Geology Reference
In-Depth Information
in 1979. Unlike the spinning Pioneer spacecraft, the
Voyagers maintained their course using small rockets and
had a stable platform for taking pictures.
The Pioneers and the Voyagers were yby scouts to
provide the reconnaissance for the next stage of explora-
tion, the Galileo project. The Galileo mission involved an
orbiter and a probe designed to plunge deep into Jupiter ' s
atmosphere. The probe was out tted with instruments to
measure the temperature, water content, composition, and
other properties of the atmosphere. The orbiter carried ten
instruments, including those used to map the composition
of Jupiter ' s clouds and the surfaces of the moons, an
imaging system, and various instruments to study small
particles and Jupiter ' s electromagnetic environment.
Getting the Galileo spacecraft to Jupiter was not easy.
Although the project began in 1977, it was 18 years before
the spacecraft arrived at Jupiter. The project was to be the
first planetary mission using NASA ' s new shuttle as a
means for leaving the Earth. However, shuttle delays
and the tragic explosion of the shuttle Challenger deferred
the launch of Galileo until 1989. Moreover, it was found
that the launch capability was less than originally planned,
and a direct path to Jupiter was not possible.
Fortunately, talented engineers at the Jet Propulsion
Laboratory found that by flying past Venus, looping
around the Sun twice, and flying past the Earth suf cient
momentum could be gained to sling-shot onward to
Jupiter. Although this saved the mission, there was also
a price to pay; during the period before leaving the inner
Solar System, the fragile high-gain antenna on the space-
craft was folded up like an umbrella. As the spacecraft left
the inner Solar System and the command to unfurl the
antenna was sent, it opened only part way and, despite
months of trying to shake it free, it remained stuck. This
meant that all communication would have to be funneled
through the inef cient low-gain antenna, which required a
complete redesign of data acquisition for it to be as effi- -
cient as possible. Painful decisions were made to decide
which observations would have to be thrown out. For
example, originally more than 70,000 pictures were to
be taken, but the number had to shrink to fewer than
2,000. The camera, however, had an adaptable computer
system, which enabled a great deal of flexibility in taking
pictures ef ciently.
By happenstance, the Galileo spacecraft was in a
unique position to view a momentous event in Solar
System activity; the collision of the comet Shoemaker-
Levy 9 with Jupiter in 1994. This comet had been discov-
ered two years earlier, and, during previous passes of
Jupiter, it was ripped into ~21 fragments, some more
than 2 km across. Analyses of the orbits of the fragments
led to predictions of impact with Jupiter. Because of the
position of the Earth with respect to Jupiter at the time of
the impact, Earth-based telescopes could not view the
actual collisions but only the impact scars left in
Jupiter ' s clouds. The Galileo spacecraft, however, was
in a position to capture the events as they occurred, en-
abling the first observations of active, large impacts in the
Solar System.
In the final stages of its journey to Jupiter, in 1995 a
beachball-size probe from Galileo plunged into Jupiter
' s
atmosphere where its instruments operated for nearly an
hour. This marked another first in Solar System explora-
tion - measurements made directly within the atmosphere
of a giant planet. As the probe relayed its data back to
Earth, the Galileo spacecraft began a series of maneuvers
to place it in orbit around Jupiter. This involved a close
flyby of Io, using gravity to slow the spacecraft and to put
it in a trajectory to begin orbiting Jupiter in 1996. The
mission lasted until 2003, completed 35 orbits around the
planet, and enabled observations of the entire Jupiter
system.
In December 2000, the Cassini spacecraft ew past
Jupiter on its way to Saturn. This provided the unprece-
dented opportunity to make simultaneous observations
from two spacecraft of a planet in the outer Solar
System. Then, in February 2007, the New Horizons space-
craft flew through the Jupiter system on its way to rendez-
vous with Pluto in 2015. During the flyby, it was possible
to obtain new data to assess changes that had occurred on
Io since the Galileo mission, monitor cloud patterns on
Jupiter, and collect other data.
8.3 Jupiter
Views of Jupiter (Fig. 8.1) show intricate clouds in a great
variety of colors. The colors result from trace amounts of
sulfur and organic materials, some of which were prob-
ably implanted by impacts. What lies below Jupiter ' s
clouds cannot be determined directly with today
s tech-
nology, but models provide insight into the characteristics
of the deep interior. Because of Jupiter ' s massive size, the
density of the gasses must increase tremendously, reach-
ing stages in which the hydrogen would first be liquid and
then transition into a metallic state. At the very center of
Jupiter, it is thought that a rocky core about the size of
Earth might be found.
'
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