the best promise is 〃human；〃 which permits us to distinguish crisply between human and robotic missions。 But every now and then； 1 find 〃human〃 not quite working； and to my dismay 〃manned〃 slips back in。
But first； let's consider the visions of a hopeful future vouchsafed by robot spacecraft out among the planets。
VOYAGER 1 AND VOYAGER 2 are the ships that opened the Solar System for the human species； trailblazing a path for future generations。 Before their launch； in August and September 1977； we were almost wholly ignorant about most of the planetary part of the Solar System。 In the next dozen years； they provided our first detailed； close…up information on many new worlds—some of them previously known only as fuzzy disks in the eyepieces of ground…based telescopes； some merely as points of light； and some whose very existence was unsuspected。 They are still returning reams of data。
These spacecraft have taught us about the wonders of other worlds； about the uniqueness and fragility of our own； about beginnings and ends。 They have given us access to most of the Solar System—both in extent and in mass。 They are the ships that first explored what may be homelands of our remote descendants。
U。S。 launch vehicles are these days too feeble to get such a spacecraft to Jupiter and beyond in only a few years by rocket propulsion alone。 But if we're clever （and lucky）； there's something else we can do： We can （as Galileo also did； years later） fly close to one world； and have its gravity fling us on to the next。 A gravity assist； it's called。 It costs us almost nothing but ingenuity。 lt's something like grabbing hold of a post on a moving merry…go…round as it passes—to speed you up and fling you in some new direction。 The spacecraft's acceleration is pensated for by a deceleration in the planet's orbital motion around the Sun。 But because the planet is so massive pared to the spacecraft； it slows down hardly at all。 Each Voyager spacecraft picked up a velocity boost of nearly 40；000 miles per hour from Jupiter's gravity。 Jupiter in turn was slowed down in its motion around the Sun。 By how much？ Five billion years from now； when our Sun bees a swollen red giant； Jupiter will be one millimeter short of where it would have been had Voyager not flown by it in the late twentieth century。
Voyager 2 took advantage of a rare lining…up of the planets： A close flyby of Jupiter accelerated it on to Saturn； Saturn to Uranus； Uranus to Neptune； and Neptune to the stars。 But you can't do this anytime you like： The previous opportunity for such a game of celestial billiards presented itself during the presidency of Thomas Jefferson。 We were then only at the horseback； canoe； and sailing ship stage of exploration。 （Steamboats were the transforming new technology just around the corner。）
Since adequate funds were unavailable； NASA's Jet Propulsion Laboratory （JPL） could afford to build spacecraft that would work reliably only as far as Saturn。 Beyond that； all bets were off。 However； because of the brilliance of the engineering design and the fact that the JPL engineers who radioed instructions up to the spacecraft got smarter faster than the spacecraft got stupid—both spacecraft went on to explore Uranus and Neptune。 These days they are broadcasting back discoveries from beyond the most distant known planet of the Sun。
We tend to hear much more about the splendors returned than the ships that brought them； or the shipwrights。 It has al； ways been that way。 Even those history books enamored of the voyages of Christopher Columbus do not tell us much about the builders of the Nina； the Pinta； and the Santa Maria； or about the principle of the caravel。 These spacecraft； their designers； builders； navigators； and controllers are examples of what science and engineering； set free for well…defined peaceful purposes； can acplish。 Those scientists and engineers should be role models for an America seeking excellence and international petitiveness。 They should be on our stamps。
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 d