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

As Galileo closes in on Callisto, preparations heat up. On Monday, a standard test of the on-board gyroscopes is performed. These gyroscopes seem to show the most sensitivity of any of the spacecraft systems to the accumulated radiation dose that the spacecraft has received over the years. The test will determine if software parameters related to this sensitivity need to be updated prior to the flyby.

Following this test a new sequence of commands begins operating the spacecraft. This sequence updates already planned activities to account for changes in the availability of ground communication antennas, and to change telecommunications parameters to account for the effects of solar conjunction. Solar conjunction is when the spacecraft appears to pass behind the Sun as seen from Earth. Since the Sun is now near the maximum in its activity cycle, the noisy and highly dynamic solar atmosphere can wreak havoc on the radio signal trying to reach Earth. Changing the way the spacecraft modulates the signal that it transmits improves the chance of properly receiving and decoding that signal when it finally reaches the ground.

On Thursday, the spacecraft closes to within a distance of 60 Jupiter radii (4.3 million kilometers, 2.7 million miles) of the giant planet. At this point the Magnetometer instrument is reconfigured to provide better sensitivity to measure this portion of the magnetosphere which lies closer to the planet.

And while all this is going on, playback of previously recorded data continues. This week we expect to see the conclusion of the calibration data that was recorded three weeks ago. The last of these data are from the Solid State Imaging camera, and were pictures taken of the Photometric Calibration Target plate mounted on the spacecraft. Following the calibration data, we will pass one final time completely through the tape, collecting bits and pieces of data that were not properly received the first time we tried to play them back.