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

Standard cruise activities continue for the Galileo spacecraft this relatively quiet week. On Monday, the spacecraft performs routine maintenance on the propulsion system. On Thursday, the spacecraft is turned 3.9 degrees to keep the communications antenna pointed towards Earth.

In the realm of real-time science data collection, the Extreme Ultraviolet Spectrometer (EUV) continues its two-month-long study of interplanetary hydrogen gas.

Just to keep things interesting, on Wednesday and Thursday the spacecraft appears to pass within approximately 0.5 degree of Earth's Moon, as seen from the ground communications antennas. When this happens, the Moon can actually be "seen" by the antennas which are tracking the spacecraft, and can interfere with the radio signal from Galileo. This effect is not nearly as severe as that seen when the spacecraft and Sun are close together in the sky, but we still make sure that no valuable telemetry is being sent during the time period when communications are affected. Not all of the complications that govern how a spacecraft is operated are caused by situations in the remote reaches of the solar system!

As part of the continuing playback of data stored on the on-board tape recorder during Galileo's May flyby of Callisto, the data expected this week are from the Solid State Imaging camera (SSI) and the suite of Fields and Particles instruments that measure the magnetic field environment of Jupiter. These instruments are the Energetic Particle Detector (EPD), Heavy Ion Counter (HIC), Magnetometer (MAG), Plasma instrument (PLS), and Plasma Wave Subsystem (PWS).

SSI will be returning the highest resolution images of Callisto ever obtained. They were taken near our closest approach, which was at 138 kilometers (85 miles) altitude. In addition, stereo pictures of a domed crater will be played back. The Fields and Particles data were recorded during a period of approximately one hour centered on the closest approach to Callisto, and will help to study the interactions between the solid body of Callisto and the electromagnetic fields and plasmas of Jupiter's magnetosphere. In addition, these data will add to our understanding of Callisto's own magnetic field. Like Europa, Callisto displays an induced magnetic field, possibly due to the presence of substantial liquid water within a hundred kilometers (62 miles) or so of its icy surface.