Today on Galileo - December 15, 1997
Today is the start of the first encounter of the Galileo Europa Mission, a two- year extension of the Galileo Mission. The encounter is the second of nine consecutive close flybys of Jupiter's moon Europa, the first of which occurred as the last encounter of Galileo's primary mission, last month. The encounter period only lasts approximately two days, a feature that will be typical of most encounters of the low cost Galileo Europa Mission. During this period the spacecraft will be an average distance of 5.5 astronomical units, about 820 million kilometers (510 million miles) from Earth. At that distance, it will take radio signals traveling at the speed of light about 46 minutes to reach Earth from Jupiter and vice-versa.
The spacecraft passes the point of closest approach to the moon Ganymede and to Jupiter during today's activities. The minimum distance to Ganymede's surface, 14,400 kilometers (8,950 miles), is reached at 1:58 a.m. The minimum distance to the center of Jupiter is reached toward the end of the day, at 10:35 p.m. This distance is approximately 8.8 times the radius of Jupiter or about 629,000 kilometers (391,000 miles).
With the start of the encounter period, the fields and particles instruments begin a "real-time" survey of the magnetic and electric environment around Jupiter. In real-time data collection, the data that is gathered by the instruments is almost immediately processed and transmitted to Earth. The fields and particles will continue this survey throughout the encounter period.
Ganymede is the first target of the encounter period. The Solid-State Imaging (SSI) camera takes a look at one of the moon's craters known as Gilgamesh. Although the observation covers a region on Ganymede, it is designed to provide data that will help determine the age of Europa's surface. The Gilgamesh crater is believed to be the youngest on Ganymede and may also mark the end of heavy bombardment in the Jovian system about 3.8 billion years ago. By counting the number of small craters in the Gilgamesh region, scientists hope to understand how many impactors have been entering the Jupiter system since the end of the bombardment period. They can then compare this number of impactors to the number of craters on Europa's surface and get an idea of its age.
The first observation of Europa is taken by the Ultraviolet Spectrometer (UVS). Scientists hope the observation will detect changes in Europa's tenuous atmosphere. These changes could be in the form of gases, like oxygen, hydrogen, and sulfur, escaping from Europa's atmosphere. Orbit-to-orbit changes in the amounts detected could be an indication of geologic activity (earthquakes, water volcanoes, etc) on Europa's surface. The observation is repeated for a second time later in the day.
Io is briefly included in today's observation schedule. The SSI instrument takes an image of Io while eclipsed from the sun by Jupiter. These eclipse images have proven to one of the best ways to discover and monitor lava temperatures and interactions between volcanic plumes and Jupiter's magnetic environment. Io's atmosphere is also monitored during the day with an observation performed by the UVS instrument. Similar to one performed on Europa, the observation is designed to look for changes in Io's atmosphere. In Io's case, the changes will more likely be due to volcanic activity or sublimation of sulfur frost.
Jupiter is also briefly included in today's schedule. The Near Infrared Mapping Spectrometer (NIMS) takes snapshots throughout the day that will, from orbit to orbit, allow scientists to track composition and temperature changes in the atmosphere over time.
Observations of Europa return late in the day as the spacecraft gets closer to the icy moon. Starting 10 hours before the point of closest approach, the radio science team starts to monitor Galileo's radio signal for changes in radio frequency due to Europa's gravitational pull. By using the Doppler effect and measuring these changes, the radio science team will be able to refine the map of the gravity field produced by Europa which was built based on information from previous orbits. The information will also allow scientists to refine their models on what the interior of Europa might look like. The day's last two observations are global observations of Europa performed by SSI and NIMS.
Note: All times of day listed today correspond to the Pacific Time zone and spacecraft event time. Radio signals indicating that an event has occured on the spacecraft reach the Earth 35 to 50 minutes later, depending on the time of year.