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

This is a relatively busy week for Galileo, considering that we are in the cruise portion of our 31st orbit of Jupiter. On Monday, the Near Infrared Mapping Spectrometer (NIMS) performs a calibration. The instrument, which can measure the temperatures of the features it looks at, will direct its gaze towards a target plate mounted on the spacecraft that has been heated to a known temperature. Measuring the signal the instrument generates will allow scientists to accurately gauge the true temperatures of the atmospheric and surface scenes that were captured by NIMS during the August 5 flyby of Jupiter and Io.

On Thursday, routine maintenance of the on-board tape recorder is performed. At least once every 30 days, the tape is wound at high speed from one end to the other and back again to help reduce the mechanical stickiness which has plagued the operation of the recorder in the past. Normal playback operations consist of many small low-speed motions back and forth, as small amounts of data are read into the spacecraft computer memory, processed, and packaged for transmission to Earth at rates of 20 to 160 bits per second. When you compare this to a typical computer modem, which can communicate at 56,000 bits per second, you can begin to see why it takes a month or two for Galileo to completely read out a full tape, which can contain nearly a gigabit (1,000,000,000 bits) of data. At the modem speed, this amount of data could be transmitted in approximately 5 hours.

On Saturday, Galileo reaches a milestone as it performs the 100th scheduled orbit trim maneuver since entering orbit around Jupiter in December of 1995. This engine burn will fine-tune the trajectory of the spacecraft, directing its path to the next flyby of Io in October. The maneuver, which could last as long as 9 hours, is preceded by an automatic drift rate calibration of the gyroscopes, which are used to maintain the attitude of the spacecraft as the thrusters fire.

On Sunday, routine maintenance of the propulsion system is performed, followed by a 4 degree turn of the spacecraft to keep the communications antenna pointed towards Earth.

Io flyby data scheduled for tape playback this week are two NIMS observations and one from the Photopolarimeter Radiometer instrument (PPR). The NIMS observations are of the Gish Bar and Amirani hot spots on Io, measuring temperatures and sulfur dioxide distribution in those areas. The PPR measurement is a temperature map of Io along a single strip that runs from the north to the south pole, along a line representing roughly noon relative to the 42-hour "day" of the satellite. These observations were taken an hour to an hour and a half after the closest approach to Io on August 5.