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. [Illustration: 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._] [Illustration: 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 the planet at about the orbit of Io. Then, both spacecraft disappeared behind Jupiter, out of view of Earth and Sun, for about two hours. During this time, measurements were taken on the planet’s dark side. Each spacecraft took over 15,000 photographs of Jupiter and its satellites. [Illustration: _Voyager spacecraft and scientific instruments._]