Today on Galileo – Sunday, August 5, 2001 – Day 2 of the Io 31 Encounter
Today is a busy day, which will end with our close flyby of Io. First up,
however, at 1 a.m. PDT (see Note 1), is a measurement by the
Photopolarimeter Radiometer instrument (PPR) of a white oval storm in
Jupiter’s atmosphere at around 30 degrees south latitude. These persistent
storm features have been a frequent target for Galileo’s scrutiny over the
past five years, and this long-term study has yielded significant insight
into their dynamics and life-cycle. This large, counter-clockwise rotating
feature, called “BA”, is about the size of the Earth. Our expectations are
that the feature, which is the product of a merger of three smaller white
ovals early in 2000, will either decay and dissipate or possibly grow
larger, similar to the Great Red Spot.
Following this, PPR turns its gaze on the north polar region of the giant
planet, an area crowded with many small storms which have a surprisingly
long life. We are observing a phenomenon first noted in ground-based
observations of Jupiter’s temperatures near its arctic pole. The polar
region has an irregular boundary that appears to form a wave. Our
observations will tell us whether we can observe this phenomenon at higher
spatial resolution to refine our plans in the next orbit’s encounter (in
October) to map the phenomenon at all longitudes. The Cassini spacecraft,
which passed Jupiter in December of last year, was able to make a far more
comprehensive study of a snapshot in time of the global atmospheric
environment than is possible from Galileo, with its limited data rate.
However, Galileo, being much closer to Jupiter, and for a far longer time,
can study specific features in much greater detail, and track their
evolution. Combining these two types of data is what keeps planetary
scientists happy, and provides significant advances to our knowledge of the
weather patterns on Jupiter.
At 4:30 a.m. PDT the tape is moved halfway down its length and back again.
Keeping the tape moving in this way helps prevent it from sticking to the
heads. The tape did stick in October of 1995 as Galileo first approached
Jupiter. This caused the project to change the mission strategy for the
December 1995 Jupiter Orbit Insertion period to guarantee capturing the
unique data provided by the Galileo Probe as it entered the Jupiter
atmosphere. This change in strategy meant forgoing recording any data from
the optical instruments during the 1995 Io flyby. Fields and particles data
was recorded, since that strategy was much simpler, and resulted in less
risk to the tape recorder. That unfortunate situation has meant that the Io
passes we have made in the extended mission have been that much more
exciting, due to the long wait for the data.
At 10:22 a.m. PDT Galileo reaches its closest point to the satellite
Ganymede. At 1,066,747 kilometers, however, it is too distant to provide
useful enough science return to trade the resources against the detailed
look at Io yet to come. In fact, for the remainder of the mission Ganymede
is too distant to provide a suitable target for observations, and so we have
looked our last upon this satellite!
At 2:50 p.m. PDT the spacecraft passes into the shadow cast by Jupiter. This
solar occultation lasts until 5:00 p.m. In addition, at 3:56 p.m. Galileo
also passes behind Jupiter as seen from Earth. This situation is used by the
Radio Science team. Sixteen minutes before the start of the occultation, the
radio signal sent by the spacecraft is changed to a pure tone, with no
telemetry modulation. By following carefully the changes to this signal as
it passes deeper into the atmosphere, the team can determine temperatures
and pressures of the different layers of gases and clouds. The physical
occultation lasts until 5:58 p.m., and the radio signal is changed back to
the normal telemetry mode 17 minutes later.
Evening is now upon us, and at last we turn our gaze to the primary target
for this orbit, the satellite Io. PPR once again takes the lead with a
series of observations between 6:05 p.m and 10:23 p.m. PDT. During this
time, the instrument takes a global temperature map of the satellite, and
nightside detailed temperature maps of the Pele and Loki volcanoes. PPR also
examines the north polar region of the satellite, which was found to be
unexpectedly warm during previous flybys. A scan from the equator to the
south pole will pass over the Pillan volcano. Finally, a map of the Lei Kung
Fluctus region is performed. Fluctus is Latin for ‘flow’, and indicates that
this area seems to be a large lava flow.
One additional unique aspect to this flyby is that this is the first time in
the nearly six years of the Galileo orbital mission that the spacecraft has
had a major encounter activity with no ground communications coverage. The
large, 70-meter diameter antenna near Madrid, Spain is currently undergoing
an extensive series of equipment modifications to provide support for future
space missions, and is unable to view Galileo during this encounter. At the
two other Deep Space Network communications complexes that are used to track
spacecraft, Galileo sets below the horizon as seen from the Canberra,
Australia site about 3 hours before closest approach, and does not rise as
seen from the Goldstone, California site until 5 hours after closest
approach. During this time, all data acquired on the spacecraft are stored
either on the tape recorder or in buffer areas of computer memory.
At 10:14 p.m. PDT, the spacecraft is only a half hour away from Io, and the
Fields and Particles instruments begin to record their data continuously at
a high rate (at least 7.68 kilobits per second, far higher than the possible
real-time rates of 20 to 60 bits per second) for the next hour.
At 10:19 p.m. PDT, the Energetic Particle Detector (EPD) instrument cycles
its power and reloads its memory to prepare for the close Io flyby. In the
event that the high radiation environment near Io and Jupiter causes the
instrument electronics to hiccup, this reset should provide the best
protection for the most critical data collection at closest approach.
Activities start coming thick and fast now, as we get closer. At 10:26 p.m.
PDT the Near Infrared Mapping Spectrometer (NIMS) joins the fray, observing
the Pele, Pillan, and Isum regions on the night side of Io, looking for
thermal anomalies and measuring any volcanic activity. While these
observations are taking place, the Solid State Imaging camera (SSI) is
cycling its power in preparation for the highest resolution images to come.
At 10:41 p.m. PDT, the spacecraft reaches its closest approach to Jupiter,
at a distance of 4.9 Jupiter radii (350,300 kilometers or 217,720 miles)
above the cloud tops. This is not the closest we’ve been to the planet. That
distinction came during our December 1995 entry into Jupiter orbit, when we
passed by at 3.0 Jupiter radii (214,500 kilometers or 133,300 miles).
Once again, PPR steps to the fore, and at 10:43 p.m. PDT directs its gaze
straight down towards the satellite. At this time, the spacecraft pointing
is fixed, and it is Io that passes before our view, as Galileo flies by at
7.1 kilometers per second (4.4 miles per second or 15,840 miles per hour!).
Our closest point to Io comes at 10:48 p.m. PDT, when Galileo is a scant 200
kilometers (124 miles) over the surface. Since we are flying over high
northern latitudes, just 13 degrees from the pole, PPR will be able to study
in great detail the unusually high temperatures seen in this region.
Two minutes later, at 10:50 p.m. PDT, SSI captures a series of six pictures
of Io in less than two minutes, looking at the volcano Tvashtar. In these
pictures, we will be able to see features as small as 3.4 meters across (11
feet). The Tvashtar volcano was active when Galileo and Cassini viewed the
area in December of 2000. The Galileo camera spied a bright lake of lava at
the site, and the Cassini camera, using an ultraviolet filter, was able to
detect a plume of erupting gaseous material reaching nearly 400 kilometers
(250 miles) above the surface. There is no direct evidence that the site is
still active at this time, but if there is any volcanic activity as we fly
by, either lava flows or eruptions, Galileo will acquire its closest look
yet at active volcanism on Io, the most geologically active body in the
solar system.
In comparison, the MISR instrument aboard NASA’s Terra satellite has
recently been observing the active eruption of Mt. Etna on the island of
Sicily in the Mediterranean Sea. Amusingly, from a linguistic standpoint,
the plume of ash and dust from Mt. Etna is now drifting across the Ionian
Sea! Is this a portent for Galileo’s view? The oracles have been mute on
this point. The Terra satellite flies at an altitude of 705 kilometers (440
miles) above the Earth, whereas Galileo’s closest point to Tvashtar is only
320 kilometers (200 miles). For pictures of Mt. Etna from the Terra
spacecraft, visit the web site http://www-misr.jpl.nasa.gov/ , and then
imagine what Galileo might be seeing from less than half that distance!
As SSI catches its breath from these pictures, NIMS takes over and maps the
distribution of sulfur dioxide (SO2) in the high northern latitudes of Io,
starting at 10:52 p.m. PDT. Then at 10:59 p.m. SSI sets its sights on the
volcano Prometheus, as this feature sits on the limb of Io as seen from the
spacecraft. When viewed in this way, any possible volcanic eruptions can be
seen silhouetted against the dark of the sky. Two minutes later, SSI
returns to gaze at Tvashtar from greater range for another six pictures.
These more distant, wider angle views of the area provide more general
context information, with a resolution of about 52 meters (170 feet),
within which the earlier pictures will give us the fine detail.
For ten minutes starting at 11:04 p.m. PDT, NIMS will be mapping the
Tvashtar region, and the instrument’s spectral measurements will provide
information about the composition of the surface materials.
At 11:21 p.m. PDT, SSI begins a series of images of various interesting
locations on Io, with exotic names like Savitr, Amirani, Maui, and Itzamna
Patera. These names, as are all the feature names on Io, have been chosen
by the International Astronomical Union to represent volcanic beings from
myth and history from many Earthly cultures.
At 11:28 p.m. PDT, NIMS begins a 16-minute observation of the Gish Bar hot
spot, looking for changes that may have occurred since we last viewed this
feature. Volcanic regions on Io are extremely variable, and a particular
site may be active on one orbit, then dormant on the next only a month
later.
PPR, which started the observation set today, neatly frames this 24-hour
period with a final observation, a single scan from the north pole to the
south pole on Io, measuring the temperatures and helping to study the
unusual polar warming on this bizarre satellite.
At this point in the flyby, we have still used up only half of the tape in
our on-board tape recorder. Once past closest approach to Io, Galileo is
able to look back and see a nearly fully sunlit hemisphere, which sets the
stage for a whole new set of observations being possible. But that is a
story for another day. 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 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:
