Status Report

Today on Galileo – Monday, August 6, 2001 – Day 3 of the Io 31 Encounter

By SpaceRef Editor
August 5, 2001
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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.

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:

SpaceRef staff editor.