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

Galileo spends this week returning the remaining portions of science data stored on its onboard tape recorder during its previous two encounters. The data will be overwritten this coming weekend when the spacecraft flies past Io for the fourth time since arriving at Jupiter in December 1995. This Io flyby will be the closest-ever, with the spacecraft dipping to only 200 kilometers (124 miles) above the fiery surface. Because Io orbits in an area of very intense radiation from Jupiter, the possibility exists that Galileo spacecraft components may be affected, particularly since Galileo has already survived more than twice the radiation it was originally designed to withstand.

Data playback is terminated on Friday, just prior to the spacecraft execution of a small flight path adjustment. Encounter commands start executing late Saturday night.

The data returned this week come from an additional pass through the data stored on the tape recorder. The additional pass allows playback planners to return new data, and also allows replay of data lost in tranmission to Earth, or reprocessing of data using different compression parameters.

Ten observations are on this week's playback schedule. One is returned by the Fields and Particles instruments (Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instruments). Two are returned by the Near-Infrared Mapping Spectrometer (NIMS). Seven are returned by the Solid-State Imaging camera (SSI).

The Fields and Particles instruments return the final portions of a 60-minute high resolution recording of the plasma, dust, and electric and magnetic fields in the vicinity of Europa. The measurements will allow scientists to refine and interpret estimates of Europa's induced magnetic field, the presence of which was detected in data received from Galileo during the actual spacecraft flyby of Europa last month. The fact that an induced magnetic field exists on Europa has provided the best evidence yet that Europa has a conducting layer beneath its surface. The most likely explanation for this layer is the presence of a salty liquid ocean beneath Europa's frozen surface.

SSI follows suit with the return of images (one image per observation) of three of Jupiter's smaller moons: Amalthea, Thebe, and Metis. These images will provide the best resolution views of these moons, almost a factor of two better than previous best resolution images in the case of Amalthea and Metis. NIMS enters the playback picture with the return of a near-global observation of the hemisphere of Io that contains the volcanic region of Loki. SSI then returns a series of color images of the Loki region, followed by images of three other regions of Io. In the first observation, SSI captures a look at two unnamed giant volcanic calderas in Io's northern hemisphere. The next observation contains an image of the Culann volcanic region, and the final observation sees a region of Io's surface near the terminator (or imaginary line dividing night from day). NIMS closes out the playback schedule with the return of a regional map of Io's surface.