Part 3

Looking back at the Saturn system as it soared upward and outward, Voyager 1 continued its observations for nearly five weeks after closest Saturn approach. The spacecraft photographed the planet’s sunlit crescent, the ring shadows falling on the planet, and Saturn’s dark hemisphere illuminated by “ringshine.” It searched for lightning and auroras on the planet’s dark side and looked for “sun dogs” resulting from ammonia crystals in the atmosphere. It continued temperature and composition measurements and searched for new satellites out to the orbit of Mimas. It measured the flow of plasma in Saturn’s magnetosphere and now, its journey far from over, Voyager 1 proceeds toward the outer boundary of our solar system, as it seeks to probe the space among the stars of our galaxy, the Milky Way.]

Departing Saturn, Voyager 1 photographed the planet from a unique perspective, clearly showing Saturn’s shadow on the rings.]

During a 40-minute period on the day of encounter, the spacecraft was itself in the planet’s shadow. At this time, the wide-angle camera acquired a photograph of this shadow line, revealing ring material in a region very close to the planet, where no material had been previously observed. This inner ring, the D-Ring, is roughly 6000 kilometers (4000 miles) wide and extends to within about 6000 kilometers of Saturn’s cloudtops.]

The Voyager Mission

Only once every 175 years are the outer planets aligned in their orbits so that we can take advantage of gravity-assist trajectories to achieve encounters with Jupiter, Saturn, Uranus, and Neptune on one mission. The gravity-assist technique uses one planet’s gravity field and motion through space to alter the spacecraft’s flight path and propel it outward toward the next planet. Voyager 1’s trajectory, which was selected to best view Titan, has now propelled the spacecraft out of the ecliptic plane, while Voyager 2’s path will remain in this plane to provide future encounters with Uranus and possibly with Neptune.

MISSION OBJECTIVES

The Voyager Project was approved in June 1972 and had as its mission objectives:

★ Exploration of the Jupiter and Saturn planetary systems, including their atmospheres, rings, satellites, and magnetospheres ★ Comparative analyses of the two systems ★ Investigation of the interplanetary medium between Earth and Saturn

A fourth objective, added in 1976, was to preserve the possibility of extending the mission to include an investigation of the planet Uranus and the interstellar medium.

With the completion of Voyager 1’s Saturn flyby, it is now clear that these objectives will be achieved.

SPACECRAFT CHARACTERISTICS

Two identical spacecraft were developed for the 1977 launch opportunity. These marvelous machines were cleverly designed to survive the rigors of long voyages in outer space and to deliver high-quality scientific information required for detailed understanding of planetary systems. The spacecraft are both complex—automatically responding to their Earth-bound monitors that remotely control them via radio commands—and highly autonomous—capable of caring for themselves in many areas through a system of sensors, computers, and spare equipment. Each spacecraft functions on about 400 watts of electrical power which is provided by nuclear generators. Broadcasts of data across a billion miles to Earth are accomplished with a spacecraft transmitter power of only about 25 watts, the amount of energy required by a small household light bulb.

Voyager’s scientific payload was carefully chosen to observe Saturn over a wide range of wave-lengths and to measure magnetic fields, charged particles, and plasma waves.

SATURN ENCOUNTER

TITAN DIONE TETHYS MIMAS ENCELADUS RHEA

Voyager 1’s Saturn encounter period began on August 22, 1980, at a range of 109 million kilometers (68 million miles) from the planet. Even at this great distance, Voyager’s images were better than any from Earth-based telescopes. During the long encounter period, which extended through December 19, 1980, continuous observations of Saturn’s realm were carried out by Voyager’s instruments. Voyager 1’s flight path through the Saturn system demanded navigation of the highest precision to meet three critical targets: (1) a close 4000-kilometer (2300-mile) flyby and occultation at Titan, (2) a precise, three-minute time period when the spacecraft was emerging from occultation at the same time Earth was in a position to receive the spacecraft signals passing through the gap between Saturn and its rings, and (3) a flight path through the E-Ring at Dione’s orbit to assure safe passage through a zone clear of potentially dangerous material. To assure these targets were achieved, small trajectory trim maneuvers were executed on October 11, 1980, and again on November 6, 1980, as Voyager 1 sped toward Saturn.

HIGH-GAIN ANTENNA (3.7-meter diameter) LOW-ENERGY CHARGED PARTICLE COSMIC RAY PLASMA IMAGING ULTRAVIOLET SPECTROMETER INFRARED INTERFEROMETER SPECTROMETER PHOTOPOLARIMETER OPTICAL CALIBRATION TARGET PLANETARY RADIO ASTRONOMY AND PLASMA WAVE ANTENNA (2) RADIOISOTOPE THERMOELECTRIC GENERATOR (3) MAGNETOMETER BOOM

By October 24, 1980, when Voyager 1 was about 30 million kilometers (19 million miles) from Saturn, the spacecraft’s narrow-angle camera could no longer capture the planet in a single picture. Thus, a period of multiple images or mosaics began. By November 2, 1980, even four-picture mosaics could no longer cover the rapidly growing scene. Voyager 1’s pace of operations reached an exciting peak during the near-encounter phase from November 11 through November 13, 1980. While still about 1.6 million kilometers (1 million miles) from closest approach to Saturn, Voyager 1 encountered Titan on November 11, 1980, and then dipped below the ring plane as it accelerated rapidly toward Saturn. On November 12, 1980, Voyager 1 came within 124,000 kilometers (77,000 miles) of the cloudtops of Saturn’s southern hemisphere, where Saturn’s gravity altered the spacecraft’s course, hurtling the spacecraft upward past the ring plane. Close observation of Saturn’s other major satellites and its rings were made during this passage.

From Earth to Saturn, Voyager 1 has traveled in the ecliptic plane, the plane in which the major planets orbit. Now, having completed its final planetary flyby, Voyager 1 is rising above this plane on a trajectory that will eventually carry it above and out of the solar system, probably before the end of this century. As it proceeds, the spacecraft will return information about the solar wind and magnetic fields in the far, unexplored reaches of our solar system and will observe cosmic rays emitted from the distant stars among which Voyager will ultimately cruise.

Scientific Highlights

Some of the most important information gathered by Voyager 1 on the Saturn system is presented pictorially in this publication and is supplemented here with brief summaries of the major discoveries, observations, and theories.

SATURN

Saturn’s atmosphere appears similar to Jupiter’s, with alternating dark belts and bright zones, circulating storm regions, and other dark and light cloud markings. Saturn’s belt and zone system extends to higher latitudes than those on Jupiter, and all of the features are muted by a thick atmospheric haze, perhaps 70 kilometers (40 miles) deep.

Wind speeds up to 1500 kilometers per hour (900 miles per hour) occur at the equator—four to five times faster than any Jovian winds. Temperatures near the cloudtops range from 86 to 92 kelvins (-305° to -294° Fahrenheit)—nearly 60 degrees colder than at Jupiter. Saturn still radiates about 2.8 times as much heat as it receives from the Sun. The coolest temperatures are found at the center of the equatorial zone.

Auroral emissions have been seen near Saturn’s poles, and auroral-type emissions have been seen in ultraviolet light near the illuminated limb of the planet.

Lightning bolts have not been seen on Saturn, but radio emissions typical of lightning discharges have been recorded. The source of these discharges is believed to be the rings rather than Saturn’s atmosphere.

RINGS

Hundreds of tiny ringlets—a few of them elliptical rather than circular—comprise the classic A-, B-, and C-Rings, once thought to be uniform disks of material. The F-Ring, which was first sighted by Pioneer 11 in 1979, was observed to be three separate, intertwined ringlets.