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

Galileo begins to transition from a purely data-gathering mode to concentrating on playing back science data stored on its onboard tape recorder. Through the end of this week, Galileo's data transmissions will be shared by the two activities. Observations continue to be performed by the Fields and Particles instruments and the Extreme Ultraviolet Spectrometer (EUV), while playback entails the return of observations made by the Solid-State Imaging camera (SSI), the Near-Infrared Mapping Spectrometer (NIMS), and the Photopolarimeter Radiometer (PPR). These latter observations are not from Galileo's most recent Ganymede encounter on May 20, but rather are from its daring fourth flyby of Io performed on February 22. That flyby was Galileo's closest-ever approach to the volcanic moon, at an altitude of only 198 kilometers (123 miles)! Prior to the Wednesday start of playback, Galileo performs routine maintenance on its onboard tape recorder.

The EUV instrument continues its remote observations of the Io torus this week. The torus is a ring-shaped region of intense plasma and radiation activity, which is maintained by Jupiter's strong electric and magnetic fields and Io's volcanic activity. The data obtained will allow scientists to continue studying the shape and energy output of the torus, and perform long term comparisons with data obtained since Galileo's arrival at Jupiter in December 1995. The Fields and Particles instruments also continue their observation of Jupiter's magnetosphere and surrounding solar wind. These are the final days of the four-week survey, which ends on Tuesday. The survey will provide measurements through the inner and outer magnetosphere and Galileo's transition into the free solar wind, and will help project planners to prepare for joint activities with the Cassini spacecraft starting in October of this year. Cassini will fly past Jupiter in December 2000 on its way to a Saturn arrival in 2004. The Dust Detector instrument (DDS) will continue to collect and return data through Monday of next week. It continues to map the distribution and character of dust streams that emanate from the Jupiter system. A recent analysis of DDS data from Galileo's primary mission through both the GEM (Galileo Europa Mission) and GMM (Galileo Millennium Mission) mission extensions has shown that these dust streams originate with volcanic activity on Io's surface.

Playback schedules for the latter half of the week contain the return of three observations. In the first, SSI returns an observation of Io's Prometheus volcanic region. The observation captures the Prometheus plume source and active lava flow at a resolution of approximately 13 meters (43 feet) per picture element. The next two observations are returned by NIMS, with additional PPR data contained in the second. An observation of the Chaac Patera region contains only NIMS data. The Chaac region includes areas on Io that are informally knows as "golf courses" because of their greenish color and their shape. The final observation, containing additional PPR data, is a mosaic of several volcanic regions on Io.