (NIMS). SSI starts with the return of portions of a 12-frame mosaic covering the Camaxtli Patera hot spot, and nearby regions to the west, including the Chaac Patera region. The remaining SSI observation and the NIMS observation both focus on the Amirani volcanic region. This region was previously known as Amirani-Maui, and was thought to contain two separate volcanoes. Galileo data have now shown that Maui is the leading edge of a lava flow that originates at the Amirani volcanic vents. The lava flow is more than 250 kilometers (160 miles) long! The SSI observation consists of color imaging of the region, while NIMS returns spectral maps.

The Fields and Particles (F&P) instruments initiate the return of data from the Ganymede flyby. The F&P instruments return portions of a 60-minute high-resolution recording of the plasma, dust, and electric and magnetic fields surrounding Ganymede. Ganymede is the only planetary moon that is known to have its own internally-generated magnetic field, and thus, its own magnetosphere.

The Plasma Wave instrument (PWS) is next on the playback schedule with the return of an observation dedicated to the detection of chorus emissions within Ganymede's magnetosphere. A chorus signal is caused by a particular type of interaction between plasmas and magnetic fields and has been observed in Earth's magnetosphere. Scientists hope to understand more about Ganymede's unique magnetosphere by detecting and analyzing chorus emissions.

Finally, SSI returns one of five observations of Ganymede designed to provide scientists with information regarding how different features and terrains came to exist on Ganymede's surface. The mosaic of images returned this week captures smooth bright terrain and grooved terrain that may be partially surrounded or engulfed by the surrounding terrain. The younger terrain types examined in this set of five observations are believed to have been created by processes internal to Ganymede, but the roles of volcanic vs. tectonic processes are not yet clear.

If some of these observations sound familiar, it is because portions of them have previously been returned to Earth. This additional pass through the data stored on the tape recorder allows for the return of additional data, replay of data lost in transmission to Earth, and/or reprocessing of data using different parameters.