All quiet on the Galileo front
Posted: May 30, 2001

An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL
The excitement of the encounter week has settled down now, and activity levels drop to the quiet murmur which is usual for the cruise portion of an orbit.

On Monday, the Extreme Ultraviolet Spectrometer (EUV) instrument begins a 68-day long study of the ionized hydrogen that permeates the solar system. The length of the observation allows scientists to watch changes in the emissions from this plasma for longer than two full rotations of the sun. This will show the interactions between solar activity and the interplanetary hydrogen gas, and contribute to knowledge of the long-term evolution of the distribution of this gas from solar minimum to solar maximum.

Also on Monday, the spacecraft performs an orbit trim maneuver. In it, the spacecraft thrusters are fired in order to nudge Galileo in its path around Jupiter, lining it up for the next encounter with the satellite Io in early August. The maneuver is preceded by a calibration of the gyroscopes, which are used to maintain the orientation of the spacecraft while the thrusters are firing and the star scanner is shuttered to prevent contamination.

On Thursday, the spacecraft is turned by 4.3 degrees to keep the communications antenna pointed more closely towards the Earth. On Sunday, routine maintenance of the spacecraft propulsion system is performed.

During this encounter, the Solid State Imaging camera (SSI) experienced a re-occurrence of an anomaly which causes its pictures to appear completely overexposed. This problem also occurred during Galileo's flyby in December, 2000, affecting something less than half of the images taken at that time. In both cases, high radiation levels near Jupiter appeared to help to trigger the anomaly. Telemetry readings from the camera suggest that all of the near-Callisto images were successfully recorded, and that some earlier observations of Io and Ganymede may have been lost. Our first priority for data playback this week is to return samples of many of those images. This will give us a better indication of exactly which observations were affected. On a subsequent pass over the tape later in the orbit, we will then be able to play back larger areas of unaffected frames to reap their scientific bounty. Ah, the woes of an aging spacecraft in a hostile environment!

On Sunday, the playback process is paused and the spacecraft is settled into a quiet state to ride out a period called Solar Conjunction. This roughly three week time span is when Jupiter, with the Galileo spacecraft in tow, appears to pass behind the Sun as seen from Earth. When the line of sight between Earth and Galileo comes too close to the Sun, radio interference from the active solar atmosphere makes reliable communication impossible.