Today on Galileo - September 13-15, 1997
The next to last science encounter of Galileo's *primary* mission starts today, Saturday, September 13th at 5 pm Pacific Time. Ending next Saturday morning, this encounter is scheduled to last almost 7 days. Galileo will fly past Callisto on Tuesday. Having flown past Callisto in the previous and third orbits, this is the third and last time Galileo will fly close to Callisto during its prime mission. During this encounter, Jupiter and the spacecraft will be an average distance of 4.3 astronomical units or about 642 million kilometers (400 million miles) from the Earth. At that distance, it will take radio communications approximately 36 minutes to travel from spacecraft to Earth, or vice-versa. Commands sent to the spacecraft earlier this week will control spacecraft activities through the middle of Wednesday. Commands for Wednesday through Saturday will be sent to the spacecraft later this week.
With the start of encounter activities, we note the continuation of the second fields-and-particles instruments "mini-tour." Initiated prior to the Ganymede - Orbit 8 encounter and maintained with little or no interruption, this collection of low time-resolution magnetospheric data will provide a record of the long term variations in Jupiter's magnetosphere. The survey is scheduled to continue through this encounter period and into cruise.
With not much time left in the day, Saturday's activities would have included the execution of the last orbit trim maneuver prior to the Callisto flyby. This maneuver has been canceled because the spacecraft is almost exactly on the desired path for this pass through the Jupiter system and over Callisto's surface. It is important to make sure that the actual path is as close as possible to the expected path to ensure that the science observations (which were planned based on the expected path) are successful. In this case, the paths match up without having to perform the final correction.
Early Sunday morning, the first observation of the encounter period is initiated. This observation of the Callisto Neutral Torus is performed by the Ultraviolet Spectromenter (UVS) and continues through the rest of Sunday and for most of Monday. The observation is designed to look for oxygen and hydrogen (among other elements) that may have been released from Callisto's surface. If present, they would form a light atmosphere around the satellite and a torus in the satellite's orbit. Changes in these measurements from orbit to orbit could give scientists clues that would tell them whether there is geologic activity (earthquakes, volcanos, etc) on the satellites surface. Also on Sunday, the spacecraft will perform regular maintenance on the tape recorder. The UVS observation does not require the tape recorder and maintenance is performed to maintain the tape pack in good operating condition.
Monday is spent completing the UVS observation of the Callisto Neutral Torus. Toward the end of the day, the UVS instrument together with the Extreme Ultraviolet Spectrometer (EUV) turns its attention to the Io Torus. During the day, flight team engineers will transmit the second set of encounter commands to the spacecraft. These commands will control all spacecraft activities through the end of the encounter schedule on Saturday.