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This Week on Galileo?
Today on Galileo
December 28, 2000
DOY 2000/363

Galileo's mission at Jupiter - Day 2 of the 29th encounter

On Day Two of Galileo's encounter with the Jupiter system, remote sensing observations of Ganymede, Callisto, Jupiter, and Io start in earnest. The spacecraft flies over the surface of Ganymede at 00:25 a.m. PST [see Note 1] at an altitude of just under 2326 kilometers (1446 miles) and a speed of 10.5 kilometers per second (6.5 miles per second, or 23,500 miles per hour). Galileo also makes its closest approaches to Callisto, Jupiter and Io at 5:42 a.m., 7:26 p.m., and 7:33 p.m. PST, at ranges of 2.3 million kilometers (1.5 million miles), 535,000 kilometers (333,000 miles), and 921,000 kilometers (573,000 miles), respectively.

First on today's observation schedule is a 60-minute high-resolution recording centered on closest approach to Ganymede and made by the Fields and Particles instruments. The recording will capture measurements of the plasma, dust, and electric and magnetic fields surrounding Ganymede, which will allow scientists to obtain a better understanding of how the magnetic fields and magnetospheres of both Ganymede and Jupiter interact. Ganymede is the only planetary moon known to have its own internally-generated magnetic field. The Fields and Particles instruments are comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.

Two Fields and Particles instruments go solo during the encounter. First, the Plasma Wave (PWS) instrument performs an observation dedicated to the detection of chorus emissions. A chorus signal is seen in the electromagnetic fields measured by PWS when plasma is being accelerated by certain kinds of wave-particle interactions. By detecting and analyzing chorus emissions, PWS scientists hope to understand more about how magnetospheres operate in the Jupiter system. Second, Galileo's Dust Detector obtain measurements as it moves through the paths of Jovian dust streams. Scientists will use measurements to establish the velocity of the particles and deduce information on the processes responsible for the dust streams, which originate at Io and fling dust particles outward from Jupiter at more than 100 kilometers per second (62 miles per second, or 224,000 miles per hour).

Throughout the day, the Photopolarimeter Radiometer instrument (PPR) makes eight observations of Ganymede. The first is a scan of temperatures from south to north across the night side of Ganymede. The data captured by the observation will allow scientists to determine how quickly different types of terrain on Ganymede cool off at night. PPR then follows with a temperature scan of Ganymede's north pole, which is likely to be one of the coldest places on Ganymede and therefore may harbor some exotic materials. PPR's next four observations are focused on mapping surface temperatures while Ganymede is eclipsed from the Sun by Jupiter. Similar to the previous south-north scan, PPR looks at various heavily-cratered regions in order to determine how different types of surface cool off while in darkness. These scan will also provide scientists with data on the fine structure and composition of these regions. The regions captured in these scans are named Tros, Barnard, Perrine, and Nicholson. Once Ganymede emerges into sunlight, PPR performs two more observations. The first is comprised of three east-west scans, and the second is a complete map of Ganymede's disk. These observations are designed to study how quickly the surface warms up with the return of the sun. In addition, the temperatures from the global map will be compared to a map of temperatures of the same regions taken on Galileo's previous encounter, when the sun had been shining on the surface for many hours.

Galileo's Solid State Imaging camera (SSI) makes three observations of Ganymede today. The first is taken while Ganymede is eclipsed from the Sun by Jupiter and is designed to capture auroral activity. Scientists hope to measure the brightness of the aurora at visible wavelengths, and the vertical structure and latitudinal and longitudinal distribution of the emissions. SSI's next observation captures color views of Ganymede's polar cap boundary, including the regions of the Tros crater and Perrine. These images will allow scientists to examine the distribution of craters in the region and fill a gap in data taken by the Voyager spacecraft during their flybys in the late 1970s. Finally, SSI makes an observation of Ganymede's Dardanus Sulcus region. This observation bridges a gap in data from the Voyager encounters, and an earlier flyby by Galileo made during its primary mission. The observation will provide better resolution coverage than either of those opportunities in addition to capturing an interesting strike-slip feature that cuts across the Dardanus region.

The Near-Infrared Mapping Spectrometer (NIMS) makes five observations of Ganymede. The first represents the final NIMS data set of Ganymede taken at moderate spatial resolution. The information captured by this observation will be used in conjunction with data acquired in previous orbits by NIMS and SSI to investigate the detailed composition of Ganymede's surface. The next four observations capture high-resolution maps of Ganymede's entire disk. These data will be used to perform global studies of material types and their distribution.

PPR is also the first to take a look at Jupiter today. In a series of observations, PPR looks at Jupiter's North Equatorial Belt, Great Red Spot, and a north-south strip. PPR's North Equatorial Belt observation will also serve to determine if there are temperature differences between Jupiter's hot spots and surrounding belt regions.

NIMS also looks at Jupiter in today's proceedings. In five different observations, NIMS captures two scans of the Great Red Spot, one of Jupiter's North Equatorial Belt, one of Jupiter's North Temperate Zone, and one of a hotspot. These observations are designed to allow scientists to study the composition and dynamics of the clouds in these different regions. NIMS is the only instrument that takes a look at Callisto during this encounter. The observation is intended to capture data on the composition of Callisto's surface.

PPR and NIMS also look at volcanic Io during the latter half of the day. In three observations, PPR captures polarimetry measurements of Io's surface. These measurements will allow scientists to learn about surface texture and small-scale surface properties. NIMS, on the other hand, performs two observations. Both are designed to monitor volcanic activity on Io. These observations will provide science planners with information needed to plan observations for future Galileo flybys of Io.

Whew, that was a long day! Come back tomorrow to learn what else is on Galileo's To Do List.

Note 1. Pacific Standard Time (PST) is 8 hours behind Greenwich Mean 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 occurred 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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