Saturn, called the wonder of the heavens by early astronomers, has been
studied from Earth for many centuries. When Galileo first focused his
telescope on Saturn in 1610, he realized that the appearance of the
planet was unusual, but he never knew its real character because the
power of his homemade telescope was far too low. He thought he was
looking at three globes, one large and two small, which seemed to change
slowly in appearance. In 1655, Huygens, after years of observing the
planet, finally realized that these projections were actually a flat
ring slightly separated from the main globe.
[Illustration: First sketch of Saturn (Galileo, 1610).]
[Illustration: First sketch showing a division in the rings
(Cassini, 1675).]
In 1675, Cassini found the first breach in the supposedly solid, rigid,
and opaque ring when he discovered that it was divided into two parts by
a dark line, now known as Cassini’s Division. In later years he also
detected some of Saturn’s moons.
[Illustration: First successful photograph of Saturn (Andrew Common,
1883). (Discovering the Universe, Colin A. Ronan, Copyright 1971 by
Colin A. Ronan, Basic Books, Inc., Publishers, New York.)]
The earliest successful photograph of Saturn was taken in 1883 by Andrew
Common. In 1895, James Keeler suggested that the rings are in fact a
swarm of particles in near-independent orbits. These rings, until the
recent discoveries of faint ring systems around Jupiter and Uranus, were
considered unique in the solar system.
Since Galileo first used his homemade telescope to view Saturn, there
have been many observers. There have also been major advances in
telescopes; a resulting modern view of Saturn is shown on the next page.
The corresponding sketch shows the nomenclature of the brightest rings.
Now the planet has been seen for the first time not from Earth, but in
much closer views by an instrument on a spacecraft, the imaging
photopolarimeter on Pioneer Saturn. The instrument separately measures
the strengths of the red and blue components of sunlight scattered from
the clouds of Saturn and converts this information into numbers. The
data are transmitted to Earth as part of the spacecraft telemetry. The
signals are then converted by computer into shades of gray on
photographic film, and the two components plus a synthesized green image
can be recombined into a color image that approximates the planet’s true
color. Some of the resulting images are shown on the pages following the
Earth-based view. These pictures were produced by a scientific team from
the University of Arizona.
These images, while helping to unravel some of the mystery surrounding
the planet, have created even more interest regarding it. We are really
just beginning to know Saturn. It is up to future spacecraft to more
completely reveal her secrets and solve her mysteries.
[Illustration: High-quality contemporary Earth-based view of Saturn
(Photo: Catalina Observatory, University of Arizona).]
[Illustration: Nomenclature of bright rings.]
OUTER (A) RING
CASSINI DIVISION
MIDDLE (B) RING
CREPE (C) RING
[Illustration: Pioneer Saturn image of Saturn and its rings from a
distance of 8,400,000 km (August 22, 1979, 10 days before closest
approach). Resolution has not yet reached Earth-based quality. Faint
banding is visible on the disk of the planet. The silhouette of the
rings can be seen in front of the planet. Slightly above this
silhouette is the shadow of the rings on the disk. Beyond the disk,
structure can be seen in the rings. The rings have a distinctly
different appearance in this and subsequent images than in
Earth-based pictures because they are illuminated from below rather
than from above as we view them from Earth. The A-ring and C-ring
are bright, and between them the B-ring is dark. The Cassini
Division at the inner edge of the A-ring is also bright, but is
blended with the ring at this distance.]
[Illustration: Pioneer Saturn image of Saturn and its rings from a
distance of 5,500,000 km (August 26, 1979, 6 days before closest
approach). Resolution of features is beginning to be approximately
equal to that of Earth-based pictures. Belts on the planet are
becoming more distinct, and considerable structure can be seen in
the rings. The small blue spot at the bottom of the planet is due to
incomplete data and will be corrected by further processing. A notch
appears at the top left of the planet, which may be a data
transmission problem or the shadow of one of Saturn’s moons.]
[Illustration: Pioneer Saturn image of Saturn and its rings from a
distance of 2,800,000 km (August 29, 1979, 72 hours before closest
approach). The Cassini Division at the inner edge of the A-ring is
clearly resolved and is bright. The A-ring is dark outside the
Cassini Division because it has more particulate matter there. Polar
belts are becoming more visible on the face of the planet.
Irregularities that appear in the ring silhouette and shadow are due
to stepping anomalies in the imaging photopolarimeter and will be
removed in further processing. The small round image that appears
above the planet is the moon Titan.]
[Illustration: These pictures of Saturn and its rings were taken by
Pioneer Saturn from a distance of 2,500,000 km (August 29, 1979, 58
hours before closest approach). The imaging photopolarimeter gathers
data using the red and blue components of the light reflected from
Saturn. These two views are from the two color components (upper,
blue; lower, red). The banded structure on the planet is
particularly evident in the upper image. Because the spacecraft is
nearing the planet, the rings are partially outside the field of
view of the instrument. The speck of light below the planet is
Saturn’s moon Rhea, which is 1450 km in diameter, about one-half the
size of Earth’s moon.]
[Illustration: Saturn and its rings: red components.]
[Illustration: This image was produced by combining the two images
on the facing page, adding a synthesized green component, and
adjusting the intensity of each to obtain an approximation to
Saturn’s true color. (The same process was used in all of the
Pioneer color images of Saturn shown here.) Although not evident on
this reproduction, scientists believe that, from detailed study of
both the uncombined and the combined images, they can begin to see
evidence of jet streams in Saturn’s upper atmosphere. The slight
blue edge is an artifact from the computer-enhancement procedure.]
[Illustration: Pioneer Saturn view showing the structure of Saturn’s
ring system in detail never before seen. The image was taken from a
distance of 943,000 km (August 31, 1979, about 44 hours before
encounter). The moon Tethys, seen at the top of the image, is 1200
km in diameter. There is a very faint unidentified Saturnian moon at
the lower right, just off the tip of the bright A-ring (may not be
visible in this print). The newly defined F-ring appears faintly
just outside the bright edge of the A-ring. The region between the
A-ring and the F-ring has been tentatively named the Pioneer
Division. This print has been processed to enhance detail of the
main rings.]
[Illustration: A color print of a version of the same image as shown
on the facing page. The blue dot at the outer edge of the C-ring is
an artifact. This image has not been computer processed to the same
extent as the facing image. Tethys, for example, is only faintly
visible, and the F-ring cannot be seen.]
[Illustration: Close-up image from Pioneer Saturn of Saturn from a
distance of 400,000 km (September 1, 1979, a few hours before
closest approach). Inset shows the location of the image on the
planet. The part of the disk shown is about 25,000 by 70,000 km. The
sawtooth pattern is an artifact. The vertical stripe on the disk is
due to a gain change in the instrument. The image has not had its
final corrections for shape. The rings and their shadows cut
diagonal swaths across the image with the upper swath being the
shadow. The rings, seen from the unlit side, are visible in the
foreground. The ring shadow shows clearly that Saturn’s rings have
two major divisions: the Cassini Division dividing the outer A-ring
from the middle B-ring, and a second division (previously
controversial) dividing the B-ring from the inner C-ring. These
divisions show as parallel pinstripes in the broad black band of the
rings’ shadow, with the upper stripe being the Cassini Division.
Some shearing in the bands and belts on Saturn’s disk is beginning
to appear, although the low contrast on the planet (due to its
high-altitude haze) does not make this highly evident.]
[Illustration: Post-encounter image of Saturn from a distance of
850,000 km (September 2, 1979, 15 hours after encounter). As
planned, during the encounter Saturn’s gravitational field turned
the spacecraft’s trajectory behind the planet and out toward the
edge of the solar system at approximately a right angle to the
inbound trajectory. Thus, the planet is now illuminated from the
side, and the view is of a crescent Saturn with the terminator on
the right of the picture. The rings appear dim when compared with
the previous inbound pictures because of the different angle between
the sun, the rings, and the spacecraft. When this picture was taken,
the spacecraft, as seen from Earth, was about to pass behind the
sun, and solar activity was interfering with spacecraft
communications. The image quality, therefore, is not as good as for
pictures taken inbound. With this farewell image from Pioneer,
Saturn waits for the Voyager spacecraft in 1980 and 1981.]
[Illustration: This image of cloud-covered Titan is one of the
“firsts” for the Pioneer Saturn mission. Titan is the largest of
Saturn’s moons. Because of its great distance from Earth, however,
Titan can be seen only as a point of light in Earth-based pictures.
The Pioneer trajectory could not be adjusted to allow passage close
to Titan, and imaging of the planet from the spacecraft was at the
outer range of capability of the imaging instrument. Thus, the fuzzy
edges and contrast variations on the moon should not be construed as
surface features.
Titan was imaged in the spirit of exploration and discovery. With
this image, which is now a part of recorded history, our visual
information on Titan is materially increased, but is still only
roughly equal to Galileo’s information on Saturn itself in 1610 at
the time that he prepared his first sketch. Better images of Titan
will be obtained by the Voyager spacecraft.]