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At each of the four giant planets—Jupiter； Saturn； Uranus； and Neptune—one or both spacecraft studied the planet itself； its rings； and its moons。 At Jupiter； in 1979； they braved a dose of trapped charged particles a thousand times more intense than what it takes to kill a human； enveloped in all that radiation； they discovered the rings of the largest planet； the first active volcanos outside Earth； and a possible underground ocean on an airless world—among a host of surprising discoveries。 At Saturn； in 1980 and 1981； they survived a blizzard of ice and found not a few new rings； but thousands。 They examined frozen moons mysteriously melted in the paratively recent past； and a large world with a putative ocean of liquid hydrocarbons surmounted by clouds of organic matter。
On January 25； 1986； Voyager 2 entered the Uranus system and reported a procession of wonders。 The encounter lasted only a few hours； but the data faithfully relayed back to Earth have revolutionized our knowledge of the aquamarine planet； its 15 moons。 its pitch…black rings； and its belt of trapped high…energy charged particles。 On August 25； 1989； Voyager 2 swept through the Neptune system and observed； dimly illuminated by the distant Sun； kaleidoscopic cloud patterns and a bizarre moon on which plumes of fine organic particles were being blown about by the astonishingly thin air。 And in 1992； having flown beyond the outermost known planet； both Voyagers picked up radio emission thought to emanate from the still remote heliopause—the place where the wind from the Sun gives way to the wind from the stars。
Because we're stuck on Earth； we're forced to peer at distant worlds through an ocean of distorting air。 Much of the ultraviolet； infrared； and radio waves they emit do not penetrate our atmosphere。 It's easy to see why our spacecraft have revolutionized the study of the Solar System： We ascend to stark clarity in the vacuum of space； and there approach our objectives； flying past them； as did Voyager； or orbiting them； or landing on their surfaces。
These spacecraft have returned four trillion bits of information to Earth； the equivalent of about 100；000 encyclopedia volumes。 I described the Voyagers 1 and 2 encounters with the Jupiter system in Cosmos。 In the following pages； I'll say something about the Saturn； Uranus； and Neptune encounters。
JUST BEFORE VOYAGER 2 was to encounter the Uranus system； the mission design had specified a final maneuver； a brief firing of the on…board propulsion system to position the spacecraft correctly so it could thread its way on a preset path among the hurtling moons。 But the course correction proved unnecessary。 The spacecraft was already within 200 kilometers of its designed trajectory…after a journey along an arcing path 5 billion kilometers long。 This is roughly the equivalent of throwing a pin through the eye of a needle 50 kilometers away； or firing your rifle in Washington and hitting the bull's…eye in Dallas。
Mother lodes of planetary treasure were radioed back to Earth。 But Earth is so far away that by the time the signal frog Neptune was gathered in by radio telescopes on our planet； the received power was only 10…16 watts （fifteen zeros between the decimal point and the one）。 This weak signal bears the same pro； portion to the power emitted by an ordinary reading lamp as the diameter of an atom bears to the distance from the Earth to the Moon。 It's like hearing an amoeba's footstep。
The mission was conceived during the late 1960s。 It was first funded in 1972。 But it was not approved in its final form （including the encounters with Uranus and Neptune） until after the ships had pleted their reconnaissance of Jupiter。 The two spacecraft were lifted off the Earth by a nonreusable Titan/Centaur booster configuration。 Weighing about a ton； a Voyager would fill a small house。 Each draws about 400 watts of power—considerably less than an average American home—from a generator that converts radioactive plutonium into electricity。 （If it had to rely on solar energy； the available power would diminish quickly as the ship ventured farther and farther from the Sun Were it not for nuclear power； Voyager would have returned no data at all from the outer Solar System； except perhaps a little from Jupiter。）
The flow of electricity through the innards of the spacecraft would generate enough magnetism to overwhelm the sensitive instrument that measures interplanetary magnetic fields。 So the magnetometer is placed at the end of along boom； far from the offending electrical currents。 With other projections； it gives Voyager alter a slightly porcupine appearance。 Cameras； infrared and ultraviolet spectrometers； and an instrument called a photopolarimeter are on a scan platform that swivels on mand so these device can be aimed at a target world。 The spacecraft must know where Earth is if the antenna is to be pointed properly and the data rereceived back home。 It also needs to know where the Sun is and at least one bright star； so it can orient itself in three dimensions and point properly toward any passing world。 If you can't point the cameras； it does no good to be able to return pictures over billions of miles。
Each spacecraft cost about as much as a single modern strategic bomber。 But unlike bombers； Voyager cannot； once launched； be returned to the hangar for repairs。 The ship's puters and electronics are therefore designed redundantly。 Much key machinery； including the essential radio receiver； had at least one backup—waiting to be called upon should the hour of need ever arrive。 When either Voyager finds itself in trouble； the puters use branched contingency tree logic to work out the appropriate course of action。 If that doesn't work； the ship radios home for help。
As the spacecraft journeys increasingly far from Earth； the roundtrip radio travel time also increases； approaching eleven hours by the time Voyager is at the distance of Neptune。 Thus； in case of emergency； the spacecraft needs to know how to put itself into a safe standby mode while awaiting instructions from Earth。 As it ages； more and more failures are expected； both in its mechanical parts and in its puter system； although there is no sign； even now； of a serious memory deterioration； some robotic Alzheimer's disease。
This is not to say that Voyager is perfect。 Serious mission…threatening； white…knuckle mishaps did occur。 Each time； special teams of engineers—some of whom had been with the Voyager program since its inception—were assigned to 〃work〃 the problem。 They would study the underlying science and draw upon their previous experience with the failed subsystems。 They would experiment with identical Voyager spacecraft equipment that had never been launched； or even manufacture a large number of ponents of the sort that failed in order to gain some statistical understanding of the failure mode。
In April 1978； almost eight months after launch； and while the ship was approaching the asteroid belt； an omitted ground mand—a human error—caused Voyager 2's on…board puter to switch from the prime radio receiver to its backup； During the next ground transmission to the spacecraft； the backup receiver refused to loc