Quiet times on Galileo probe
Posted: June 5, 2001

An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL
Galileo, the spacecraft, is now settling into a 3-week-long period of extreme rest, even while Galileo, the flight team, is gearing up in planning for the next flyby in early August.

On Monday, June 4, the spacecraft enters a period called solar conjunction. Each year there comes a time when Jupiter, with Galileo in orbit around it, appears to pass behind the Sun. On Monday, the angle between the Sun and Galileo with Earth at the apex shrinks to less than 7 degrees. At this point, even in the best of circumstances, the noise from the solar atmosphere interferes with the radio signal from Galileo, making reception of the science and engineering data doubtful. This year in particular, with the Sun reaching the time in its activity cycle called solar maximum, the interference is particularly bad, and some of our planned data return has already been lost in the noise.

On Wednesday, June 13, the apparent separation between Galileo and the Sun as seen from Earth reaches its minimum of a third of a degree.

So for now, the spacecraft systems have been battened down and prepared for the long dry spell. The routine maintenance activities are complete, the playback of data from the on-board tape recorder is paused, and the computer routines that look for regular communications from Earth have been told not to expect any messages for the duration. There is no increased risk to the spacecraft during this time, but our inability to see what's going on makes us wary of performing any activities.

The sole exceptions to this enforced quiet are for the Extreme Ultraviolet Spectrometer (EUV) instrument, and passive support of a radio science experiment. EUV is continuing to look for variations in the Sun's output by looking for light reflected off of interplanetary hydrogen gas. This is a simple task, however, which only involves storing the science data in buffer areas of computer memory. Radio science investigators in Germany take this opportunity to study the way the radio signal is affected by its journey through the Sun's atmosphere. This provides information about the structure, content, and dynamics of the gases streaming out from the Sun. This study relies on the primary radio signal itself, not on the correct reception of the ones and zeros which are the meat and potatoes of the remaining science telemetry.