Geology Reference
In-Depth Information
CHAPTER 8
The Jupiter system
8.1 Introduction
planets and stars in the 1500s by Nicolaus Copernicus
suggested that this might not be true. Care had to be taken,
however, because such views were contrary to popular
beliefs and were considered heretical. In fact, the Italian,
Giordino Bruno, was too outspoken in expressing his
negative views of a Sun-centered system. In 1600, he
was tried at an Inquisition in Rome, found guilty of heresy,
and was burned at the stake.
So here was Galileo, announcing to the world that he
had found four objects orbiting something other than the
Earth. Could this also be the heretical rantings of another
trouble maker? Clearly, Galileo had to walk a
fine line
between maintaining scienti
c integrity (he knew what he
had seen) and not upsetting the establishment. But Galileo
also had some very powerful friends, the Medici family,
who controlled much of central Italy at the time. Galileo
'
s
work was directly or indirectly supported by the Medici,
and, when he discovered the moons of Jupiter, he had the
good political sense to refer to them as the Medician
“
Jupiter, one of the brightest objects in the sky, was named
after the mightiest of the Roman gods because of its
dominance. More massive than the other planets com-
bined, Jupiter with its rings and satellites has been likened
to a
“
miniature Solar System.
”
More than 400 years of
telescopic observations and, more importantly,
flights of
the Pioneer, Voyager, Galileo, Cassini, and New Horizons
spacecraft have yielded images and other data for the
Jovian system that are among the most spectacular in the
Solar System.
8.2 Exploration
Scienti
c exploration of the Jupiter system was begun in
1610 by Galileo Galilei. He had been waiting many days
for the night-time skies to clear so that he could try out a
new, technically advanced instrument. But it was January in
the town of Padua in northern Italy where he worked and
winter skies were frequently cloudy. Then, on January 7, a
break in the weather brought a sparkling clear night, and
Galileo was able to use the new invention to discover a
fascinating set of worlds. These discoveries not only
brought Galileo much acclaim but also led to a series of
military contracts to put the invention to other uses. The
invention was the telescope, and, although Galileo did not
invent it, he was probably the
first to use the telescope to
study the heavens, leading to his discovery of the four large
moons of Jupiter.
Galileo
'
s discovery was pretty heady stuff and not with-
out substantial controversy. The late 1500s and early
1600s saw the emergence of ideas that the Earth might
not be the center of the Universe. Egocentric humans
naturally assumed that planets, as well as stars, circled
around the all-important Earth. But careful tracking of the
”
Improvements in telescopes following Galileo
'
s obser-
vations enabled re
ned views of Jupiter and de
nition of
the moons
'
orbits and other general characteristics.
Surprisingly, the innermost moons, Io and Europa, were
found to have densities suggesting rocky materials, while
data for the outer two moons, Ganymede and Callisto,
suggested a large proportion of water or water-ice.
The Space Age ushered in the
first visits by spacecraft to
the outer planets. In 1973 and 1974, two spacecraft,
Pioneers 10 and 11, respectively, streaked past Jupiter and
obtained the
first close-up observations, including the
rst
spacecraft images of Jupiter
'
s moons. The picture quality
was rather limited, however, because the spacecraft had to
spin like bullets to keep on course. This was something
like trying to take pictures while turning somersaults
stars.
not
an easy task, even in the Space Age. The
first high-quality
pictures were taken by the Voyager 1 and 2 spacecraft
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