Today on Galileo - November 6, 1997
Today the spacecraft performs a close flyby of Europa and reaches the minimum distance from Jupiter for this encounter period. Today's scheduled science activity focuses on these two bodies but also includes a few observations of Io, Ganymede and some of the minor satellites. The first set of instructions for the encounter period completes its run this morning, timing out at about 2 AM Pacific Standard Time (PST) at which time the second set begins to execute.
At 12:32 PM PST, the spacecraft will fly over the surface of Europa at a distance of 2042 kilometers (1276 miles) and a speed of 5.7 kilometers per second (12,750 miles per hour)! The flyby of Europa is followed by passage through the point of closest approach to Jupiter, which occurs just over 4 hours later at 4:42 PM PST, at a distance of about 9 Jupiter Radii, 641,000 kilometers (398,000 miles).
In the morning the Ultraviolet Spectrometer (UVS) continues to monitor Io's plasma torus for changes in shape, size or activity level. This observation is followed by the first of a series performed by the Photopolarimeter Radiometer (PPR). This series is aimed at Jupiter and has a goal of constructing a 360 degree map of a strip of Jupiter centered at 13 degrees north latitude. The observation series continues toward the end of the day and a few more portions are completed tomorrow. The Near Infrared Mapping Spectrometer (NIMS) also looks at Jupiter this morning. In a series of six observations, NIMS continues to gather data on Jupiter's brown barges.
Starting approximately 10 hours before the Europa flyby, the radio science team begins to monitor Galileo's radio signal for changes in radio frequency due to Europa's gravitational pull on the spacecraft. As the spacecraft passes Europa, its velocity is affected by Europa's gravitational pull and these changes are seen as frequency changes in Galileo's radio signal. By using the Doppler effect and measuring these changes, the radio science team will be able to construct a map of the gravity field produced by Europa.
A couple of hours prior to noon (PST), the instruments turn their attention to Europa and the approaching close flyby. The UVS instrument is the first to look at Europa by performing a couple of observations. The first captures data on selected portions of Europa's surface while the second looks at Europa's edge. This observation is followed with one by the PPR instrument designed to look for and detect hot spots on Europa's surface. This is only the first observation in a campaign that will be continued into GEM (Galileo Europa Mission).
The NIMS and Solid-State Imaging (SSI) instruments follow closely with a series of observations of different types of regions on Europa. Included in these region types are impact craters named Pwyll and Mannann'an and wedge-shaped areas that seem to indicate places where the surface has been pulled apart. Also covered by these observations are areas characterized by features known as triple-bands where the surface is thought to have split apart and material from beneath the surface has flowed onto the surface. A region of bright plains is also observed. These plains are thought to be smooth regions that may have recently been resurfaced. Finally, a high-resolution observation of previously unexplored terrain is performed by SSI. This observation is performed under lighting conditions that are ideal for determining the shape of the region. The observation is unique in that the same lighting conditions are not expected to be available during GEM.
In addition to these remote sensing observations, the fields and particles instruments perform a high-time resolution recording of the interaction between Europa and Jupiter's magnetospheric environment. This observation starts about 30 minutes before the Europa flyby. The recording continues for about 2 hours after Europa closest approach in order to also record the magnetic environment as the spacecraft passes through Jupiter's magnetic equator.
Amidst the series of Europa flyby observations, the SSI instrument takes a break from Europa and performs a light intensity observation (photometry) of Io. Once this series is completed, other targets trickle back in to the observation schedule. UVS looks at Ganymede several times, SSI looks at Io again in an observation designed to provide surface shape data (topography), and SSI takes a series of observations of the minor moons Thebe, Amalthea, Adrastea and Metis. These minor moon observations are designed to help determine the global shape of each of these bodies. The evening's observation set also includes more NIMS observations of Jupiter's brown barges as well as PPR's set of global observations of Jupiter. Finally, NIMS performs a thermal observation of Jupiter's atmosphere.