  The Voyager mission is focused on the exploration of the Jupiter and Saturn systems. The alignment of these large planets permits the use of a gravity-assist trajectory in which the gravity field of Jupiter and Jupiter’s motion through space may be used to hurl the spacecraft on to Saturn. In 1977, a rare alignment (once every 176 years) of our four outer planets—Jupiter, Saturn, Uranus, and Neptune—may permit a gravity-assist trajectory to Uranus and even to Neptune for Voyager 2. Voyagers 1 and 2 began their journeys in the late summer of 1977, catapulted into space by a Titan/Centaur launch vehicle from Cape Canaveral, Florida. With them went the hopes and dreams of thousands of people who had worked to create them and their mission. The Voyager spacecraft are unique in many respects. Since their journeys are taking them far from the Sun, the Voyagers are nuclear powered rather than solar powered. The Voyagers are the fastest man-made objects ever to have left Earth. In fewer than ten hours, they had crossed the Moon’s orbit. This compares to about three days for an Apollo flight and one day for the Mariner and Viking spacecraft. Their launches marked the end of an era in space travel—the end of the planned use of Titan/Centaur launch vehicles. With the advent of the Space Shuttle in the 1980s, future spacecraft will be launched from the Shuttle Orbiter. Voyager 1 was launched 16 days after its sister ship, but because of a different trajectory, it arrived at Jupiter four months ahead of Voyager 2. Both spacecraft spent more than nine months crossing the asteroid belt, a vast ring of space debris circling the Sun between the orbits of Mars and Jupiter. During their 16- and 20-month journeys to Jupiter, the spacecraft tested and calibrated all of their instruments, exercised their scan platforms, and measured particles and fields in interplanetary space. As the spacecraft neared the planet, the cameras showed the dramatic visible changes that had taken place in the five years since Jupiter had been photographed by Pioneer 11. And for the first time, we got a close look at some of Jupiter’s moons: Amalthea, Io, Europa, Ganymede, and Callisto. Targeted for the closest look at Io, Voyager 1 flew the more hazardous course, passing between Jupiter and Io, where the radiation environment is the most intense. Voyager 2’s flight path gave Jupiter and its intense radiation a much wider berth. Unlike Voyager 1, which encountered the five innermost satellites as it was leaving Jupiter, Voyager 2 encountered the satellites as it was approaching the planet, thus providing closeup photography of opposite sides of the satellites. March 5, 1979. Voyager 1’s unique flight path allowed scientists to study at close range 5 of Jupiter’s 13 known satellites. Each is shown at its closest point to the trajectory of Voyager 1’s outbound flight away from Jupiter. Closest approach was 280,000 kilometers (174,000 miles) from Jupiter. July 9, 1979. Voyager 2’s closest approach to Jupiter was 645,000 kilometers (400,000 miles) from the planet. Voyager 2 encountered the satellites on its inbound journey to Jupiter, which enabled the spacecraft to photograph the opposite sides of the satellites. Arriving at Jupiter from slightly different angles, both spacecraft measured the large, doughnut-shaped ring of charged sulfur and oxygen ions, called a torus, encircling Voyager spacecraft and scientific instruments. From the moment of launch, the Voyager spacecraft have been monitored by a worldwide tracking system of nine giant antennas strategically located around the world in California, Spain, and Australia to ensure constant radio contact with the spacecraft as the Earth rotates. Radio contact with Voyagers 1 and 2 has not been instantaneous, however. When Voyager 1 flew past Jupiter, radio signals between Earth and the spacecraft took 37 minutes; when Voyager 2 arrived, the signals took 52 minutes because by then the planet was farther from Earth. The pictures in this book were taken by a shuttered television-type camera. Each picture is composed of 640,000 dots, which were converted into binary numbers before being radioed to Earth. When the signals reached Earth, they were reconverted by computer into dots and reassembled into the original image. Most of the color pictures are composed of three images, each one taken through a different color filter: blue, orange, or green. The images were combined and the original color was reconstructed by computer. The computer eliminated many of the imperfections that crept into the images, and enhanced some of the images by emphasizing different colors. Designed to provide a broad spectrum of scientific investigations at Jupiter, the science instruments investigated atmospheres, satellites, and magnetospheres. The scientific investigations for the Voyager mission and their Jovian encounter objectives are shown in the table on page 40. After their closest approaches to Jupiter, both spacecraft fired their thrusters, retargeting for their next goal, the Saturn system. Scientists will still be studying the wealth of new information about Jupiter when Voyager 1 reaches Saturn in November 1980, and Voyager 2 follows in August 1981. After Voyager 1 encounters Saturn, Voyager 2 may be retargeted to fly past Uranus in 1986. Upon completion of their planetary missions, both spacecraft will search for the outer limit of the solar wind, that boundary somewhere in our part of the Milky Way where the influence of the Sun gives way to other stars of the galaxy. Voyagers 1 and 2 will continue to study interstellar space until the spacecraft signals can no longer be received. |
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