THE VOYAGER SPACECRAFT are bound for the stars. They are on escape trajectories from the Solar System, barreling along at almost a million miles a day. The gravitational fields of Jupiter, Saturn, Uranus, and Neptune have flung them at such high speeds that they have broken the bonds that once tied them to the Sun.
Have they left the Solar System yet? The answer depends very much on how you define the boundary of the Sun's realm. If it's the orbit of the outermost good-sized planet, then the Voyager spacecraft are already long gone; there are probably no undiscovered Neptunes. If you mean the outermost planet, 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 comets 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, completing 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 composition 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 accompanied by planets. The instructions on the record jackets, written in what we believe to be readily comprehensible 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 incomprehensible, 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 incomprehensibilities 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 come upon them. Five billion years is a long time. In five billion years, all humans will have become extinct or evolved into other beings, none of our artifacts will have survived on Earth, the continents will have become 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 common. 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 Commandment 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 come from? If it's a commonplace 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 completely 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, to dark blue and purple, and then to the completely black color of the sky. It is a very beautiful transition.
Clearly, the daylit sky—all that blue— is somehow connected with the air. But as you look across the breakfast table, your companion is not (usually) blue; the color of the sky must be a property not of a little air, but of a great deal. If you look closely at the Earth from space, you see it surrounded by a thin band of blue, as thick as the lower atmosphere; indeed, it is the lower atmosphere. At the top of that band you can make out the blue sky fading into the blackness of space. This is the transition zone that Simons was the first to enter and Gagarin the first to observe from above. In routine spaceflight, you start at the bottom of the blue, penetrate entirely through it a few minutes after liftoff; and then enter that boundless realm where a simple
breath of air is impossible without elaborate life-support systems. Human life depends for its very existence on that blue sky. We are right to consider it tender and sacred.
We see the blue in daytime because sunlight is bouncing off the air around and above us. On a cloudless night, the sky is black because there is no sufficiently intense source of light to be reflected off the air. Somehow, the air preferentially bounces blue light down to us. How?
The visible light from the Sun comes in many colors—violet, blue, green, yellow, orange, red—corresponding to light of different wavelengths. (A wavelength is the distance front crest to crest as the wave travels through air or space.) Violet and blue light waves have the shortest wavelengths; orange and red the longest. What we perceive as color is how our eyes and brains read the wavelengths of light. (We Might just as reasonably translate wavelengths of light into, say, heard tones rather than seen colors—but that's not how our senses evolved.)
When all those rainbow colors of the spectrum are mixed together, as in sunlight, they seem almost white. These waves travel together in eight minutes across the intervening 93 million miles (150 million kilometers) of space from the Sun to the Earth. They strike the atmosphere, which is made mostly of nitrogen and oxygen molecules. Some waves are reflected by the air back into space. Some are bounced around before the light reaches the ground and they can be detected by a passing eyeball. (Also, some bounce off clouds or the ground back into space.) This bouncing around of light waves in the atmosphere is called "scattering."
But not all waves are equally well scattered by the molecules of air. Wavelengths that are much longer than the size of the molecules are scattered less; they spill over the molecules, hardly influenced by their presence. Wavelengths that are closer to the size of the molecules are scattered snore. And waves have trouble ignoring obstacles as big as they are. (You can see this in water waves scattered by the pilings of piers, or bathtub waves from a dripping faucet encountering a rubber duck.) The shorter wavelengths, those that we sense as violet and blue light, are more efficiently scattered than the longer wavelengths, those that we sense as orange and red light. When we look up on a cloudless day and admire the blue sky, we are witnessing the preferential scattering of the short waves in sunlight. This is called Rayleigh scattering, after the English physicist who offered the first coherent explanation for it. Cigarette smoke is blue for just the same reason: The particles that make it up are about as small as the wavelength of blue light.
So why is the sunset red? The red of the sunset is what's left of sunlight after the air scatters the blue away. Since the atmosphere is a thin shell of gravitationally bound gas surrounding the solid Earth, sunlight must pass through a longer slant path of air at sunset (or sunrise) than at noon. Since the violet and blue waves are scattered even more during their now-longer path through the air than when the Sun is overhead, what we see when we look toward the Sun is the residue—the waves of sunlight that are hardly scattered away at all, especially the oranges and reds. A blue sky makes a red sunset. (The noontime Sun seems yellowish partly because it emits slightly more yellow light than other colors, and partly because, even with the Sun overhead, some blue light is scattered out of the sunbeams by the Earth's atmosphere.)
It is sometimes said that scientists are unromantic, that their passion to figure out robs the world of beauty and mystery. But is it not stirring to understand how the world actually works—that white light is made of colors, that color is the way we perceive the wavelengths of light, that transparent air reflects light, that in so doing it discriminates among the waves, and that the sky is blue for the same reason that the sunset is red? It does no harm to the romance of the sunset to know a little bit about it.
Since most simple molecules are about the same size (roughly a hundred millionth of a centimeter), the blue of the Earth's sky doesn't much depend on what the air is made of—as long as the air doesn't absorb the light. Oxygen and nitrogen molecules don't absorb visible light; they only bounce it away in some other direction. Other molecules, though, can gobble up the light. Oxides of nitrogen—produced in automotive engines and in the fires of industry—are a source of the murky brown coloration of smog. Oxides of nitrogen (made from oxygen and nitrogen) do absorb light. Absorption, as well as scattering, can color a sky.
