Geology Reference
In-Depth Information
CHAPTER 10
The Uranus and Neptune systems
flybys in 1986 and 1989, respectively, the trajectories of
which were optimized for coverage of the moons. There
were, however, some critical aspects of the
ybys that had
to be taken into account. At Uranus
10.1 Introduction
Sir William Herschel was an amateur astronomer in Bath,
England, who built his own telescopes to view stars. In
March 1781 he observed a point of light that did not
behave like a normal star. At
first, he thought that it was
a comet, but, with further observations, the object was
shown to be the seventh planet
-
the
first to be discovered
in historical times. Although initially he suggested that it
be named Georgium Sidus (George
distance of 3 billion
kilometers from the Sun, and an even greater distance for
Neptune, the amount of light illuminating the satellites is
extremely low, making it dif
cult for Voyager
'
s vidicon
imaging system to take pictures. Some camera exposures
had to be as long as 96 seconds, but, because the space-
craft was traveling so fast, the long exposures would have
resulted in image
“
smear
”
(imagine trying to snap a pic-
ture of an oncoming car through the window of your car
with a camera exposure time of nearly two minutes).
Fortunately, the talented engineers at JPL devised an
image motion compensation routine in which the space-
craft was rotated while staying locked in position on the
target, thus enabling high-quality images to be taken
despite the long exposure times.
'
s star) after the reign-
ing King George III, it was subsequently named Uranus
after the Roman god of the heavens.
Following the discovery of Uranus, Neptune
'
s exis-
tence was predicted before it was actually seen.
Observations of the orbit of Uranus and the application
of the laws of physics suggested that some object ought to
exist beyond Uranus to account for its motions. On the
basis of independent calculations by the British mathema-
tician John Couch Adams and the French mathematician
Le Verrier, a German astronomer, Johann Gottfried Galle,
used a telescope at the Berlin Observatory to search for the
proposed object. On the very
first night of his search,
September 23, 1846, Galle found the suspected planet
within one degree of the predicted position.
Herschel continued viewing Uranus and in 1787 dis-
covered the two largest satellites, later named Titania and
Oberon. Subsequently, in 1851, Umbriel and Ariel were
found, but it was not until nearly 100 years later that the
fifth large moon of Uranus was discovered by Gerard Kuiper
and named Miranda. The discovery of Neptune
'
10.2 Uranus and Neptune
While Jupiter, Saturn, Uranus, and Neptune are referred to
as giant planets because they are so large, there are sig-
nificant differences between the sets Jupiter
Saturn and
Uranus
-
Neptune with regard to their interiors. As noted in
Chapters 8
and
9
, Jupiter and Saturn are composed pri-
marily of hydrogen in various forms (including metallic
hydrogen) and helium, while Uranus and Neptune include
substantial amounts of water in both frozen and liquid
states, along with ammonia, methane, and proportionally
large rocky cores. Such a con
guration allows electrical
dynamo conditions, enabling the generation of magnetic
fields.
Uranus takes 84 Earth years to orbit the Sun once and is
unique among the major planets in our Solar System in
that its axis of rotation is tilted about 82°, making it nearly
horizontal with the ecliptic plane (
Fig. 10.1
). Although no
-
slarge
moons followed a similar path to the discovery of those of
Uranus. Only 17 days after the discovery of Neptune in
1846, its largest moon, Triton, was found by William
Lassell. Again, it was more than a century until the other
large moon, Nereid, was found in 1949, again by Kuiper.
Most information on the geology of the uranian and
neptunian satellites comes from the Voyager 1 and 2
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