The Pioneer 10 Plaque

On the title page of this chapter is shown the message. It is etched on a 6-inch by 9-inch gold-anodized aluminum plate, attached to the antenna support struts of Pioneer 10. The expected erosion rate in interstellar space is sufficiently small that this message should remain intact for hundreds of millions of years, and probably for a much longer period of time. It is, thus, the artifact of mankind with the longest expected lifetime.

The message itself intends to communicate the locale, epoch, and something of the nature of the builders of the spacecraft. It is written in the only language we share with the recipients: Science. At top left is a schematic representation of the hyperfine transition between parallel and antiparallel proton and electron spins of the neutral hydrogen atom. Beneath this representation is the binary number 1. Such transitions of hydrogen are accompanied by the emission of a radiofrequency photon of wavelength about 21 centimeters and frequency of about 1,420 Megahertz. Thus, there is a characteristic distance and a characteristic time associated with the transition. Since hydrogen is the most abundant atom in the Galaxy, and physics is the same throughout the Galaxy, we think there will be no difficulty for an advanced civilization to understand this part of the message. But as a check, on the right margin is the binary number 8 (1---) between two tote marks, indicating the height of the Pioneer 10 spacecraft, schematically represented behind the man and the woman. A civilization that acquires the plaque will, of course, also acquire the spacecraft, and will be able to determine that the distance indicated is indeed close to 8 times 21 centimeters, thus confirming that the symbol at top left represents the hydrogen hyperfine transition.

Further binary numbers are shown in the radial pattern comprising the main part of the diagram at left center. These numbers, if written in decimal notation, would be ten digits long. They must represent either distances or times. If distances, they are of the order of several times 1011 centimeters, or a few dozen times the distance between the Earth and the Moon. It is highly unlikely that we would consider them useful to communicate. Because of the motion of objects within the Solar System, such distances vary in continuous and complex ways.

However, the corresponding times are on the order of 1/10 second to 1 second. These are the characteristic periods of the pulsars, natural and regular sources of cosmic radio emission; pulsars are rapidly rotating neutron stars produced in catastrophic stellar explosions (see Chapter 38). We believe that a scientifically sophisticated civilization will have no difficulty understanding the radial burst pattern as the positions and periods of 14 pulsars with respect to the Solar System of launch.

But pulsars are cosmic clocks that are running down at largely known rates. The recipients of the message must ask themselves not only where it was ever possible to see 14 pulsars arrayed in such a relative position, but also when it was possible to see them. The answers are: Only from a very small volume of the Milky Way Galaxy and in a single year in the history of the Galaxy. Within that small volume there are perhaps a thousand stars; only one is anticipated to have the array of planets with relative distances as indicated at the bottom of the diagram. The rough sizes of the planets and the rings of Saturn are also schematically shown. A schematic representation of the initial trajectory of the spacecraft launched from Earth and passing by Jupiter is also displayed. Thus, the message specifies one star in about 250 billion and one year (1970) in about 10 billion.

The content of the message to this point should be clear to an advanced extraterrestrial civilization, which will, of course, have the entire Pioneer 10 spacecraft to examine as well. The message is probably less clear to the man on the street, if the street is on the planet Earth. (However, scientific communities on Earth have had little difficulty decoding the message.) The opposite is the case with the representations of human beings to the right. Extraterrestrial beings, which are the product of 4.5 billion years or more of independent biological evolution, may not at all resemble humans, nor may the perspective and linedrawing conventions be the same there as here. The human beings are the most mysterious part of the message.

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In the same way, the relative distances of the planets from the Sun, shown by binary notation at the bottom of the plaque, indicate that we use base-10 arithmetic. From the fact that we have 10 fingers and 10 toes – drawn with some care on the plaque – I hope any extraterrestrial recipients will be able to deduce that we use base-10 arithmetic and that some of us count on our fingers. From the stumpiness of our toes they may even be able to deduce that we evolved from arboreal ancestors.

Carl Sagan describes everything that went into the symbolism of this message sent to the stars.