  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. First sketch of Saturn (Galileo, 1610). 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 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. High-quality contemporary Earth-based view of Saturn (Photo: Catalina Observatory, University of Arizona). Nomenclature of bright rings.
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. 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. 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. 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. Saturn and its rings: red components. 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. 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. 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. 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. 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. 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. |
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