Voyager is the name of two identical probes sent to explore the outer planets. The Voyager project, originally called Mariner Jupiter Saturn, was begun in 1972. The two probes, Voyager 1 and Voyager 2, started their journey of exploration 1n 1977. Voyager 2 was launched first, on August 20; Voyager 1 on September 5. Voyager 1 overtook its sister craft and reached Jupiter and then Saturn before it. It made a host of discoveries at each planet, before starting to make its way out of the solar system. Voyager 2 also visited Jupiter and Saturn, but then went on to explore Uranus and Neptune. It was taking advantage of a line up of the outer planets that happens about once every 175 years. It used the gravitational pull of each planet to increase its speed and fling it off in the direction of the next planet. This technique, called gravity assist, was first used on the Mariner 10 mission to Venus and Mercury. Each Voyager probe is the same. It has a mass of about 815 kg, and its biggest feature is its dish antenna, which measured 3.7 metres across. It carries most of its instruments on a science boom, the top part of which, the scan platform, is movable. The platform carries two TV cameras and other instruments. A longer boom carries two magnetometers. The instruments are powered by a nuclear 'battery', a set of radioisotope thermoelectric generators (RTGs). Voyager 2 has been the most successful of all probes. During its 12-year voyage of discovery through the outer solar system, it visited four planets. By the time it reached the last, Neptune, in August 1989, it had travelled over 7,000 million km. Yet it had been guided with such precision that it was able to swoop within 4,900 km of Neptune's surface. It was so far away at the time that its radio signals, travelling at the speed of light, took over four hours to make their way back to Earth.
