This Week on Galileo - July 21-28, 1997
The second recording of fields and particles high-time resolution observations of Jupiter's magnetosphere, and the first distant satellite occultation of the current cruise period are both scheduled to occur this week on Galileo. Interwoven with these events is the continued transmission to Earth of data from Galileo's previous close encounter with the Jupiter system.
On Wednesday, as Galileo passes 130 Jupiter Radii, or about 9.3 million kilometers (5.8 million miles), from Jupiter, normal data playback from the tape recorder will be paused and the fields-and-particles instruments will perform another high-time resolution observation of Jupiter's magnetosphere. Usually, the fields and particles data that are taken during cruise contain information that is gathered at a rate of a few bits per second. The recorded high-time resolution observations contain data gathered at a rates of several hundred bits per second. This higher density data gives scientists a more detailed view of the behavior of Jupiter's magnetic environment. This observation is part of the data set that will be constructed by periodic observations taken as the spacecraft travels into and away from a region of Jupiter's magnetosphere known as the magnetotail (where the solar wind "pushes" Jupiter's magnetic field into a long, streaming tail). The observations are performed on this orbit because it is the point where the spacecraft will travel deepest into the magnetotail -- 143 Jupiter radii -- during Galileo's mission. The data set is expected to provide exciting new information regarding how the magnetotail evolves and interacts with the middle and inner magnetosphere and the solar wind.
Early Sunday morning, Galileo will pass behind Ganymede as seen from Earth. The radio science team will take the opportunity to measure changes in Galileo's radio signal as it is blocked by Ganymede. As the spacecraft sneaks behind Ganymede and as it reappears on the moon's opposite side, Ganymede's atmosphere, however tenuous, will act as a lens and will bend Galileo's radio signal. This bending will cause changes in the characteristics of the radio signal and will allow scientists to learn about the density, temperature and pressure of the atmosphere. In addition, by measuring the time at which the radio signal is lost and the time at which it is regained, scientists can determine Ganymede's diameter.
Playback plans for this week include the first observations from Galileo's close flyby of Callisto, the first since November 1996. Recall that the spacecraft flew over Callisto's surface at a distance of only 415 kilometers (258 miles). The Callisto observations returned this week include a global observation and a high resolution observation of the Anarr crater region performed by NIMS (Near Infrared Mapping Spectrometer) and designed to obtain data on the composition and distribution of material on Callisto's surface. Observations by SSI (Solid State Imaging camera) include an image of the domed crater Har and a mosaic of a large, unnamed, central pit crater. The latter observation will be used for determining crater statistics and examining crater shapes on Callisto's icy surface. A bright limb observation performed by UVS (Ultraviolet Spectrometer) will be used to get more information on the distribution of hydrogen and oxygen as a function of altitude in Callisto's tenuous atmosphere. For the final remote sensing observation of this week, PPR (Photopolarimeter Radiometer) returns a polarimetry map of Callisto's surface. Also returned this week is a fields and particles instrument high-time resolution recording of the closest approach passage of Callisto. This 45 minute recording will provide the basis for the study of the processes associated with Callisto's interaction with Jupiter's magnetosphere.
The playback schedule also includes observations of Jupiter's Great Red Spot, a plume head region, regional maps and north-south strips performed by NIMS and PPR. A single polarimetry observation of Ganymede by PPR, an Io eclipse observation by SSI and an Io monitoring observation by NIMS round out the satellite observations and the playback plans for this week.