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The scheme required electronic components of such durability that they could survive in the cold, hostile environment of space for the mission's entire twelve-year duration. Because it would have to range so far from the sun that the solar system's great central fire would be no more than a faint glow in the darkness of space, the ship could not be solar powered. The answer: a miniature nuclear power plant that converted the heat produced by the radioactive decay of plutonium directly into electricity. Most challenging of all, the spacecraft had to be able to react to emergencies without human help. If its small directional-control rockets accidentally began firing in the vicinity of Uranus, for example, a warning signal, traveling at the speed of light, would take about three hours to reach earth 3 billion km (2 billion miles) away. By that time the ship might have exhausted its precious fuel, wrecking the mission. To avert such a breakdown, J.P.L. began designing on-board computer systems that could check out spacecraft functions and, if necessary, command corrections entirely on their own.
Scientists found the journey to the outer reaches of the solar system the most exciting kind of exploration. But the Grand Tour soon lost the war of the budgetary worlds to more favored space agency projects, notably the still delayed space shuttle. In 1972 the Grand Tour was, as Voyager Project Manager Raymond Heacock puts it in NASA jargon, "rescoped"; what had started out as a plan to survey all the major outer planets was reduced to less costly inspection of just two: Jupiter and Saturn. The initial work was not all wasted, however. Much of the proposed Grand Tour hardware went into Voyager, including the long-lived components, the nuclear power pack and the array of self-correcting computers. In addition, so much functional overlap was built into the ship that if there was a failure in one component, another would often be available to perform its duties.
Voyager reached Saturn with ten different scientific instruments functioning, ranging from cosmic-ray detectors and magnetometers (for measuring the strength of magnetic fields in space) to infrared and ultraviolet spectrometers (used for remote temperature readings and the search for key chemicals). Only one instrument, the photopolarimeter, had failed. By beaming radio signals through planetary clouds and atmospheres, the spacecraft can also use its radio transmitters for scientific investigation. The effect of particles on radio signals, for example, provides clues to such things as the density and makeup of an atmosphere.
Voyager's most useful instruments may be two high-resolution television cameras, one with a wide-angle lens, the other telephoto. The cameras can be pointed in virtually any direction—up, down, to the side, even backward. Their optics are so precise that the cameras can spot features only five miles across from a distance of a million miles. To produce color images, the cameras make successive scans through red, green and blue filters. Transmitted back to earth as three separate sets of signals, the pictures are reassembled by computer from the digital data and combined on color film.