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Today on Galileo?
November 25, 1999

Galileo Threads Through the Heart of the Jupiter System

Galileo deftly threads its way through the heart of the Jupiter system today, diving deep into the intense radiation environment that surrounds the solar system's biggest planet. The spacecraft has distant flybys of Jupiter and Ganymede, with a relatively close flyby of Europa, and an extremely close flyby of Io. The geometries of both the Europa and Io flybys are somewhat unique in that they provide good views of the polar regions of these bodies. The onboard instruments spend the day performing observations of the latter two Galilean satellites.

Eleven hours and 36 minutes prior to closest approach to Io, the spacecraft passes closest approach to Europa at a distance of 8,642 kilometers (5,371 miles) from the moon's surface. Nine hours and 40 minutes later, the spacecraft flies past Jupiter at a distance of 5.7 Jupiter Radii (406,000 kilometers or 252,000 miles). The close flyby of Io is next on the flyby schedule. The spacecraft passes 300 kilometers (186 miles) from the surface of Io at 8:05 pm PST-SCET (8:40 pm PST-ERT). Three hours and nine minutes later, the spacecraft flies past Ganymede at a distance of 611,000 kilometers (380,000 miles) from the moon's center.

Observations of Europa occupy the first half of the day. In addition to the views of Europa's north pole, the flyby provides unique geometry in that Galileo can see the hemisphere of Europa that faces Jupiter. Such observations are difficult, if not impossible, to perform from Earth. The Photopolarimeter Radiometer (PPR) makes the first observations of the day, looking at Europa's dark side. The observation will be compared to day side observations of the same regions performed on previous orbits in hopes of identifying heat sources or thermal anomalies originating from within the icy moon. In the second observation, PPR takes polarimetry measurements of Europa's surface. The measurements will allow scientists to study Europa's surface texture and thermal properties.

The Solid-State Imaging camera (SSI) is next on the observation schedule, and is followed closely by an observations performed by the Near-Infrared Mapping Spectrometer (NIMS). Both of these instruments take a look at Europa's north pole. These observations will provided the highest resolution views of the polar region to date. SSI follows these observations with two more observations of features on Europa's surface. The first captures images of a pair of dark bands first detected in September 1996. The region is believed to be the site of relatively recent faulting and relative movement of blocks of Europa's crust. The second observation captures a view of mottled (or blotchy-looking) terrain believed to be related to ice volcanic flows. NIMS also looks for evidence of plate tectonics on Europa's surface.

PPR returns to the observation schedule with a dayside thermal map of Europa. The map captures a region that was also observed on Europa's night side in September 1998. The two sets of data covering this region will be compared in hopes of determining if the anomalous temperatures that were detected in the first observation are due to Europa's heat retention characteristics (or thermal inertia) or possibly due heat from volcanic activity inside Europa.

Just after 10:40 am PDT-ERT, the radio science team begins to carefully measure changes in the frequency of Galileo's radio signal. The changes are caused by Io's gravitational pull on the spacecraft, and the resulting Doppler shift in Galileo's radio signal. The radio scientists will track these changes for almost 20 hours, centered on the point of closest approach to Io, and will use the measurements to refine models of Io's gravity field and internal structure.

SSI is next to observe Europa and does so by taking a global scale image of the icy moon. PPR follows with another polarimetry map, followed by a regional observation performed by NIMS. The latter observation will aid scientists in constructing complete global maps of Europa. This observation also brings to a close the Europa portion of the encounter's observing campaign.

The Fields and Particles instruments perform the first of the Io-related observations. Seven hours and 17 minutes before closest approach to Io, the instruments begin a six hour and 40 minute high resolution recording of the Io torus. The Io torus is a region of intense plasma and radiation activity, in which there are strong magnetic and electric fields. The recording will gather data down through closest approach to Jupiter (5.7 Jupiter Radii), the third deepest Torus passage of Galileo's primary and Europa missions. The data acquired during this recording will be used to understand the structure and dynamics of plasma, dust, and electric and magnetic fields in the torus region. The data will also be important for understanding the overall dynamics of the Jovian magnetosphere.

PPR is the first instrument to perform remote sensing of Io. Its first observation of Io captures a global dark side map. The map will describe night time thermal emissions on Io and will aid scientists in the development of heat flow models. The second observation also captures Io's night side, but is regional in resolution and contains data on the southern hemisphere hot spots of Babar Patera, Sengen Patera, and Ulgen Patera. PPR's final observation captures a high resolution view of Pele's volcanic vent. The observation should allow scientists to pinpoint the exact location of the volcano's vents.

Starting 18 minutes prior to closest approach to Io, the Fields and Particles instruments record their data to the tape recorder for 49 minutes. As they did during the Io torus recording, the instruments acquire measurements describing the plasma, dust, and electric and magnetic fields surrounding Io. The primary purpose of this observation is to determine if Io possesses its own internally-generated magnetic field, similar to both the Earth and to another Galilean satellite, Ganymede.

Galileo's flight path takes the spacecraft above Io's south polar region and was chosen in order to make this observation. In addition, measurements made by individual instruments will be combined to better understand processes such as particle pickup by the magnetic field, and thermal and non-thermal plasma interactions near Io.

SSI and NIMS follow PPR's remote sensing observations with a series of observations designed to characterize Io's surface at resolutions higher than ever before. By collaborating, the instruments will provide a wealth of useful information on the morphology, thermal state, and composition of surface materials.

NIMS makes the first observation of the series by looking at a hot spot called Tiermes. Both SSI and NIMS then take a look at Io's south pole. Io's south polar region is relatively unknown, and these observations will provide new information about this area. SSI follows next with a solo observation of a feature that was seen to be "sapping" in an observation that was made in June 1999. Sapping is the natural process of erosion along the base of a cliff by which soft layers are worn away. The erosion removes the support for the upper part of the cliff which then breaks off in large blocks and falls from the cliff face. NIMS then takes a look at Io while the Prometheus volcanic region is on the limb as seen from the spacecraft. The observation will be used to examine atmospheric materials at various heights above Io's surface.

SSI and NIMS join again for the next several observations. In the first, the instrument pair looks again at Io's south pole. In the next couple of observations, the pair looks at Emakong Patera in hopes of catching active lava flowing from the hot spot. Next, SSI and NIMS look at Tupan Patera, a caldera-like feature from which information about the form and distribution of the hottest materials on Io's surface is expected to be gleaned. In another pairing, SSI and NIMS jointly look at the sapping feature detected in June 1999.

SSI continues the observation campaign by looking a hot spot called Shamshu Patera. This is followed by a solo observation from NIMS of the Tupan Patera region. SSI and NIMS then join back up and make another observation of Emakong Patera. Together with the previous images, scientists will be able to construct stereo views of Emakong.

As the spacecraft recedes from Io, NIMS and SSI perform another joint observation. In it, the two instruments look at two unnamed giant volcanic calderas in Io's northern hemisphere. This activity is followed by observations of the Culann volcanic region. SSI contributes a color image of the region to the growing Io data set. In the last joint observation, the instrument pair look at a region of Io's surface near the terminator (or line dividing night from day). The last observation of the day is performed by NIMS and contains a regional map of Io's surface.

Come back tomorrow and learn of Galileo's remaining observations and initial plans to return all the data stored on board!

Note 1. Pacific Standard Time (PST) is 8 hours behind Greenwich Meridian Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occured is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 35 minutes to travel between the spacecraft and Earth.

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