This Week on Galileo - February 2-8, 1998
On this week's schedule, Galileo will transmit to Earth pictures and science information that were on the tail end of last week's schedule. This data, gathered during the Dec. 16 Europa flyby, when the spacecraft dipped 200 kilometers (124 miles) above the surface, was stored on the spacecraft's onboard tape recorder. It includes science information obtained by Galileo's camera of the Gilgamesh region of Jupiter's moon Ganymede. This is followed by a global observation of Europa performed by the Near Infrared Mapping Spectrometer, then the final part of the week contains the processing and transmission of two more camera observations. Both are of Europa, with one showing a region containing unusual fretted shaped, and the other depicting a region of dark lines.
Galileo is scheduled to perform a spacecraft turn and a flight path maneuver this week, both of which will exercise the spacecraft's attitude control system that has behaved anomalously on two occasions since December 1997.
Flight team members and JPL experts have confirmed that the cause of the two incidents was a hardware error in one of the spacecraft's two gyroscopes. Analysis efforts are now focused on understanding the characteristics of the error and the conditions under which the behavior may deteriorate.
Remember that the gyroscopes are the part of the attitude control system that can be used to control where the spacecraft is pointing. This control is typically provided by another part of the subsystem (the star scanner) that scans the sky surrounding the spacecraft for visible stars. The gyroscopes are used when the stars that are visible are not good enough to be used for this purpose, when the star scanner view must be covered to avoid over-exposure and damage due to the presence of bright light, or when the spacecraft is performing a turn and the star scanner is changing the stars it is using as a reference. In addition, the gyroscopes are needed for accurate remote sensing instrument pointing. They provide data on motion of the spacecraft that is small enough to not affect the operation of the star scanner (and control of where the spacecraft is pointing), but are large enough to affect where the instrument pack is pointing. The gryoscopes, which are mounted on the instrument pack platform, can detect this small motion, compensate for it and keep the instruments pointed where desired.
The commands for this weeks' spacecraft turn and maneuvers are being built with extra safeguards. These safeguards will protect the spacecraft from the anomalous behavior of the gyroscope and will ensure that the spacecraft remains pointed where desired by the flight team. The turn is being performed to keep the spacecraft's radio antenna pointing near the Earth. Turns like this one are required periodically to keep up with the Earth's motion across the sky. This week's flight path maneuver is the last orbit path correction prior to the spacecraft's close flyby of Europa scheduled for next week.