Today on Galileo - April 4, 1997
Two more flybys are on today's schedule of activities for the spacecraft. The first, occurring early in the morning, occurs when the spacecraft reaches its closest point to Jupiter at a range of 9.1 Rj, or Jupiter radii (1 Rj is about 71490 kilometers). The second occurs late at night, just prior to midnight (PST), when the spacecraft will fly by Ganymede at an altitude of only 3095 kilometers.
The first three quarters of the day are mainly spent performing various observations of Jupiter. A few more hot spot observations are taken by the NIMS (Near Infrared Mapping Spectrometer) and the PPR (Photopolarimeter Radiometer)instruments. Regional and east-west strip observations are performed through the day by the PPR instrument. This provides context information for the small red spot observations performed during this encounter, a new set of which are taken toward the end of the day. The PPR instrument will also perform a couple of observations of the Great Red Spot. The non-Ganymede observations for today are rounded out by a few observations monitoring Io's volcanic activity and images of Adrastea and Metis. The latter couple of observations, like yesterday's observation of Thebe, are designed to help in the construction of global maps by the end of Galileo's primary mission.
Observations of Ganymede are scheduled to start about 5 pm (PST) and continue through tomorrow morning. These observations will serve primarily to fill in regions that have not been previously observed. They cover a variety of bright, dark and dark-rayed (like the Kittu crater) regions with the goal of determining surface composition and studying the effects of high energy impacts. Observations by NIMS and SSI (Solid State Imaging camera) focus on these high energy impact regions (circles, domes, grooves) and what roles the impactors have played in their formation and characteristics. The observations will also provide information on any role volcanism might have played in the resurfacing of craters. There is even an opportunity to perform some comparative planetology with craters on Mars.
Global observations by the NIMS and UVS (Ultraviolet Spectrometer) instruments will allow for the determination of the surface composition of Ganymede which can be used to compare Ganymede with the other satellites. PPR darkside and dayside maps performed are part of campaigns to map surface thermal properties over the entire globe. High resolution samples taken by PPR are expected to provide information about the temperature and physical properties of Ganymede's surface and help determine whether any material is being lost to space. A recording of fields and particles instrument data taken during closest approach continues the investigation of the interaction between Ganymede and the magnetosphere.