Today on Galileo - December 18, 1996
The big day is here! After traveling approximately 50 minutes, signals from Galileo will be received on Earth at about 11:43 pm PST, indicating that the spacecraft has flown by Europa. The spacecraft will fly by this icy satellite's surface at a distance of only 698 km! That is over 250 times closer than the Voyager spacecraft and over 45 times closer than Galileo's closest previous pass during the C3 encounter. However, before getting to Europa, Galileo will first fly past the moon Io at an altitude of 321,000 km (the second closest flyby of the mission after the previous pass during the C3 encounter which was at 243,000 km) and then past Jupiter at the closest distance for this orbit, a range of 655,000 km.
This exciting day starts early with color SSI images being taken of the inner satellites Amalthea and Thebe. These observations will provide information on color, shape and geology at resolutions of 12 km/pixel and 8 km/pixel, respectively. Io is observed throughout the day by SSI, NIMS and PPR. These observations are part of the continued monitoring of Io's surface which will allow scientists to measure how volcanic activity occurs over time. Toward the end of the day, the inner moon Adrastea is imaged by SSI. These observations are performed through a clear filter and will provide information on the geology of Adrastea at a resolution of about 6 km/pixel.
Jupiter hot spot observations continue today with a set of observations performed during the midmorning. A few hours later, a PPR observation, known as a North-South stripe, will look at the regions of Jupiter's atmosphere north and south of the hot spot region. This type of observation is geared toward allowing scientists to place the hot spot observations in context with what was happening on the rest of the planet.
The first observation of Europa during this orbit is executed in the morning. Performed about 11 hours before closest approach, PPR will acquire a global map of Europa in its radiometry mode at a resolution of 500 km. Most of the observations of Europa, however, are taken in the second half of the day and into tommorrow. The images and data obtained during the distant encounter with Europa during the C3 encounter have Galileo science and team members very excited about this close-up look at Europa. It is hoped that these observations will provide information on the characteristics of the different surfaces, how craters are eroded or removed from the surface, whether there are small amounts of gases at the surface of Europa, whether there is enough energy flowing around in Europa to produce "ice volcanos" and, finally, whether there is evidence of tectonic (earthquakes, etc.) activity on Europa. In addition, measurements by the fields and particles instruments will contribute toward understanding the interaction between Europa and Jupiter's magnetosphere. Might they find that Europa too has its own magnetic field?
The encounter with Europa provides a unique opportunity for the Galileo radio science team. For about 18 minutes, as Galileo flies by Europa, its radio signal will be blocked from reaching the Earth. This is the first time during Galileo's tour of the Jovian system that one of Jupiter's satellites has caused a spacecraft occultation. In tracking what happens to the radio signal as the spacecraft passes behind Europa, scientists will be looking for evidence of a tenuous atmosphere and, based on when the signal is lost and recovered, will be measuring the diameter of Europa expecting to get an accuracy of about 1 km.