Today on Galileo - September 16, 1997
Galileo's close flyby of Callisto occurs today at approximately 5:19 pm, Pacific Time. The flyby altitude is only 538 km (334 miles) from the satellite's surface and is the second closest of the three flybys of Callisto. Most of today's observation schedule is dedicated to Callisto, including observations of the Asgard basin and Valhalla regions.
This morning is spent completing a remote observation of the Io torus performed by the Ultraviolet Spectrometer (UVS) and Extreme Ultraviolet Spectrometer (EUV). This observation is one of many performed during Galileo's orbital mission. Together, the series of observations is designed to gather information on the types and number of particles (oxygen, sulfer, hydrogen, electrons) that are expelled from Io and trapped in Io's orbit. Sprinkled through the day are a few distant observations of Europa. Four of these, taken by the UVS, are performed from views not available from Earth. The Near Infrared Mapping Spectrometer (NIMS) examines Europa's surface characteristics with the other Europa observation of the day.
Today's observations of Callisto are performed by NIMS, the Photopolarimeter Radiometer (PPR), UVS, and the Solid State Imaging (SSI) camera. The NIMS instrument will observe the Valhalla impact basin to determine differences that may exist between the materials that make up the basin and the materials that make up the rings surrounding the basin. Also on the NIMS schedule are a few observations of a region of Callisto known as Gipul Catena. These observations will be looking for residual traces of objects that may have hit Callisto and affected the formation of the region.
SSI, on the other hand, will focus on obtaining high-resolution images of Callisto's Asgard basin. These images are expected to provide more information regarding the structure and makeup of the different layers of Callisto's surface in this region. A detailed look at some of the smooth plains on Callisto's surface is also planned by the SSI. These images should provide clues as to how these surfaces were formed. PPR will dedicate its observing time to obtaining thermal maps of Callisto's day and night sides. These maps will provide data on how well Callisto's surface can retain heat and how compact the surface is on Callisto.
The UVS instrument rides along with a number of NIMS observations, but the EUV instrument performs a unique 15 hr long observation of Callisto's surface. This observation will be looking for the production of ions and possible outgassing of material from Callisto's surface. Near closest approach to Callisto, the fields and particles instruments perform a high time-resolution recording of the environment around Callisto.
During this encounter's flyby, the spacecraft will pass through the wake created by Callisto in Jupiter's magnetosphere. This recording will provide important data on the interaction of Callisto with the magnetosphere in the creation of this wake. Finally, for 20 hours surrounding the point of closest approach with Callisto, the radio science team will measure changes in the frequency of the spacecraft's radio transmissions to Earth. These measurements can then be turned into a map of the gravity field around Callisto.