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This Week on Galileo?
Today on Galileo
Monday, August 6, 2001
DOY 2001/218

Day 3 of the Io 31 Encounter

The day begins with our focus still firmly on Io, with Galileo only a little over an hour past closest approach, and the view is now of a nearly fully sunlit hemisphere of the volcanic satellite. The Near Infrared Mapping Spectrometer (NIMS) starts today's observations at 12:08 a.m. PDT [See Note 1] with a view of the Amirani and Maui region, looking at the temperatures of a hot spot at Amirani and surveying the distribution of sulfur dioxide in the area.

The Solid State Imaging camera (SSI) then follows at 12:25 with a medium-resolution look at Masubi Fluctus (or flow). This is our first view of this area at this resolution (393 meters per pixel or 1270 feet per pixel). The five images taken here also capture the site of a new hot spot first seen by NIMS during our previous encounter in May.

The ball is still in the SSI court, as it next views two other regions of flow -- Lei-Zi Fluctus and Kanehekili Fluctus. During the approximately three minutes that these pictures are being taken, Galileo is moving further away from Io at 7.1 kilometers per second (4.4 miles per second). As the distance increases, our resolution drops, and by the end of these observations, we can only see objects 410 meters across (1330 feet). Finally, the camera views the terminator, or day-night boundary of Io, looking at three areas, called Surya, Tohil, and Culann. By viewing areas near sunrise (or sunset) it is possible to use any shadows cast by the features to determine their relative heights.

NIMS again comes to the fore, and spends the next hour constructing temperature maps of several regions on the satellite, including the Prometheus volcano and Emakong. These maps can be compared with data obtained during previous orbits, such as our most recent Io flyby in February 2000. The comparison will show if any changes have taken place in the distribution of material, and will show if any new active regions have cropped up.

Next, SSI views the tiny inner satellite Amalthea, with the highest resolution we have yet achieved on this side of the body. However, given our distance from Amalthea at the time (635,000 kilometers or 395,000 miles), this still limits us to only being able to see features larger than 6.5 kilometers (4 miles). This picture will also be used to help pin down the exact location of Amalthea, using a technique called optical navigation. By improving our knowledge of the orbit of this satellite, we also improve our ability to maneuver the spacecraft to a planned close flyby in November of next year. At that time, we will be viewing the same side of the satellite that this view shows.

The Photopolarimeter Radiometer (PPR) instrument now rejoins the observing armada, as it spends the next two hours mapping the entire visible surface of Io for the first time in daylight. By comparing daytime temperatures of features with nighttime temperatures obtained in earlier observations, scientists will be able to determine the total amount of volcanic heat flowing out of Io, and thus better understand the processes by which Jupiter's tides heat Io and cause the volcanic activity. PPR then continues with a second, half-hour map of the entire disk of Io, this time measuring the polarization of the light, rather than the temperatures. This data provides insight into the detailed physical structure of the surface materials.

By now, it's 7:18 a.m. PDT, and time for another quick picture of Amalthea, to help triangulate its position. In the four and a half hours since the previous look, both the spacecraft and the satellite, which only takes about 10 hours to completely circle Jupiter, have moved considerably.

At 8:00 a.m. PDT PPR directs its focus on Callisto for its second of three brief studies of the polarization of light from that satellite during this orbit. The final observation comes at 7:37 p.m. PDT this evening.

There are longer pauses between the observations now, and it isn't until 10:00 a.m. PDT that NIMS looks again at Jupiter's atmosphere, measuring the cloud dynamics and compositional variations in the turbulent area lying just behind the Great Red Spot.

At 1:16 p.m. PDT, Galileo reaches this orbit's closest point to the icy satellite Europa, which was the focus of our observations throughout most of 1998. Our distance this time is 609,000 kilometers (380,000 miles), and, as with Sunday's distant pass by Ganymede, Europa is too far away to warrant any science observations.

Between 6 and 8 p.m. PDT, the Energetic Particle Detector (EPD) reloads its memory. In the past, this instrument has shown susceptibility to the radiation seen near Jupiter, which causes the microprocessor controlling the instrument to stop operating correctly. Though such an upset is certainly not guaranteed, this pre-emptive reload will restore the instrument to a proper operating state without needing the ground controllers to intervene.

By 9:20 p.m. PDT, the spacecraft has receded sufficiently from the fierce radiation field of Jupiter to allow the attitude control software to look once again for four stars to guide our way. For the past two days, we have been relying on a single bright star, whose signal in the star tracker sensor rises above the noise caused by the radiation. That noise has now subsided, and fainter stars can also be reliably viewed. More stars provide the software greater accuracy in determining the orientation of the spacecraft.

During this entire day, the suite of instruments which measure the fields and particles in the Jupiter system have been actively, quietly, and continuously collecting data and storing the results in the large science data memory buffer in the spacecraft computer. These data are then relayed to Earth whenever a ground tracking antenna views the spacecraft. The continuity of this data is important in understanding the detailed structure of the Jupiter environment as Galileo slices deeply into, and then back out of, the dynamic magnetosphere of the giant planet.


Note 1. Pacific Daylight Time (PDT) is 7 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 49 minutes to travel between the spacecraft and Earth. All times quoted above are in Earth Received Time.

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