Cosmic Connection (3 page)

When I look out into the night sky, I cannot discern the outline of a lion in the constellation Leo. I can make out the Big Dipper, and, if the night is clear, the Little Dipper. I am at a loss to make out much of a hunter in Orion or a fish in the constellation of Pisces, to say nothing of a charioteer in Auriga. The mythical beasts, personages, and instruments placed by men in the sky are arbitrary, not obvious. There are agreements about which constellation is which–sanctioned in recent years by the International Astronomical Union, which draws boundaries separating one constellation from another. But there are few clear pictures in the sky.

These constellations, while drawn in two dimensions, are fundamentally in three dimensions. A constellation, such as Orion, is composed of bright stars at considerable distances from Earth and dim stars much closer. Were we to change our perspective, move our point of view–with, for example, an interstellar space vehicle–the appearance of the sky would change. The constellations would slowly distort.

Largely through the efforts of David Wallace at the Laboratory for Planetary Studies at Cornell University, an electronic computer has been programmed with the information on the three-dimensional positions from the Earth to each of the brightest and nearest stars–down to about fifth magnitude, the limiting brightness visible to the naked eye on a clear night. When we ask the computer to show us the appearance of the sky from Earth, we see results of the sort displayed in the accompanying figures: One for the northern circumpolar constellations, including the Big Dipper, the Little Dipper, and Cassiopeia; one for the southern circumpolar constellations, including the Southern Cross; and one for the broad range of stars at middle celestial latitudes, including Orion and the constellations of the zodiac. If you are not a student of the conventional constellations, you will, I believe, have some difficulty making out scorpions or virgins in the picture.

We now ask the computer to draw us the sky from the nearest star to our own, Alpha Centauri, a triple-star system, about 4.3 light-years from Earth. In terms of the scale of our Milky Way Galaxy, this is such a short distance that our perspectives remain almost exactly the same. From α Cen the Big Dipper appears just as it does from Earth. Almost all the other constellations are similarly unchanged. There is one striking exception, however, and that is the constellation Cassiopeia. Cassiopeia, the queen of an ancient kingdom, mother of Andromeda and mother-in-law of Perseus, is mainly a set of five stars arranged as a W or an M, depending on which way the sky has turned. From Alpha Centauri, however, there is one extra jog in the M; a sixth star appears in Cassiopeia, one significantly brighter than the other five. That star is the Sun. From the vantage point of the nearest star, our Sun is a relatively bright but unprepossessing point in the night sky. There is no way to tell by looking at Cassiopeia from the sky of a hypothetical planet of Alpha Centauri that there are planets going around the Sun, that on the third of these planets there are life forms, and that one of these life forms considers itself to be of quite considerable intelligence. If this is the case for the sixth star in Cassiopeia, might it not also be the case for innumerable millions of other stars in the night sky?

One of the two stars that Project Ozma examined a decade ago for possible extraterrestrial intelligent signals was Tau Ceti, in the constellation (as seen from Earth) of Cetus, the whale. In the accompanying figure, the computer has drawn the sky as seen from a hypothetical planet of T Cet. We are now a little more than eleven light-years away from the Sun. The perspective has changed somewhat more. The relative orientation of the stars has varied, and we are free to invent new constellations–a psychological projective test for the Cetians.

I asked my wife, Linda, who is an artist, to draw a constellation of a unicorn in the Cetian sky. There is already a unicorn in our sky, called Monoceros, but I wanted this to be a larger and more elegant unicorn–and also one slightly different from common terrestrial unicorns–with six legs, say, rather than four. She invented quite a handsome beast. Contrary to my expectation that he would have three pairs of legs, he is quite proudly galloping on two clusters of three legs each, one fore and one aft. It seems quite a believable gallop. There is a tiny star that is just barely seen at the point where the unicorn’s tail joins the rest of his body. That faint and un-inspiringly positioned star is the Sun. The Cetians may consider it an amusing speculation that a race of intelligent beings lives on a planet circling the star that joins the unicorn to his tail.

When we move to greater distances from the Sun than Tau Ceti–to forty or fifty light-years–the Sun dwindles still further in brightness until it is invisible to an unaided human eye. Long interstellar voyages–if they are ever undertaken–will not use dead-reckoning on the Sun. Our mighty star, on which all life on Earth depends, our Sun, which is so bright that we risk blindness by prolonged direct viewing, cannot be seen at all at a distance of a few dozen light-years–a thousandth of the distance to the center of our Galaxy.

M
ankind’s first serious attempt to communicate with extraterrestrial civilizations occurred on March 3, 1972, with the launching of the
Pioneer 10
spacecraft from Cape Kennedy.
Pioneer 10
was the first space vehicle designed to explore the environment of the planet Jupiter and, earlier in its voyage, the asteroids that lie between the orbits of Mars and Jupiter. Its orbit was not disturbed by an errant asteroid–the safety factor was estimated as 20 to 1. It approached Jupiter on December 3, 1973, and then was accelerated by Jupiter’s gravity to become the first man-made object to leave the Solar System. Its exit velocity is about 7 miles per second.

Pioneer 10
is the speediest object launched to date by mankind. But space is very empty, and the distances between the stars are vast. In the next 10 billion years,
Pioneer 10
will not enter the planetary system of any other star, even assuming that all the stars in the Galaxy have such planetary systems. The spacecraft will take about 80,000 years merely to travel the distance to the nearest star, about 4.3 light-years away.

But
Pioneer 10
is not directed to the vicinity of the nearest star. Instead, it will be traveling toward a point on the celestial sphere near the boundary of the constellations Taurus and Orion, where there are no nearby objects.

It is conceivable that the spacecraft will be encountered by an extraterrestrial civilization only if such a civilization has an extensive capability for interstellar space flight and is able to intercept and recover such silent space derelicts.

Placing a message aboard
Pioneer 10
is very much like a shipwrecked sailor casting a bottled message into the ocean–but the ocean of space is much vaster than any ocean on Earth.

When my attention was drawn to the possibility of placing a message in a space-age bottle, I contacted the
Pioneer 10
project office and NASA headquarters to see if there were any likelihood of implementing this suggestion. To my surprise and delight, the idea met with approval at all steps up the NASA hierarchy, despite the fact that it was–by ordinary standards–very late to make even tiny changes in the spacecraft. During a meeting of the American Astronomical Society in San Juan, Puerto Rico, in December 1971, I discussed privately various possible messages with my colleague Professor Frank Drake, also of Cornell. In a few hours we decided tentatively on the contents of the message. The human figures were added by my artist wife, Linda Salzman Sagan. We do not think it is the optimum conceivable message for such a purpose: There were a total of only three weeks for the presentation of the idea, the design of the message, its approval by NASA, and the engraving of the final plaque. An identical plaque has been launched in 1973 on the
Pioneer 11
spacecraft, on a similar mission.

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.