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.

For more information on the Galileo spacecraft and its mission to Jupiter,
please visit the Galileo home page at one of the following URL’s: