An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

Cruise activities continue for the Galileo spacecraft this week. On Tuesday, the Near Infrared Mapping Spectrometer (NIMS) performs an instrument calibration using the Radiometric Calibration Target. This target is a plate that is mounted on the spacecraft and can be heated to a specific, known temperature. This allows the NIMS instrument to determine the accuracy of its heat-measuring sensors. The last such calibration was done in early April. By periodically checking out the instrument in this manner, scientists can determine accurately how the signal from the instrument is changing as the detectors age.

On Friday, the spacecraft performs an Orbit Trim Maneuver (OTM), the 97th such activity planned since Galileo entered orbit around Jupiter in December of 1995, and the second of three planned for this orbit. This burn of the spacecraft's propulsive thrusters adjusts the path the spacecraft in order to accurately reach our next close flyby of Io in early August.

On Saturday, routine maintenance of the on-board tape recorder is performed. Throughout the week, the Extreme Ultraviolet Spectrometer (EUV) continues its two-month-long study of interplanetary hydrogen gas.

This week's scheduled playback of data from the tape recorder includes observations from NIMS, the Solid State Imaging camera (SSI), and the suite of Fields and Particles instruments that measure the magnetic field environment of Jupiter. These instruments are the Energetic Particle Detector (EPD), Heavy Ion Counter (HIC), Magnetometer (MAG), Plasma instrument (PLS), and Plasma Wave Subsystem (PWS). All of the data to be returned this week were recorded during the close flyby of Callisto in May.

NIMS will be returning global observations of the atmosphere of Jupiter this week. SSI will be returning the highest resolution images of Callisto taken near our closest approach, which was at 138 kilometers (85 miles) altitude. The Fields and Particles data were recorded during a period of approximately one hour centered on the closest approach to Callisto, and will help to study the interactions between the solid body of Callisto and the electromagnetic fields and plasmas of Jupiter's magnetosphere. In addition, these data will add to our understanding of Callisto's own magnetic field. Like Europa, Callisto displays an induced magnetic field, possibly due to the presence of substantial liquid water within a hundred kilometers (62 miles) or so of its icy surface.