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

The pace of activity onboard Galileo picks up a bit this week. On Wednesday, routine maintenance is performed on the spacecraft propulsion system. On Friday, routine maintenance is performed on the tape recorder. Both of these activities are done periodically during the relatively quiet cruise portion of an orbit in order to maintain the health of the thrusters and of the tape recorder for when they are needed the most -- during the intense activities of the close satellite encounters.

On Sunday, some special science instrument calibrations are performed. Because of the limits on the amount of data that Galileo can return in a given orbit, calibrations, though very important, are rarely done. The preferred data to return are the detailed science observations made during the close flybys. However, because of the intense radiation environment the spacecraft has been living in over the years, and the simple fact that the spacecraft has been in space for over 10 years, the science instruments have slowly been degrading with age.

In order for the science data to accurately measure specific physical quantities (100 photons, for example), we must calibrate the instruments so that we know how they respond to a given input signal (the instrument receives 100 photons, but only reports 85 of them). Also, even in the absence of input signals (looking at black sky, for example) most instruments will report seeing some signal, mostly as a result of electrical noise in the control circuits.

The Solid State Imaging camera (SSI) will be taking pictures through different color filters of two star fields with stars of known intensity which have been measured before by the Galileo instrument. By comparing these pictures with those taken earlier in the mission, scientists can see how the sensitivity of the camera at different wavelengths may have changed. Also, pictures taken without opening the camera shutter will determine how much noise there is in the camera's electronic circuits. The Near Infrared Mapping Spectrometer (NIMS) will be looking at dark sky, and at the star Sirius, the brightest in the sky.

This is the last time in the mission that we plan on calibrating SSI and NIMS in this way. These measurements will be stored on the spacecraft tape recorder and played back over the next month, prior to the next flyby of Callisto near the end of May.

Leading up to this activity, playback of data recorded during the last flyby of Ganymede in December will continue. At this point, the data are mostly a replay of data that were lost in transit during an earlier playback attempt.

SSI will be filling gaps in an observation of Ganymede taken when that satellite was in Jupiter's shadow. These images were looking for the glow of an aurora on the satellite, which was the highest priority observation for this instrument on this orbit. Bits of other pictures to be returned are from Ganymede's polar cap boundary, and the Dardanus Sulcus region of this largest of Jupiter's moons. Also expected are pictures of a stormy area near the Great Red Spot on Jupiter itself, and of Jupiter's ring.

NIMS will be completing some mapping of Ganymede, both at a global scale, and at somewhat higher spatial resolution. An observation of Io will also be returned, keeping track of the volcanoes and hot spots on this satellite. On Jupiter itself, an observation of hot spots in the atmosphere near 7 degrees North latitude will be played back, as well as measurements taken of the North Temperate Zone and of the aurora in the south polar region of the planet.