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

During the next two weeks Galileo completes weeks four and five of the 14-week long continuous survey of the Jovian magnetosphere being performed by the spacecraft's Fields and Particles instruments. In addition, in the latter of the two weeks, the spacecraft will start playing back data which were recorded earlier in the survey period on Galileo's onboard tape recorder. Galileo also performs a pair of engineering activities in the coming days. On Friday, November 24, the spacecraft performs standard maintenance on its onboard tape recorder. On Saturday, December 2, the spacecraft performs standard maintenance on its propulsion systems.

The Fields and Particles instruments are the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument. Their survey is part of a dual-spacecraft observation campaign with the Cassini spacecraft, which will pass by Jupiter in December on its way to arrival at Saturn in 2004. Cassini instruments will measure the characteristics of the solar wind during this period, while Galileo's flight path takes it from the solar wind, into the depths of the Jovian magnetosphere, and back out into the solar wind. Scientists will be able to use both data sets to study how changes in the solar wind (Cassini's measurements) affect the outer edges of the Jovian magnetosphere and its interior (Galileo's measurements).

The continuity of Galileo's survey is still a high priority during the next couple of weeks. As described in previous editions of This Week on Galileo, the spacecraft is using its onboard tape recorder to store the contents of a data buffer when insufficient communications time is scheduled for use by Galileo. Without recording the contents of the data buffer, the survey data would be lost. However, Galileo also starts to play back some of the survey data that were recorded during the earlier weeks of the survey. You might be asking, "What happened to not being able to do playback during the same time period that data are being recorded?". The answer is that Galileo has been recording the buffered data in a relatively narrow region of its tape. As a result, the tape will be commanded to move back and forth from unrecorded portions (where new buffer dumps can be laid down) to the previously recorded area (where data are being played back). By allowing the two activities of playback and recording to "take turns," Galileo is able to maximize its usage of the available communications to Earth, and make its survey of the magnetosphere more extensive.