Today on Galileo - November 2, 1997
Today is the start of the last encounter of Galileo's primary mission. This encounter, however, does not mark the end of Galileo's activity at Jupiter. The Galileo spacecraft is scheduled to continue studying Jupiter's system for two more years through a follow-on mission called the Galileo Europa Mission or GEM. This encounter period features the moon Europa with a close flyby on Thursday. Although this is the last flyby of Europa during the primary mission, it is only the first in nine consecutive flybys of the moon. The next eight of these will occur in the first phase of the GEM.
Galileo will spend the next eight days gathering data from Jupiter, its moons and the surrounding environment. Most of the data is stored on the spacecraft's on-board tape recorder for transmission to Earth later in Galileo's orbit. During the eight days of the encounter period, the spacecraft will be an average distance of five astronomical units, about 750 million kilometers (465 million miles) from Earth. At that distance, it will take Galileo's signal approximately 41 minutes to travel from Jupiter to Earth.
Galileo's activity is controlled by command sequences or instruction sets that are sent to it by flight team members here on Earth. The first set of commands was sent to the spacecraft last week and will begin to execute today at about 8 AM Pacific Standard Time (PST). This instruction set terminates its run on Thursday at which time a new command sequence will take over and run through the end of the encounter period on Sunday.
The start of the encounter also marks the continuation of a data survey being taken by the fields and particles instruments. This data survey has provided a near-continuous record of activity in Jupiter's magnetosphere since the middle of the seventh orbit. The magnetosphere is the part of space in which Jupiter's magnetic field dominates the magnetic field of the solar wind. This survey will continue uninterrupted until the end of the encounter period, when the survey will stop. These multiple orbit surveys are not planned for GEM.
The activity level on Galileo for the first day of the encounter period is relatively light. It includes observations made by the Ultraviolet Spectrometer (UVS). These data are returned to Earth in real-time, which means that they are almost immediately transmitted to Earth and are not stored on the tape recorder. All of today's observations are of Callisto. The first looks at Callisto while it passes through an eclipse of sunlight by Jupiter. The second looks at Callisto at a solar phase angle of 97 degrees (solar phase angle varies between 0 and 180 degrees, at 0 degrees the target is completely illuminated by the sun and at 180 degrees the target is completely dark). The final observation of the day is not of Callisto itself, but of what is known as Callisto's neutral torus. This observation starts this evening and continues well into tomorrow. All of these observations are different ways to look for the presence of oxygen and hydrogen on or around Callisto. These observations have been repeated in many orbits. Changes in the amounts of oxygen and hydrogen would provide an indication of the presence of geologic activity on Callisto.
Near the end of the day, standard maintenance is performed on the spacecraft's tape recorder. The maintenance, which will be completed tomorrow morning, is performed to keep the tape recorder in good operating condition. These maintenance activities are typically performed twice per orbit, with one maintenance usually scheduled just prior to an encounter period where good tape recorder performance is important for storing the encounter data.