October 18 - 24, 1999
Galileo Continues to Return Science Information from Historic Io Flyby
Galileo continues to process and return to Earth science information acquired during its historic
flyby of Io a couple of weekends ago. The data are stored on the spacecraft's onboard tape recorder.
In the process of playback, the spacecraft's main computer reads the tape, processes and packages the
data and sends them to Earth to be received by NASA's Deep Space Network antennas. Galileo's
playback is interrupted twice this week. On Tuesday, the spacecraft performs regular maintenance
on its propulsion system. On Friday, a standard calibration is performed on the Near-Infrared Mapping
Flight engineers continue to investigate the computer memory anomaly that caused the spacecraft
to enter into safe mode just hours prior to the Io flyby. Jupiter's intense radiation belts,
through which the spacecraft flew, caused an error in the memory of Galileo's onboard computer,
causing the spacecraft to enter safe mode. However, flight controllers were able to scramble and
restore most functions in time for the flyby.
All of the recorded science that was planned for the Io encounter was safely stored on the tape
recorder except for a 5 hour Fields and Particles observation of the Io torus and some
stand-alone Photopolarimeter Radiometer (PPR) observations. The Fields and Particles recording
was lost as the spacecraft was in the process of being recovered from safe mode by ground
controllers. The PPR observations were deleted as they would have accessed the faulty area of
memory and caused the spacecraft to enter safe mode again.
Investigations have now identified the affected area of memory, and have determined that it is
safe to play back observations performed by the Solid-State Imaging camera (SSI). However, the
affected area lies in a part of memory used to record three types of data (PPR and two types
from the Near-Infrared Mapping Spectrometer). The investigation has shown that there is a single
bit in memory that is permanently damaged. Efforts are now being focused on developing software
changes to not use the affected bit.
The Near-Infrared Mapping Spectrometer (NIMS) returns most data in the first half of the week.
The second half of the week is devoted to returning observations made by the SSI. Small amounts
of ride-along data recorded by the PPR during several SSI observations are also included on this
week's schedule. A small snippet of data is also returned by the Fields and Particles instruments.
The data come from the instruments' 65-minute high resolution recording of the magnetospheric
environment (plasma, dust, and electric and magnetic fields) surrounding Io. The data will
assist scientists with studies of the Io ionosphere and its interaction with the Jovian magnetosphere.
This week's NIMS observations contain information on several different regions of Io's surface.
All of the observations were taken while the features were on Io's day side, allowing NIMS to
provide scientists with data describing the composition of the surface. The data consist of
spectral signatures of the surface which contain information on the absorption and emission of
light by the surface materials. These spectra allow scientists to identify these materials by
their unique signatures. In addition, the NIMS data is expected to reveal the presence of thermal
emissions due to active volcanic flows and other volcanic features. This will tell scientists
which targets were actively erupting or had recently erupted at the time the observations were
The first NIMS observation captures a look at Dorian Mons, a mountain-like feature whose
geological structure, origin and history are presently unknown. The area is characterized by
greenish colored deposits. Next returned is a scan of the Amirani, Skythia, and Gish Bar regions,
followed by a look at a region of Io's surface near the sun terminator (or line dividing the
night from day). The region also contains the Hi'iaka caldera.
In hopes of catching a volcanic plume in action, the next observation captured the Pillan hot
spot while it was situated on Io's limb as seen from the spacecraft. If Pillan was active during
the observation, its plume would be observed against the dark sky above the limb. In a similar
observation, the NIMS also looked at the plume of the Pele volcano. The geometry of the
observation was such that the plume, again, if present, will be on Io's limb, but this time with
Jupiter's disk in the background. The NIMS then returns a couple of regional scans of Io's surface.
The playback schedule then cycles back up to the beginning of the observation sequence, in order
to return SSI observations. First, the NIMS returns two observations capturing the Loki and
Pele volcanic regions while on Io's night side. Taken in darkness, the observations were designed
to search for thermal emissions from the volcanic calderas. The first SSI data consist of
high-resolution images of the Pele region. The images are taken with Pele in darkness in the
hope of catching hot glowing lava near Pele's volcanic vent. Capturing daybreak on Io, the
next observation consists of high-resolution images of the Pillan volcanic region, taken with
oblique viewing geometry, but good low-sun illumination.
The next series of SSI observations are taken in full sunlight. The first captures the Colchis
Montes region. Next is the return of images of the Zamama volcanic vent, followed by the
Prometheus volcanic vent and associated lava flows. A comparison of clear and green filter
images of Prometheus are expected to reveal unresolved lava and allow scientists to determine
SSI then returns another set of images of Colchis Montes, with a wider, lower resolution look
that should provide a context for the higher resolution images. This is followed by an
observation of Tohil Mons, and a return to Prometheus. Tohil Mons and Colchis Montes, like
Dorian Mons, are mountains, whose geological structure, origin, and history are presently
unknown. The Prometheus images are in full color, and will be combined with the previous set
of images to provide stereo coverage of the region. The NIMS also returns spectral scans of
Prometheus at this point in the playback schedule.
SSI then returns a second observation of the Zamama volcanic vent, providing coverage of a
wider region, again as context for the higher resolution observations performed earlier. The
next observation contains a look at Dorian Mons, which as mentioned earlier, is characterized
by greenish colored deposits. Finally, SSI returns moderate resolution images of the Amirani,
Skythia, and Gish Bar regions.
On a final note, Monday, October 18, is Galileo's 10 year anniversary of launch. Carried into
orbit by Space Shuttle Atlantis (STS-34), Galileo was propelled out of Earth orbit by an
Inertial Upper Stage booster rocket. It arrived at Jupiter in December 1995. En route to Jupiter,
Galileo flew past Venus once and Earth twice, using gravity assists to give the spacecraft
enough energy to get to Jupiter. Galileo obtained the first close-up images of an asteroid
(Gaspra). Flying by the asteroid Ida, Galileo also discovered the first known moon of an
asteroid (Dactyl). Galileo gave us our only direct views of the crash of Comet Shoemaker-Levy
9 into Jupiter's atmosphere. Upon arrival, Galileo's probe penetrated Jupiter's atmosphere and
returned a Jovian weather report on temperature, pressure, composition, winds and lightning.
Since then, the orbiter has flown past Jupiter's Galilean moons a total of 25 times: Callisto
(7), Ganymede (4), Europa (12) and Io (2). During its tour of the Jovian system, Galileo has
made discoveries that include extreme high temperatures at Io's volcanoes, indications that
there may be liquid water inside Europa, an internal magnetic field for Ganymede, and has even
raised the possibility of liquid water inside Callisto. Since launch, the spacecraft has
traveled a total of 5,539 million kilometers (3,442 million miles) and returned over 3.8
gigabits of data.