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

It is going to be yet another quiet week for the Galileo spacecraft. Long cruise periods between encounters can be that way! On Tuesday, the spacecraft performs standard maintenance on its tape recorder system.

Playback of the data stored on the tape from the December 27 flyby of Ganymede continues. At the time of this writing, we are running about a day ahead of schedule on our playback plan. This can happen when the data compress more efficiently on the spacecraft than we had planned for, and therefore take less time to transmit to the ground. If the trend continues, we will be able to take advantage of the time gained and retrieve more data, either by specifying additional time periods or more wavelengths to play back, choosing larger fractions of images (we don't always play back the whole frame), or asking the spacecraft to compress the data less. Since our image compression technique is similar to the popular JPEG image format used throughout the World Wide Web, when we specify a high level of compression, the image will look much blockier, but will take fewer bits and less time to transmit. If we have more bits available, we can specify a lower level of compression, and the image will provide much more detail to the scientists.

This week we will continue to receive pictures of the Ganymede aurora taken by the camera while the satellite was in shadow. Additional Photopolarimeter Radiometer data taken during the artificial night of the Ganymede eclipse, and data taken when the satellite returned to the daylight, will show how various surface areas warm up, and will complement the data taken earlier, which showed how those areas cooled down when entering the shadow.

Most of the week, however, will be taken up with a set of color pictures of the boundary area of Ganymede's North Polar Cap. Since this flyby of Ganymede occurred near 60 degrees North latitude, this will provide us with a view of the North Polar Cap which we haven't seen since our seventh orbit in April of 1997, during the Prime Mission. It is equivalent to flying by the Earth just south of Anchorage, Alaska. Only our second flyby of Ganymede in September of 1996 was at a higher latitude (80 degrees).

Near the end of the week, we should begin to see additional images taken of an equatorial region of Ganymede named Dardanus Sulcus. Also expected are infrared data taken at moderate spatial resolution of Ganymede. This is the final such data from Ganymede planned to be taken by the Near Infrared Mapping Spectrometer instrument for the remainder of Galileo's mission at Jupiter.