anet； it may be that there are other—perhaps Triton…like—planets far beyond Neptune and Pluto； if so； Voyager 1 and Voyager 2 are still within the Solar System。 If you define the outer limits of the Solar System as the heliopause—where the interplanetary particles and magnetic fields are replaced by their interstellar counterparts—then neither Voyager has yet left the Solar System； although they may do so in the next few decades。* But if your definition of the edge of the Solar System is the distance at which our star can no longer hold worlds in orbit about it； then the Voyagers will not leave the Solar System for hundreds of centuries。
* Radio signals that both Voyagers detected in 1992 are thought to arise from the collision of powerful gusts of solar wind with the thin gas that lies between the stars。 From the immense 。power of the signal （over 10 trillion watts）； the distance to the heliopause can be estimated： about 100 times the Earth's distance from the Sun。 At the speed it's leaving the Solar System； Voyager 1 might pierce the heliopause and enter interstellar space around the year 2010。 If its radioactive power source is still working； news of the crossing will be radioed back to the stay…at…homes on Earth。 The energy released by the collision of this shock wave with the heliopause makes it the most powerful source of radio emission in the Solar System。 It makes you wonder whether even stronger shocks in other planetary systems might he detectable by our radio telescope。
Weakly grasped by the Sun's gravity； in every direction in the sky； is that immense horde of a trillion ets or more； the port Cloud。 The two spacecraft will finish their passage through
the Oort cloud in another 20；000 years or so。 Then； at last； pleting their long good…bye to the Solar System； broken free of the gravitational shackles that once bound them to the Sun； the Voyagers will make for the open sea of interstellar space。 only then will Phase Two of their mission begin。
Their radio transmitters long dead； the spacecraft will wander for ages in the calm； cold interstellar blackness—where there is almost nothing to erode them。 Once out of the Solar System； they will remain intact for a billion years or more； as they circumnavigate the center of the Milky Way galaxy。
We do not know whether there are other space…faring civilizations in the Milky Way。 If they do exist； we do not know how abundant they are； much less where they are。 But there is at least a chance that sometime in the remote future one of the Voyagers will be intercepted and examined by an alien craft。
Accordingly； as each Voyager left Earth for the planets and the stars； it carried with it a golden phonograph record encased in a golden； mirrored jacket containing； among other things； greetings in 59 human languages and one whale language； a 12…minute sound essay including a kiss； a baby's cry； and an EEG record of the meditations of a young woman in love； 116 encoded pictures； on our science； our civilization； and ourselves； and 90 minutes of the Earth's greatest hits—Eastern and Western； classical and folk； including a Navajo night chant； a Japanese shakuhachi piece； a Pygmy girl's initiation song； a Peruvian wedding song； a 3；000…year…old position for the ch'in called 〃Flowing Streams；〃 Bach； Beethoven； Mozart； Stravinsky； Louis Armstrong； Blind Willie Johnson； and Chuck Berry's 〃Johnny B。 Goode。〃
Space is nearly empty。 There is virtually no chance that one of the Voyagers will ever enter another solar system—and this is true even if every star in the sky is acpanied by planets。 The instructions on the record jackets； written in what we believe to be readily prehensible scientific hieroglyphics； can be read； and the contents of the records understood； only if alien beings； somewhere in the remote future； find Voyager in the depths of interstellar space。 Since both Voyagers will circle the center of the Milky Way Galaxy essentially forever； there is plenty of time for the records to be found—if there's anyone out there to do the finding。
We cannot know how much of the records they would understand。 Surely the greetings will be inprehensible； but their intent may not be。 （We thought it would be impolite not to say hello。） The hypothetical aliens are bound to be very different from us—independently evolved on another world。 Are we really sure they could understand anything at all of our message？ Every time I feel these concerns stirring； though； I reassure myself。 Whatever the inprehensibilities of the Voyager record； any alien ship that finds it will have another standard by which to judge us。 Each Voyager is itself a message。 In their exploratory intent； in the lofty ambition of their objectives； in their utter lack of intent to do harm； and in the brilliance of their design and performance； these robots speak eloquently for us。
But being much more advanced scientists and engineers than we—otherwise they would never be able to find and retrieve the small； silent spacecraft in interstellar space—perhaps the aliens would have no difficulty understanding what is encoded on these golden records。 Perhaps they would recognize the tentativeness of our society； the mismatch between our technology and our wisdom。 Have we destroyed ourselves since launching Voyager； they might wonder； or have we gone on to greater things？
Or perhaps the records will never be intercepted。 Perhaps no one in five billion years will ever e upon them。 Five billion years is a long time。 In five billion years； all humans will have bee extinct or evolved into other beings； none of our artifacts will have survived on Earth； the continents will have bee unrecognizably altered or destroyed； and the evolution of the Sun will have burned the Earth to a crisp or reduced it to a whirl of atoms。
Far from home； untouched by these remote events； the Voyagers； bearing the memories of a world that is no more； will fly on。
CHAPTER 10 SACRED BLACK
Deep sky is； of all visual impressions； the nearest akin to a feeling。
—SAMUEL TAYLOR COLERIDGE； NOTEBOOKS （1805）
The blue of a cloudless May morning； or the reds and oranges of a sunset at sea； have roused humans to wonder； to poetry； and to science。 No matter where on Earth we live； no matter what our language； customs； or politics； we share a sky in mon。 Most of us expect that azure blue and would； for good reason； be stunned to wake up one sunrise to find a cloudless sky that was black or yellow or green。 （Inhabitants of Los Angeles and Mexico City have grown accustomed to brown skies； and those of London and Seattle to gray ones—but even they still consider blue the planetary norm。）
And yet there are worlds with black or yellow skies； and maybe even green。 The color of the sky characterizes the world。 Plop me down on any planet in the Solar System； wsithout sensing the gravity； without glimpsing the ground； let me take a quick look at the Sun and sky； and I can； I think； pretty well tell you where I am。 That familiar shade of blue； interrupted here and there by fleecy white clouds； is a signature of our world。 The French have an expression； sacre…bleu！； which translates roughly as 〃Good heavens！〃* Literally； it means 〃sacred blue！〃 Indeed。 If there ever is a true flag of Earth； this should be its color。
* Like 〃gosh…darned〃 and 〃geez；〃 this phrase was originally a euphemism for those who considered Sacre…Dieu！； 〃Sacred God！；〃 too strong an oath； the Second mandment duly considered； to be uttered aloud。
Birds fly through it； clouds are suspended in it； humans admire and routinely traverse it； light from the Sun and stars flutters through it。 But what is it？ What is it made of？ Where does it end？ How much of it is there？ Where does all that blue e from？ If it's a monplace for all humans； if it typifies our world； surely we should know something about it。 What is the sky？
In August 1957； for the first time； a human being rose above the blue and looked around—when David Simons； a retired Air Force officer and a physician； became the highest human in history。 Alone； he piloted a balloon to an altitude of over 100；000 feet （30 kilometers） and through his thick windows glimpsed a different sky。 Now a professor at the University of California Medical School in Irvine； Dr。 Simons recalls it was a dark； deep purple overhead。 He had reached the transition region where the blue of ground level is being overtaken by the perfect black of space。
Since Simons' almost forgotten flight； people of many nations have flown above the atmosphere。 It is now clear from repeated and direct human （and robotic） experience that in space the daytime sky is black。 The Sun shines brightly on your ship。 The Earth below you is brilliantly illuminated。 But the sky above is black as night。
Here is the memorable description by Yuri Gagarin of what he saw on the first spaceflight of the human species； aboard Vostok 1； on April 12； 1961：
The sky is pletely black； and against the background of this black sky the stars appear somewhat brighter and more distinct。 The Earth has a very characteristic； very beautiful blue halo； which is seen well when you observe the horizon。 There is a smooth color transition from tender blue； to blue； t