This Week on Galileo - February 24 - March 2, 1997
The first week after its second encounter with Europa is still quite busy for the Galileo spacecraft. Having just flown through the Jovian system, the spacecraft will pass through three Earth occultations (by Europa, Jupiter and Io) and one Solar occultation (Jupiter/Io). An occultation is a period of time when the view to one celestial body is blocked by the body of another. Earth occulations are of particular interest to the Radio Science investigators. When Galileo starts passing behind Jupiter or one of its moons, its radio signal must travel through the outermost layers of the body's atmosphere. The atmosphere refracts (essentially, bends) and weakens Galileo's radio signal. As the spacecraft moves more and more behind the body, the radio signal must travel through more and more layers of atmosphere, which weaken the signal even more until it can no longer be detected (of course, when the spacecraft is behind the planet or moon itself, that completely blocks the signal). On the other side, the radio signal will gain strength and will be bent less as the spacecraft re-emerges. The data gathered while monitoring Galileo's radio signal along with trajectory information will be processed by the investigators and ultimately result in temperature and wind profiles of the body's atmosphere.
At the beginning of the week, the spacecraft is scheduled to perform the first orbit trim maneuver after its encounter with Europa. Each of these post-encounter maneuvers are important because they are used to correct the amount of "gravity assist" energy transferred to the spacecraft during the satellite flyby. For example, to examine only one of the variables, if the flyby is a little lower in altitude than planned the spacecraft receives more energy and if the flyby is a little higher than planned the spacecraft receives less energy. These maneuvers are used to add or subtract energy as required by the flyby conditions and send the spacecraft on the right path toward the next encounter.
Galileo continues gathering magnetospheric data from its fields and particles instruments through the middle of the week. In addition, the first observations from the encounter are returned to earth. In general, the order of playback follows the order in which the data was recorded on the tape recorder. As such, this week's playback focuses mainly on observations of Io and Jupiter. Europa observations start to be returned toward the end of the week and into next week. A single image of Thebe is returned about half-way through the week.
The observations of Io included in this week's playback concentrate mainly on the continual effort to monitor plume activity and surface changes. Observations of Pele, Loki and Ra Patera were specifically taken in hopes of observing a plume in action. Surface changes are hoped to be characterized by determining the changes in the chemistry of Io's surface as well as the spatial, temporal and spectral variability of the surface.
Jupiter observations returned this week are all of the selected atmospheric feature for this encounter--White Ovals. Remember, from last week's Today on Galileo, white ovals are huge storms easily seen from Earth. They are usually found in the boundary regions between two opposing zonal jet streams and some of these are very long lived. What provides energy to these storms is only one of the many questions scientists hope to answer with Galileo's observations.
The Europa observations returned toward the end of this week are mostly regional in resolution. Global and regional observations performed by SSI and NIMS are part of campaigns that span the different Europa enconters (orbit 4 and orbit 11) and are designed to result in complete global images and maps. An observation performed by SSI of a region showing the intersection of two dark lineae is the final observation on this week's playback schedule. This region is believed to represent a stress-controlled eruption or intrusion of material originating from beneath Europa's surface.
More Europa observations are on next week's schedule.