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This Week on Galileo?
Today on Galileo
Thursday, May 24, 2001
DOY 2001/144

Galileo's Mission at Jupiter - Day 3 of the Callisto 30 Encounter

Today has an action-packed morning and a laid-back afternoon. There is one close encounter of the satellite kind today, which is with Ganymede, the largest of Jupiter's moons, at 5:10 a.m. PDT [See Note 1]. Though the distance from Galileo to Ganymede is a seemingly remote 358,700 kilometers (222,930 miles), this is slightly less than the distance from Earth to our Moon, and still close enough to command the attention of several of the science instruments.

The Photopolarimeter Radiometer instrument (PPR) leads the way with a global scale temperature map of the night side of Ganymede. Such observations show how the surface materials heat and cool when compared with other observations taken when the surface is in sunlight. This gives scientists information about the structure of those materials; how dusty or rocky, fluffy or solid, rough or smooth. This observation uses the radiometer, the portion of the instrument that measures temperatures. The polarimeter portion of PPR comes into play in four observations of Ganymede taken through PPR's polarizing filters. These observations occur throughout the day and catch the satellite under different lighting conditions. By measuring how much light is scattered off of the surface materials at different angles, scientists gain additional insight into the fine structure of those materials.

About an hour after the Ganymede closest approach, the Solid State Imaging camera (SSI) looks back at the morning terminator, or day-night boundary of the satellite. Even at this distance, it still takes two pictures laid side by side to capture the entire lit hemisphere of the body. Pictures taken when the sun is low on the local horizon are useful for revealing the topography of a region. This will allow scientists to perform global-scale mapping of the grooves and furrows along the terminator. Also, fortuitously placed in the field of view are two prominent bright-ray craters and a crater chain, which were discovered earlier in the mission by Galileo imaging.

Sharing observing time with these Ganymede studies, the Near Infrared Mapping Spectrometer instrument (NIMS) concentrates on viewing Jupiter itself. First up is a look at the white oval storm that PPR measured yesterday. Where PPR measured the temperatures, NIMS will be looking at the compositional variations in the clouds that make up the storm. The various Galileo instruments have been studying this feature and its precursors for the past four years, following the evolution of these long-lived and dynamic tempests. One of the questions that these measurements may help to answer is how these storms can survive for so long. The Great Red Spot, a giant high-pressure storm in the southern hemisphere of the planet, has survived in much the same form for over 330 years.

An area just downwind of the Great Red Spot is also the target of two NIMS observations today. This region in the wake of this most well-known of Jupiter's storms has been shown to be quite turbulent and variable, and these NIMS measurements will provide data on the composition and dynamics of the clouds.

Also under NIMS scrutiny today are two areas in the northern hemisphere that are populated by storms called 'brown barges'. These relatively long-lived features, which appear only in the North Equatorial Belt on the planet, were first seen in pictures taken by the two Voyager spacecraft when they flew by Jupiter in 1979. They have been studied from Earth since 1997 and seem to be rich in methane gas, but whether they are clouds of methane, or holes in the clouds through which deeper concentrations of the gas can be seen is still a mystery. Never tiring of the details of Jupiter's atmosphere, NIMS also views a band of hot spots in the northern hemisphere. These spots have been popular targets for all of the optical instruments on Galileo over the course of the mission.

By noon PDT, the spacecraft has receded far enough from Jupiter, at least 15 Jupiter radii or 1 million kilometers (660,000 miles) that radiation levels have dropped considerably. Radiation-related interference in the Star Scanner has decreased to the point that fainter stars can again be detected reliably, and the software begins to use three stars to calculate the orientation of the spacecraft. Since the encounter sequence began on Tuesday, the spacecraft had been relying on a single bright star to provide this information. The use of three stars provides a more accurate calculation over long periods of time.

NIMS occupies Galileo's time in the afternoon by making a global observation of Jupiter. This is the first of three limb-to-limb, pole-to-pole maps by NIMS to look for compositional variation in the atmosphere over an entire Jovian rotation of just under 10 hours.

Finally, at 10 p.m. PDT, the second period of continuous data collection begins for the six Fields and Particles instruments. These instruments are the Energetic Particle Detector (EPD), Heavy Ion Counter (HIC), Magnetometer (MAG), Dust Detector (DDS), Plasma instrument (PLS), and Plasma Wave Subsystem (PWS). This period will cover the closest approach to the primary target for this orbit, the satellite Callisto, which is coming up tomorrow.

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 50 minutes to travel between the spacecraft and Earth.

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