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

The week of Hallows Eve finds an 11-year old Galileo spending a relatively quiet week mapping the Jovian magnetosphere. The spacecraft was launched on October 18, 1989, arrived at Jupiter on December 7, 1995, and has spent the last 5 years exploring Jupiter, its moons and rings, and its magnetospheric environment.

Galileo's Fields and Particles instruments are in the initial days of a 100-day continuous survey that starts in the solar wind, continues through the Jovian magnetosphere (when Galileo flies through the heart of the Jupiter system in December) and proceeds back out into the solar wind. The survey is part of a joint investigation with the Cassini spacecraft, which is now approaching Jupiter and will pass by in December en route to Saturn.

All planned data playback from previous encounters was completed in time to support the Fields and Particles survey. When Deep Space Network (DSN) antennas are scheduled for use by Galileo, survey data are immediately packaged and transmitted to Earth. But when these antennas are being used by other projects, Galileo makes use of an onboard data buffer (a section of computer memory) to store data until an antenna is next made available for use by Galileo. The data buffer, however, only holds about seven hours of data, and tough negotiations for DSN antennas have resulted in larger DSN coverage gaps for Galileo than the data buffer can handle without overflowing. With data playback complete, the tape recorder can be used to record the contents of the data buffer, and prevent loss of valuable data.

Why are these data so valuable? It is a rare and valuable circumstance when two spacecraft are in the same region of space, simultaneously examining the same phenomena from different viewpoints. The dual-spacecraft observations will allow scientists to observe both the solar wind and the interior of the Jovian magnetosphere at the same time. With this unique opportunity, scientists will be able to see how changes in the solar wind can affect the interior of the magnetosphere. Galileo's tape recorder is used six times this week to record the contents of its data buffer, once each on Monday, Thursday, Friday, and Saturday, and twice on Sunday.