by Carl Sagan
There was one catch. Though NASA felt that the project was a good idea, time constraints forced them to insist on receiving the finished record in about six weeks. Not only did they require a formal proposal with actual submissions of pictures, voices, sounds and music, they needed an approved and fabricated record, ready to bolt onto the spacecraft. This meant that we had something like a month to find all the photographs, prepare them, secure legal permission for their use, draw all the diagrams, photograph everything to format, convert the pictures into a sound signal suitable for recording on a phonograph record, and devise an instruction sheet for the cover of the record to tell anybody who found it how to play it.
Since NASA was not prepared to sink a lot of money into this endeavor, Drake had to find some cheap system that already existed for converting pictures into sound. He worked with Valentin Boriakoff, a research associate at the National Astronomy and Ionosphere Center, who located a suitable system belonging to Colorado Video, a company that was willing to convert the pictures into sound on their equipment free of charge as a public service. It is a tribute to Frank and Val’s combined ingenuity and expertise that within a week they had reduced the record time required to record a picture from close to a minute per picture to eight seconds per picture, had solved the problem of how to record in color, and had worked out a way of recording different pictures on each of the two stereo channels without getting crossover “ghost” images.
Our original proposal to NASA included this picture of two nude human beings to show recipients how our bodies look. We wanted to be neither sexist, pornographic, nor clinical. After looking through medical textbooks and anatomy books, we chose this photograph as the best and most inoffensive compromise. NASA refused to include it, perhaps because of possible adverse public reaction. We decided to keep the silhouette of this picture in the package because we felt that without it the continuity of the human reproduction sequence would be broken.
It was soon apparent that we could fit over a hundred pictures in the few minutes of the record allotted to the picture package. Even so, we didn’t know exactly how many pictures we would send. A message of only black-and-white pictures could have three times as many pictures as a message of only color pictures, since a color picture took three times as long to record as a black-and-white one. We decided to compromise and send a mix, but up until the last minute we didn’t know what the proportion of black and white to color would be. Where we felt color was vital (as in the solar spectrum) or desirable to give the best picture of our planet (by showing human skin tones or the color of trees, for example), we recorded in color. Of the one hundred and eighteen pictures in the finished package, twenty are in color.
The team that worked on selecting the pictures consisted of Frank Drake, me, Amahl Shakhashiri of the National Astronomy and Ionosphere Center, and Wendy Gradison, of the Laboratory for Planetary Studies at Cornell, who assisted in finding pictures and securing permissions. Technical support was provided by Dan Mitler, NAIC engineer, and Herman Eckelmann, the NAIC staff photographer, who had the tedious and frustrating job of photographing all pictures to format—and often rephotographing them several times as various problems came up. Eckelmann also took several photographs composed especially for the message. Draftsman Barbara Boettcher assisted me in preparing most of the diagrams in the package.
Wendy and I scoured the Cornell and local public libraries and amassed a stack of coffee-table and picture books that would have done credit to a major bookstore at Christmastime. The History of Toys, Birds of North America, The Family of Man, Plant-Devouring Insects, The Age of Steam, and a hundred others teetered in precarious stacks alongside every issue of the National Geographic back to 1958.
There were a few topics that we intentionally avoided. We reached a consensus that we shouldn’t present war, disease, crime, and poverty. It would be naive to deny the importance of these phenomena in human culture and history—after all, more human beings have killed one another or starved to death than have written string quartets. Yet we felt that we were making something that would survive us and our time—something that might be the only token of Earth the universe would have. We decided that the worst in us needn’t be sent across the galaxy. Also, we wanted to avoid any sort of political statement in this message, and a picture of Hiroshima or My Lai—or of a noble and heroic warrior, for that matter—seemed more an ideological statement than an integral part of an image of Earth. Nor did we want any part of the message to seem threatening or hostile to recipients (“Look how tough we are”), which is why we didn’t send a picture of a nuclear explosion.
Similarly, we decided not to include any picture that was specifically religious. The music of Bach or the ch’in piece on the record certainly convey something of human spirituality and our sense of awe, but there are so many human religions that if we had shown any, we felt we would have to give equal time to all. If we’d included a picture of a cathedral, we felt we would also have to include one of a mosque, a synagogue, a lamasery, and so forth. Since there was no way of explaining each religion, inclusion of all faiths would be merely a political sop to people on Earth viewing our work.
Finally, we decided not to include artwork—mostly because we didn’t feel competent to decide what art should be sent. A great deal of human art is shown in the music, which comprises the bulk of the record, but there was enough time to gather a panel of musicological experts to advise on balance and selection. We were so rushed in putting together the picture message that we couldn’t assemble experts in all the various visual arts and have them agree. And we thought extraterrestrials would have enough trouble interpreting photographs of reality or simple diagrams without our including a photograph of a painting, which is itself an interpretation of reality. Even though we have some acknowledged “great art” in the pictures (Ansel Adams, for example, is generally considered one of the world’s great photographers), the criterion for the picture message was informative, not aesthetic, value.
As we plowed through material, we began contacting individuals and organizations who had access to cross-indexed picture libraries that contained some of the subjects we were looking for. Of the greatest assistance was the National Geographic Society, which provided us with transparencies of published and unpublished material and was in general invaluable. In a way, they do routinely and on a larger scale what we were trying to do—give a full picture of Earth and its inhabitants. The UN picture library was also extremely cooperative. Sports Illustrated and NASA’s photo services were contacted as well for specific shots.
I found myself increasingly playing the role of extraterrestrial, a mental exercise I had done in fun for many years (while playing Frisbee, for example, I’d ask myself, “What would ETI make of this?”). Only now it was in earnest. I would look at pictures and try to imagine that I’d never seen the subject before. How could the photograph be misinterpreted? What was ambiguous? How could scale be deduced? That bird in the distance flying past the man, a wingtip partly obscured by the man’s outflung arm—I knew that the bird was a second creature in the distance, but if I didn’t, couldn’t it be a growth on the man’s arm? The late anthropologist and poet Loren Eiseley wrote perceptively that “one does not meet oneself until one catches the reflection from an eye other than human.” These words echoed in my mind during this whole process.
I was much influenced, in trying to adopt this mode of thought, by physicist Philip Morrison and by the science-fiction author Robert Heinlein. Each had pointed out in correspondence to us that the concept of “picture” as we understand it is by no means “universal” even on Earth, and that human beings from cultures that don’t use pictures have to be educated to the concept before they see photographs as Westerners do. How dangerous to assume that ETI could understand pictures, even if they were tremendously intelligent!
It may be an insoluble problem, especially in the unlikely case that those who find Voyager (whom I will refer to as “recipients”) have no sens
es as we understand them. In choosing pictures, we were faced with two contradictory demands: the pictures should contain as much information as possible, and they should be as easy to understand as possible. It seemed to me that one solution would be to have on board some pictures with very little information, primarily to help the recipients understand how to see pictures. So the first two pictures in the sequence are of objects elsewhere on Voyager—two of the engravings on the record’s cover. As engravings, they can be perceived by senses other than vision. We hope that beginning with these will give the recipients a way of comparing a photograph with an object they can touch.
It also occurred to me that silhouettes of photographs might be a kind of insurance. A silhouette maximizes the figure/background contrast and might show how we separate the various objects in a photograph by their outlines. It’s a way of saying “This is what we want you to see in this picture.” So in a number of places I drew silhouettes of photographs and inserted them in sequence.
In two cases, the pressure of events caused us to depart from a regular sequence of silhouette/photograph. Last-minute permission problems led to a change in the photograph of the fetus (originally there was a photograph of a fetus and embryo that matched the silhouette), and NASA decided not to include the photograph of two human nudes (who appear in silhouette in picture 32). We decided to keep this silhouette anyway because it showed that the fetus was inside the mother, and in the time that remained before the pictures were to be recorded (a few hours), we couldn’t find another representation. The vetoed photograph is reproduced on this page.
It also seemed a good idea to use recurrent images, such as the elephant in pictures 66 and 67 and the circles of people in pictures 36, 74, and 81. I’ve enumerated some of these “links” in the individual descriptions of pictures. It may be an interesting parlor game for people to find some of the others.
Here, in sequence, are the Voyager pictures:
1. Calibration Circle
Physicist Philip Morrison of M.I.T. suggested that the first picture be of some very simple geometrical form. Although we believe that the reconstruction of pictures from the audio signal should be simple for any civilization able to find Voyager in the first place, it seemed wise to begin with something easy. The diagram on the cover of the record, which shows how the audio signal is to be reconverted back into video, ends with a picture of a circle. Thus if recipients follow the instructions correctly, the first picture they reproduce will be the circle shown on the cover. This will tell them they are proceeding correctly. A circle also has the advantage of confirming the correct ratio of height to width in the picture raster.
2. Solar Location Map
In a way, this repeats the idea of the circle, since the solar location map, which shows the position of the sun relative to some astronomical “landmarks” called pulsars, also appears on the cover of the record. The original idea was to reproduce the whole map. But the resolution of the pictures on the record is equivalent only to the resolution of a television picture, and in that resolution the binary code, which gives the characteristic period of each pulsar, falls just below the resolution limit of the television picture. We wanted to make sure that the recipients recognize this picture as equivalent to the map on the cover, so the binary notation had to be clear. We therefore reproduced just a part of the map and added to it, as another reference point, a picture of M31, the Andromeda galaxy in its position relative to our sun at the time of the launch.
The Andromeda galaxy is our galaxy’s nearest large neighbor in space, and the close-up view of the core and dust lanes should make it an obvious landmark. In fact, if the recipient race is very old or has good astronomical measurements of the internal motions of Andromeda or of the motion of its companion galaxy M32, or has access to the records of some other very old civilization, this image of Andromeda may help date the age in which Voyager was launched. Andromeda should appear about the same from any viewing position in our own galaxy, and millions of years from now the pattern of stars and dust will have changed slightly in Andromeda. Recipients may be able to look up (or even remember!) when it was that Andromeda looked the way it does in our photograph; in fact, it may be the only object in the whole package of pictures that both we and the recipients have seen firsthand.
This picture also provides a check to the “handedness” of all the pictures—that is, it will tell the recipients which is supposed to be the left-hand side and which the right-hand. Since it must match the map on the cover and the real appearance of Andromeda in the sky, this picture assures recipients that they haven’t processed all the pictures backwards.
3. Dictionary—Mathematical Definitions
The cover of the record uses binary notation and units equal to the period of the 21-centimeter emission characteristic of neutral atomic hydrogen to tell recipients how fast to play the record, how to reconstruct the pictures, and which pulsars we are using as reference points. We wanted to superimpose measurement and notation on the pictures to indicate the sizes, weights, and so forth of various objects, but we felt that the “hydrogen binary” was too clumsy to use throughout. (For example, the length unit of the hydrogen emission is 21 centimeters; to express the diameter of the earth in units of 21 centimeters requires a very long binary number!) So our “dictionary” introduces a more convenient notation. The first picture shows groups of dots (one dot, two dots, three dots, and so forth) and their equivalent notation in both binary and Arabic numerals. We then show how our numbers are used in exponents, fractions, arithmetical operations, and so forth. So now if we want to say that something is 1½ units long, recipients will understand what we mean by 1½. In some cases, we give more than one example of a particular usage so there will be something to confirm their hypotheses.
4. Dictionary—Physical Unit Definitions
The second picture in our dictionary is a conversion table that defines common units in terms of the “hydrogen binary” units of length, time, and weight. The two drawings at the top of the picture represent the hydrogen atom (whose mass is 1 M) undergoing a change of energy states that emits radiation at a frequency which is the reciprocal of 1 t with a wavelength of 1 L. From these three units, we derive metric units of weight and distance (plus the angstrom as a distance unit in notating atomic and molecular diagrams); we invented a unit called e to measure one earth mass, and employed units of time (seconds and years). We had now defined a plethora of symbols, with more to come when we introduced symbols for elements, atomic number and nucleotide base pairs. We began to worry that things might be getting confusing, so to help matters a bit we distinguished all the units of measure by underlining them.
5 and 6. The Solar System
Frank Drake, who devised the dictionary, also did this diagram of the solar system, which shows our sun and planets along with the diameter, distance from the sun, mass, and period of rotation of each. The poor TV quality resolution of the pictures made it necessary to use two pictures for this. Coincidentally, since the first picture ends at Mars and the second begins at Jupiter, we also indicated the partitioning of the sun’s family into an inner solar system (composed of small and rocky planets) and an outer solar system (composed of large and gassy planets). Note that the recently discovered rings of Uranus, omitted in the Pioneer 10 and 11 plaques, have been included.
7. The Sun
This Hale Observatory photograph shows four views of the sun taken through different kinds of filters, showing sunspots and the granular texture of the surface. Together with the next picture this should indicate the nature of our star to extraterrestrials.
8. Solar Spectrum
Once extraterrestrials begin to reconstruct the pictures, they will notice a curious thing: most of the pictures are recorded in one burst of information, as shown on the cover. Yet about twenty of the pictures, beginning with this one, are recorded not once, but three times in a row. The only difference between the three is in the relative values of the grays. Recipients will wonder wh
at this means. The answer is that these triple-imaged pictures are the ones we recorded in color; each shot represents a color separation (similar to that used in photo-offset color printing) indicating the amount of red, blue, or green in each picture. But how do we tell them that?
The solution we came up with relies on a fact of stellar astronomy that should be common knowledge to astronomers throughout the galaxy. The atmosphere of a star, any star, contains elements that absorb some of the light the star is emitting. When you look at the spectrum of a star, you see that the rainbow pattern is broken by an array of dark lines called absorption lines. These lines have been studied and mapped with precision by astronomers; they serve as a kind of fingerprint that tells a great deal about the star emitting the light. In particular, they tell the precise temperature of the stellar surface, and thus how much light the star emits at each wavelength. The “color” of the star is given exactly by these lines. Their displacement by the Doppler shift (called the red shift when the star is moving away from the sun) tells about the motion of the stars and galaxies emitting the light and provides much of the observational basis for studies of other galaxies and of the structure of the universe as a whole.
We think that the absorption lines in the picture of the solar spectrum, even when reproduced in black and white, should shout out “G 2 star! G 2 star!” loud and clear. So the recipients will know that our sun is a G 2 star—and they should also know exactly what the spectrum of this kind of star “really” looks like in color. Even if the recipients’ eyes don’t utilize exactly the same portion of the electromagnetic spectrum we call visible light, the absorption lines will tell them that we wish to indicate something about this portion of the spectrum. Their task is to combine the three-color separations of the solar spectrum into one picture that presents the spectrum as it really is—as they know from their own observations of common yellow stars like the sun. They have to work backward from information they already know to understand our concept of color separations. Then they’ll be able to reconstruct the other color pictures correctly and see flowers, coral reefs, and skin tones in the colors of the original photographs.