Today on Galileo - May 7, 1997
On this fourth day of the encounter period, Galileo passes the point of closest approach to Io at a distance of 956,000 kilometers (about 594,000 miles) before speeding by the surface of Ganymede at an altitude of only 1596 kilometers (about 990 miles) and a speed of 8.6 kilometers per second (over 19,000 miles per hour). Early in the afternoon, the viewing geometry of Io will have changed sufficiently so that the spacecraft can be turned back to an earth pointed orientation and the instruments no longer have to look through the spacecraft booms to see Io.
As can be expected, the day's science activities focus primarily on this close flyby of Ganymede. On the way toward Ganymede approximately two-thirds of the satellite is illuminated by the sun. After closest approach, only one-third is lit up. The remote sensing instruments perform high-resolution studies of a variety of different surface morphological (shape) and albedo (reflected light) features. The regions included in these observations are Osiris (dome structure), Uruk Sulcus (craters, grooves and furrows), a multi-ringed structure, light-dark material, caldera-like features, palimpsest and dark floor craters. The fields and particles instruments perform a high-resolution recording of the magnetosphere for about 45 minutes centered at closest approach. This particular recording is unique as the orbital path takes the spacecraft through a region upstream from the satellite wake (the wake is the region created in front of a satellite as the charged particles that co-rotate with the magnetosphere sweep past the satellites). The observation will provide additional data on Ganymede's magnetic field and will help further understand the interaction between Ganymede and the Jovian magnetosphere. Finally, because of the geometry of the encounter, Ganymede blocks the spacecraft's view of the earth and the sun for about 7 minutes. The radio science team will take this opportunity to measure the changes in Galileo's radio signal through this period. These measurements will allow scientists to further study the temperature and wind profiles in Ganymede's tenuous atmosphere.
Also on today's observing schedule are Io observations, as Galileo continues monitoring surface changes and plume activity. Another remote Io torus observation is performed by the UVS (Ultraviolet Spectrometer). Observations of Jupiter's south pole region also continue today, and another north-south strip is added to the growing map of Jupiter's atmosphere. Finally, a few observations of Europa are performed by UVS, NIMS (Near Infrared Mapping Spectrometer) and PPR (Photopolarimeter Radiometer).