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Getting to Io won't be easy. As the spacecraft nears Jupiter
it will be exposed to high-energy particles trapped in the giant
planet's radiation belts. Although mission planners expect the
spacecraft to survive, the threat of damage is considerable.
"We're not doing much observing at Callisto on this encounter," noted JPL's Duane Bindschadler, the manager of Galileo's Science Planning and Operations Team, "just some radio science experiments to probe Callisto's atmosphere and its internal mass distribution. The primary science goals for this orbit are related to the Io torus and Jupiter's magnetosphere. All of the Fields and Particles instruments will be turned on and collecting data on plasma, dust, and electromagnetic fields. These are in situ instruments, which means that they sense the environment right around the spacecraft." One of these instruments - the plasma wave detector - will be listening for eerie-sounding "chorus" emissions. Chorus signals are low frequency plasma waves that produce strange whistling and popping sounds when converted to audio frequencies (see below). Not only are they entertaining to listen to, but they also convey important scientific information. By analyzing these signatures of plasma waves, scientists can understand how energy flows between Jupiter's magnetic field and the material in the Io torus. Below: This colorful spectrogram shows Chorus Emission from the satellite Ganymede recorded by the Plasma Wave Instrument on Galileo as it sailed past the moon in 1996. Chorus signals are plasma waves that can't be heard directly with the human ear, but they can be converted to audio frequencies. To hear chorus emission from Earth's magnetosphere recorded during a severe magnetic storm in Vancouver Island, BC, Canada on 21 Feb. 1994, click here for a RealAudio version or a WAV version.  "We
haven't been this far into the torus since 1995," says Bindschadler.
"In '95 the radiation levels we saw were relatively low,
but apparently that's not the case now. Galileo flew through
the outer edges of the torus last month [August 99] and ran into
a pretty intense
radiation environment. Something's happened in the inner
magnetosphere of Jupiter since 1995. We're not sure what."
"One possibility is solar activity," he continued. "In 1995 the sunspot cycle was near minimum, but now we're almost on top of the solar maximum. A particle event on the sun could have injected particles into the Jovian magnetosphere and caused high radiation levels. It's also possible that Io's to blame. In July, astronomers at the Wyoming Infrared Observatory were monitoring Io and they saw a big IR outburst. That means there was a volcanic eruption that might have substantially increased the local plasma density in the torus." If energetic particles were injected into Jupiter's magnetic field last month, will radiation levels still be high now as Galileo heads into the inner magnetosphere? "That's a really interesting question," says Claudia Alexander, a JPL plasma physicist. "We don't know how long it takes for energetic particles in Jupiter's magnetosphere to diffuse away. On Earth, high-energy particles from the solar wind enter our magnetosphere on the Sun-facing side, and then electromagnetic forces make them flow around to the magnetotail. From there, some particles head for the poles and trigger aurora, but most are simply trapped inside the magnetosphere. They can't remain trapped forever -- otherwise the magnetic field would fill up with plasma. During geomagnetic storms these trapped particles are ejected into space in the form of giant blobs of plasma called plasmoids." "It took 25 years of study to figure that out for Earth. We're just starting on that 25 year journey of understanding for Jupiter."  Left: A ground-based telescopic image of
the Io torus, showing emissions from singly ionized sulfur. Jupiter
is at the center and the Io torus is the faint ring extending
to the orbit of Io at 5.9 Jupiter radii. "Here's an example of how far we have to go," she continued. "In November, 1998 Galileo was scheduled to fly through Jupiter's magnetotail on the nightside of Jupiter, at about 90 Rj [Rj=Jupiter radii]. The center of the magnetotail contains something called the plasma sheet -- a thick layer of ionized gas that extends from Jupiter itself to beyond 150 Rj. The plasma sheet is not very massive, but it's big. At 90 Rj, it takes hours to cross through. We got ready to take the measurements, but when the spacecraft arrived the plasma sheet wasn't there! What a surprise. Imagine missing something so large." "There was probably a coronal mass ejection that hammered Jupiter and it changed the configuration of the magnetosphere," she explained. "As it turned out the absence of the plasma sheet from where we thought it should be taught us a lot. We had a lot of missed predictions in the beginning of this mission, but Galileo is really improving our understanding of Jupiter's magnetic environment."
Below: Two sulfurous eruptions are visible on Jupiter's volcanic moon Io in this color composite Galileo image. On the left, over Io's limb, a new bluish plume rises about 86 miles above the surface of a volcanic caldera known as Pillan Patera. In the middle of the image, near the night/day shadow line, the ring shaped Prometheus plume is seen rising 45 miles above Io while casting a shadow to the right of the volcanic vent. Io is about the size of the Moon. More information.  To
survive long enough for a joint rendezvous with Cassini, Galileo
has to first survive the next few months. The suspense will be
high in November when Galileo flies over the volcano Pillan Patera
and possibly right through its active sulfurous plume. As the
spacecraft flies closer and closer to Jupiter radiation levels
will climb. Although the intense radiation in the vicinity of
Io is strong enough to kill a human, Galileo is expected to continue
operating. Nevertheless, it could be subjected to enough radiation
to pepper the camera's light detector with blinding hits to many
pixels, and potentially cause the computer's bits to flip in
random ways, causing Galileo to enter safe mode and discontinue
data taking. "We're nervous," says Duane Bindschadler, "but not excessively so. We've had some minor radiation damage and degradation of subsystems throughout the mission. The things we begin to get concerned about now that we're approaching Io are catastrophic failures of critical subsystems. We can't absolutely predict what the radiation environment will be like and if there's an unexpected radiation storm there's not much we can do." "Nevertheless, there are some things we can prepare for. During our last pass through the torus, for example, we saw a number of [anomalous] events in various systems on the spacecraft due to radiation. The first was a series of bus resets. Those used to be very, very serious because they would cause the spacecraft to enter safe mode, and we couldn't revive it for at least 48 hours. You lose most of the observations during an encounter when that happens. But now we've implemented a patch to the spacecraft's software to prevent unnecessary safing. That should help us out as we go deeper into the Jovian system."  With the first Io encounter just
over a month away, mission scientists are excited and cautiously
optimistic that Galileo will survive and complete its mission.
Left: Ron Baalke and David Seal of NASA's Jet Propulsion Laboratory have prepared several computer-generated animations of today's Callisto flyby. Click here for viewing options. "It doesn't get much better than this," said one NASA scientist anticipating the Io images. To scientists and the public alike, October and November 1999 will be a suspenseful and exciting time for planetary exploration. Today's Callisto flyby is just the beginning. Galileo has been orbiting Jupiter and its moons since December
1995. Its primary mission ended in December 1997. The spacecraft
is currently near the end of a two-year extended mission that
will culminate in two daring flybys of volcanoes on Io later
this year. More information about the Galileo mission is available
at: http://www.jpl.nasa.gov/galileo/
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