Today on Galileo - June 25, 1997
Today, just past 7 A.M. PDT, Galileo passes by Callisto at a distance of only 415 km (258 miles)! This is only slightly higher than the altitude by which the space shuttle typically travels around the Earth (approximately 300 km or 186 miles). Most of the day is dedicated to observations of Callisto, but the day also includes observations of Jupiter, Io and Europa. Two spacecraft turns are performed prior to and after the Callisto closest approach. These turns are small and improve the remote sensing instrument view of the satellite.
The flyby of Callisto provides a darkside pass, with about half of the satellite illuminated by the sun on approach and departure. The primary feature observed during the flyby is known as the Valhalla multi-ringed structure with several remote sensing instruments coordinating their observational efforts. Additional remote sensing observations include studies of a bright crater (Skuld), the northern latitudes, a dark-limb observation and continuation of dayside and darkside thermal mapping. For about 47 minutes surrounding the closest approach, the fields and particles instruments will perform a high-time-resolution recording of the magnetospheric environment. This recording will help determine if Callisto has its own magnetic field and will provide information on the interaction between Callisto and Jupiter's magnetosphere.
During the flyby, the spacecraft's radio signal will be blocked from reaching Earth by Callisto. This is the first time in the orbital tour that Callisto will cause an occultation. As the spacecraft's radio signal passes behind Callisto and comes out the other side, the radio science experimenters will look for changes, if any, in Galileo's radio signal. These changes could be indicators of the presence of a tenuous atmosphere around Callisto. The occultation event also allows the experimenters to measure the diameter of the satellite. The accuracy of their measurement is dependent on the geometry of the flyby and how well the navigators can reconstruct the orbit. An accuracy of about 1 kilometer (0.62 miles) is expected.
The remaining observations performed today include a regional observation of Jupiter performed by PPR (Photopolarimeter Radiometer), Europa, Io Torus, Jupiter aurora and darkside observations performed by UVS (Ultraviolet Spectrometer), Jupiter hot spot and aurora observations performed by NIMS (Near Infrared Mapping Spectrometer), and, the first of the primary features of Jupiter's atmosphere selected for this orbit, a Great Red Spot observation performed by SSI (Solid State Imaging camera).