An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

Galileo is about 815 million kilometers (507 million miles) from Earth this week, continuing its trek around Jupiter, the solar system's largest planet. The spacecraft spends the week playing back science data that is stored on its onboard tape recorder. This week's data set was acquired during Galileo's May flyby of Ganymede, the largest of Jupiter's moons.

Data playback is limited only by the amount of time scheduled for Galileo's use of the Deep Space Network's 70-m (230-foot) diameter radio antennas. That diameter is just about the same length as a Boeing 747 jumbo jet. Those are some pretty big ears! The Deep Space Network (DSN) consists of three communications facilities placed at longitudes approximately 120 degrees apart around the world. The facilities are located at Goldstone, in California's Mojave Desert; near Madrid, Spain; and near Canberra, Australia. This strategic placement permits constant observation of a spacecraft as the Earth rotates. Galileo's schedule this week includes daily "tracks" from Goldstone and Canberra, averaging just over 6 hours from each location. On Sunday, Galileo also gets about 5-1/2 hours at the Madrid radio antenna.

This week's playback schedule includes an observation by the Fields and Particles instruments, one by the Plasma Wave instrument (PWS), and two by the Solid-State Imaging camera (SSI). Continuing from last week, the Fields and Particles instruments are first on this week's schedule with portions of a 60-minute high-resolution recording of the plasma, dust, and electric and magnetic fields surrounding Ganymede. Ganymede is the only planetary moon that is known to have its own internally-generated magnetic field, and thus, its own magnetosphere. The data taken during this observation will allow scientists to obtain a more complete understanding of the unique interactions between the magnetospheres of Ganymede and Jupiter.

Next, PWS returns an observation dedicated to the detection of chorus emissions within Ganymede's magnetosphere. A chorus signal is seen in the electromagnetic fields measured by PWS. The chorus signal is present when plasma is being accelerated by a particularly efficient type of wave-particle interaction. Scientists hope to understand more about Ganymede's unique magnetosphere by detecting and analyzing chorus emissions.

Toward the end of the week, data from the two SSI observations are processed, packaged, and transmitted to Earth. These observations are part of a set of five designed to provide scientists with information regarding how different features and terrains came to exist on Ganymede's surface. The younger terrain types examined in this set of observations are believed to have been created by processes internal to Ganymede, but the roles of volcanic vs. tectonic processes are not yet clear. The first mosaic of images captures smooth bright terrain and grooved terrain that may be partially surrounded, or "engulfed" by the surrounding terrain. The second observation looks at a transition region between bright and dark terrain.