Galileo Executes Two Engineering Activities This Week

The spacecraft executes two engineering activities this week. On Monday, the spacecraft makes a small turn, or attitude adjustment maneuver, which is used to keep the communications antenna pointed towards Earth. On Friday, there is a routine maintenance activity of the propulsion system.

This week will see the conclusion of the tape playback of the magnetospheric survey data that was acquired as the spacecraft travelled outbound from the December encounter with Jupiter and its moons. When that is complete, a second pass through the recorded data will begin. When playing back data from the tape recorder after one of its close encounters, Galileo employs a two-pass data return strategy. The first time through the tape, some high priority observations are returned, and small samples or highly compressed versions of data from other observations are usually taken. This gets important data to the scientists as soon as possible. Also, since the data are processed and filtered on board the spacecraft before they are transmitted to Earth, this gives the scientists the opportunity to react to any unusual data that they might see, and change the way the data are processed the second time they are played back, enhancing the information content.

A second pass through the tape also gives scientists a chance to play back data which were lost in transmission the first time. This can happen because of minor technical glitches at the ground communications antennas, or, more usually, because of bad weather at the site, which can interfere with the signal. After a journey of 800,000,000 kilometers (500,000,000 miles) through space from Jupiter, the last few miles through the Earth's atmosphere can be the most treacherous to a radio signal.

This week's second pass through the tape mostly contains data from the Photopolarimeter Radiometer (PPR), the Solid State Imager (SSI), and the Near Infrared Mapping Spectrometer (NIMS). PPR is playing back data from its Ganymede eclipse observation. SSI is returning data from its Ganymede eclipse aurora observation, its color observation of Ganymede's polar cap boundary, and its observation of the strike/slip fault region of Ganymede called Dardanus Sulcus. NIMS is filling in gaps and returning additional wavelength data from its regional Ganymede map.