7/7/79 390,000 km (245,000 mi)
Callisto is the most heavily cratered planetary body in our solar system. In this Voyager 2 nine-frame mosaic, a special computer filter was used to provide high contrast in the surface topography. The impact structure visible at the upper right edge of the satellite is smaller than the largest one found by Voyager 1 but more detail is obvious; it is estimated that 15 concentric rings surround the bright center. Many hundreds of moderate-sized craters are also visible, a few with bright ray patterns. The limb is smooth, which is consistent with Callisto’s icy composition.
3/6/79 200,000 km (125,000 mi)
This high-resolution image of Callisto, photographed by Voyager 1, shows details of the large ring structure surrounding the remains of the ancient impact basin visible on page 35. The surface area shown in this image is at the right edge and slightly above the center of the picture on [page 35]. The relatively undisturbed region on the right shows the shoulder-to-shoulder large impact craters typical of most of Callisto’s surface. A decrease in crater density toward the center of the structure (to the left) is evident, and is caused by the destruction of very old craters by the large impact that formed the ring
The Voyager Mission
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.