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

The cold dark vacuum of space envelops Galileo as it continues its journey around our solar system's largest planet, Jupiter. Galileo is not alone on this journey, however, as a dedicated crew of people here on Earth monitor its health and care for its every need. In return, Galileo has provided humanity with a vast amount of knowledge about Jupiter, its atmosphere, the largest four of its 17 moons, its rings and minor moons, and the charged particles and fields of its magnetosphere.

The spacecraft spends this week continuing the return of science data acquired in May when it flew past Ganymede, Jupiter's largest moon. The data are stored on an onboard tape recorder. During playback, the spacecraft's main computer retrieves the data from the tape recorder, compresses and packages it, and then transmits it to Earth. The large radio antennas of the Deep Space Network then receive Galileo's transmissions and forward them to the Jet Propulsion Laboratory in Pasadena, California, for decoding. Four observations from that May flyby are returned to Earth this week, and playback is interrupted twice to perform engineering activities. On Wednesday, the spacecraft performs a small turn to keep its radio antenna pointed toward Earth. On Friday, Galileo performs standard maintenance on its onboard tape recorder.

Galileo's Solid-State Imaging camera (SSI) returns parts of two observations this week. The first is the last in a series of five high-resolution observations dedicated to providing scientists with information to help explain how different features and terrains came to exist on Ganymede's surface. Other such observations have been returned in previous weeks, and this week's mosaic of images captures caldera-like features. Then, SSI returns a portion of the first of five mosaics centered at the same locations as the previous high-resolution observations, but covering a much wider area of the surface. These wider images will provide the geologic context for the high-resolution samples. In addition, the motion of the spacecraft along its flight path at the time these observations were taken will allow stereo images to be produced by combining data from the high-resolution and context images. The context observation returned this week contains a region of smooth bright terrain and grooved terrain that may be partially surrounded, or 'engulfed' by the nearby terrain.

The Near-Infrared Mapping Spectrometer returns the next observation. Also of Ganymede, the observation contains a spectral scan of a dark crater. Surrounding the crater are ice and background dark regions. The scan will map the spatial distributions of the different surface types within and adjacent to the crater. The data will also enable scientists to determine the grain sizes of the surface materials, an important clue in all investigations of geological processes.

As during the previous weeks, the Fields and Particles instruments continue to return portions of a 60-minute high-resolution recording of the plasma, dust, and electric and magnetic fields surrounding Ganymede. The Fields and Particles instruments are comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument. The data taken during this observation will allow scientists to obtain a more complete understanding of the unique interactions between the magnetospheres of Jupiter and Ganymede. Ganymede is the only planetary moon that is known to have its own internally-generated magnetic field, and thus, its own magnetosphere.