Arecibo Interstellar Message

To help inaugurate the powerful radio/radar telescope of the Arecibo Observatory in the tropical jungles of Puerto Rico, an interstellar message of friendship was beamed to the fringes of the Milky Way galaxy. On November 16, 1974, this interstellar radio signal was transmitted toward the Great Cluster in Hercules (Messier 13 or M13, for short), which lies about 25,000 light-years away from Earth. The globular cluster M13 contains about 300,000 stars within a radius of approximately 18 light-years.

This transmission, often called the Arecibo Interstellar Message, was made at the 2,380-megahertz (MHz) radio frequency with a 10 hertz (Hz) bandwidth. The average effective radiated power was 3 x 1012 watts (3 terawatts [TW]) in the direction of transmission. The signal is considered to be the strongest radio signal yet beamed out into space by our planetary civilization. Perhaps 25,000 years from now, a radio telescope operated by

Arecibo Observatory

The Arecibo Observatory is the world's largest radio/radar telescope, with a 1,000-foot- (305-m-) diameter dish. It is located in a large, bowl-shaped natural depression in the tropical jungle of Puerto Rico. This huge facility is the main observing instrument of the National Astron omy and Ionosphere Center (NAIC), a national center for radio and radar astronomy and ionospheric physics that is operated by Cornell University under contract with the National Science Foundation. The observatory operates on

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Arecibo Telescope
The Arecibo Observatory is the world's largest radio/radar telescope. The facility, located in a large, bowl-shaped natural depression in the tropical jungle of Puerto Rico, has a diameter of 1,000 feet (305 m). (NASA)

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a continuous basis, 24 hours a day every day, providing observing time and logistic support to visiting scientists.

When the giant telescope operates as a radio-wave receiver, it can listen for signals from celestial objects at the farthest reaches of the universe. As a radar transmitter/receiver, it assists astronomers and planetary scientists by bouncing signals off the Moon, off nearby planets and their satellites, off asteroids, and even off layers of Earth's ionosphere.

The Arecibo Observatory has made many contributions to astronomy and astrophysics. In 1965, the facility (operating as a radar transmitter/receiver) determined that the rotation rate of the planet Mercury was 59 days rather than the previously estimated value of 88 days. In 1974, the facility (operating as a radio-wave receiver) supported the discovery of the first binary pulsar system. This discovery led to an important confirmation of Albert Einstein's theory of general relativity and earned the American physicists Russell A. Hulse (1950- ) and Joseph H. Taylor, Jr., (1941- ) the 1993 Nobel Prize in physics. In the early 1990s, astronomers used the facility to discover extra-

solar planets in orbit around the rapidly rotating pulsar B1257+12.

In May 2000, astronomers used the Arecibo Observatory as a radar transmitter/receiver to collect the first-ever radar images of a main-belt asteroid named 216 Kleopatra. Kleopatra is a large, dog-bone-shaped minor planet about 135 miles (217 km) long and 58.4 miles (94 km) wide. Discovered in 1880, the exact shape of Kleopatra was unknown until early this century. Astronomers used the telescope to bounce radar signals off Kleopatra. Then, with sophisticated computer analysis techniques, the scientists decoded the echoes, transformed them into images, and assembled a computer model of the asteroid's shape. This activity was made possible because the Arecibo radio telescope underwent major upgrades in the 1990s—improvements that dramatically improved its sensitivity and made feasible the radar imaging of more distant objects in the solar system.

The Arecibo Observatory is also uniquely suited to search for signals from extraterrestrial life, by focusing on thousands of star systems in the 1,000 MHz to 3,000 MHz range. To date, no confirmed SETI signals have been found.

members of an intelligent alien civilization somewhere in the M13 cluster will receive and decode this interesting signal. If they do, they will learn that intelligent life had evolved here on Earth.

The Arecibo Interstellar Message of 1974 consisted of 1,679 consecutive characters. It was written in a binary format—that is, only two different characters were used. In binary notation, the two different characters are denoted as "0" and "1." In the actual transmission, each character was represented by one of two specific radio frequencies, and the message was transmitted by shifting the frequency of the Arecibo Observatory's radio transmitter between these two radio frequencies in accordance with the plan of the message.

The message itself was constructed by the staff of the National Astronomy and Ionosphere Center (NAIC). It can be decoded by breaking up the message into 73 consecutive groups of 23 characters each and then arranging these groups in sequence one under the other. The numbers 73 and 23 are prime numbers. Their use should facilitate the discovery by any alien civilization receiving the message that the above format is the right way to interpret the message. The figure on page 182 shows the decoded message: The first character transmitted (or received) is located in the upper right-hand corner.

This message describes some of the characteristics of terrestrial life that the scientific staff at the National Astronomy and Ionosphere Center felt would be of particular interest and technical relevance to an extraterrestrial civilization. The NAIC staff interpretation of the interstellar message is as follows.

The Arecibo message begins with a "lesson" that describes the number system being used. This number system is the binary system, where numbers are written in powers of 2 rather than of 10, as in the decimal system used in everyday life. NAIC staff scientists believe that the binary system is one of the simplest number systems and is particularly easy to code in a simple message. Written across the top of the message (from right to left)

This artist's rendering, entitled "Call," represents SETI alien style. The interesting painting shows an alien scientist operating a large radio transmitter/receiver telescope array on a planet in a distant solar system. The alien is sending radio signals out into the Milky Way galaxy in an attempt to make contact with another intelligent species. The intelligent being is also using the facility to listen patiently for any radio signals that may arrive from other intelligent civilizations. (© Pat Rawlings, the artist; used here with permission)

This artist's rendering, entitled "Call," represents SETI alien style. The interesting painting shows an alien scientist operating a large radio transmitter/receiver telescope array on a planet in a distant solar system. The alien is sending radio signals out into the Milky Way galaxy in an attempt to make contact with another intelligent species. The intelligent being is also using the facility to listen patiently for any radio signals that may arrive from other intelligent civilizations. (© Pat Rawlings, the artist; used here with permission)

Decoded form of the Arecibo Message of 1974. (Frank D. Drake and the staff of the National Astronomy and Ionosphere Center, operated by Cornell University under contract with the National Science Foundation)

Decoded form of the Arecibo Message of 1974. (Frank D. Drake and the staff of the National Astronomy and Ionosphere Center, operated by Cornell University under contract with the National Science Foundation)

Binary numbers 1-10

Atomic numbers of hydrogen, carbon, nitrogen, oxygen, and phosphorus

Chemical formulas for sugars and bases in nucleotides of deoxyribonucleic acid (DNA) molecule

Double helix of DNA

Human being

Height of human being

Size of human population Solar system with Earth displaced toward human

Arecibo telescope, which sent message

Diameter of telescope

Binary numbers 1-10

Atomic numbers of hydrogen, carbon, nitrogen, oxygen, and phosphorus

Chemical formulas for sugars and bases in nucleotides of deoxyribonucleic acid (DNA) molecule

Double helix of DNA

Human being

Height of human being

Size of human population Solar system with Earth displaced toward human

Arecibo telescope, which sent message

Diameter of telescope are the numbers 1 through 10 in binary notation. Each number is marked with a number label—that is, a single character, which denotes the start of a number.

The next block of information sent in the message occurs just below the numbers. It is recognizable as five numbers. From right to left these numbers are: 1, 6, 7, 8, and 15. This otherwise unlikely sequence of numbers should eventually be interpreted as the atomic numbers of the elements hydrogen, carbon, nitrogen, oxygen, and phosphorus.

Next in the message are 12 groups on lines 12 through 30 that are similar groups of five numbers. Each of these groups represents the chemical formula of a molecule or radical. The numbers from right to left in each case provide the number of atoms of hydrogen, carbon, nitrogen, oxygen, and phosphorus, respectively, that are present in the molecule or radical.

Since the limitations of the message did not permit a description of the physical structure of the radicals and molecules, the simple chemical formulas do not define in all cases the precise identity of the radical or molecule. However, these structures are arranged as they are organized within the macromolecule described in the message. Intelligent alien organic chemists somewhere in the M13 cluster should eventually be able to arrive at a unique solution for the molecular structures being described in the message.

The most specific of these structures, and perhaps the one that should point the way to interpreting the others correctly, is the molecular structure that appears four times on lines 17 through 20 and lines 27 through 30. This is a structure containing one phosphorus atom and four oxygen atoms, the well-known phosphate group. The outer structures on lines 12 through 15 and lines 22 through 25 give the formula for a sugar molecule, deoxyribose. The two sugar molecules on lines 12 through 15 have between them two structures: the chemical formulas for thymine (left structure) and adenine (right structure). Similarly, the molecules between the sugar molecules on lines 22 through 25 are: guanine (on the left) and cytosine (on the right).

The macromolecule or overall chemical structure is that of deoxyribonucleic acid (DNA). The DNA molecule contains the genetic information that controls the form, living processes, and behavior of all terrestrial life. This structure is actually wound as a double helix, as depicted in lines 32 through 46 of the message. The complexity and degree of development of intelligent life on Earth is described by the number of characters in the genetic code, that is, by the number of adenine-thymine and guanine-cytosine combinations in the DNA molecule. The fact that there are some 4 billion such pairs in human DNA is illustrated in the message by the number given in the center of the double helix between lines 27 and 43. Note that the number label is used here to establish this portion of the message as a number and to show where the number begins.

The double helix leads to the "head" in a crude sketch of a human being. The scientists who composed the message hoped that this would indicate connections among the DNA molecule, the size of the helix, and the presence of an "intelligent" creature. To the right of the sketch of a human being is a line that extends from the head to the feet of the "message human." This line is accompanied by the number 14. This portion of the message is intended to convey the fact that the "creature" drawn is 14 units of length in size. The only possible unit of length associated with the message is the wavelength of the transmission, namely about five inches (12.6 cm). This makes the creature in the message some five feet nine inches (176 cm) tall. To the left of the human being is a number, four billion. This number represents the approximate human population on planet Earth when the message was transmitted.

Below the sketch of the human being is a representation of humans's solar system. The Sun is at the right, followed by nine planets with some coarse representation of relative sizes. The third planet, Earth, is displaced to indicate that there is something special about it. In fact, it is displaced toward the drawing of the human being, who is centered on it. Hopefully, an extraterrestrial scientist in pondering this message will recognize that Earth is the home of the intelligent creatures that sent it.

Below the solar system and centered on the third planet is an image of a telescope. The concept of telescope is described by showing a device that directs rays to a point. The mathematical curve leading to such a diversion

You Say to a Little Green Man?

What Do

Suppose there is an intelligent extraterrestrial species out there, say 75 light-years away, who is not only capable of receiving our radio messages but also wants to communicate. If humans really are successful at SETI, just what will we talk about with this alien race? Remember, radio waves travel at the speed of light—so it will take 75 years for a message to travel each way in this hypothetical interstellar communication. There are going to be some very long pauses in this extraterrestrial "phone call." In fact, the first interstellar conversation with all the excitement it will cause on Earth might start something like this:

EARTH: This is Earth calling. Is anyone there? (—very long pause of about 150 years—) THEM: (possible response number one) Yes, we are here. Who (or what) are you? or

THEM: (possible response number two) Yes, someone is here. What do you want?

or finally

THEM: (possible response number three)

Yes, someone is here. Who and where are you?

Well, just how should the people of Earth respond to any of these hypothetical replies? Furthermore, who should speak for Earth?

Maybe we could just send out some basic data and news about the human race—much like the Arecibo interstellar message sent in 1974. But that is almost like a holiday greeting card with a little, nonpersonalized printed newsletter tucked inside. People of Earth really have to choose the items selected for transmission carefully to guarantee the durability of the information. For example, if scientists decide to send news about their latest technical achievements, after the passage of a century and a half or so, humans could become very embarrassed by the "backward" civilization that these data appear to represent. Just think of the technical changes that have taken place on Earth in the last century and a half. Perhaps human beings might decide to send of paths is crudely indicated. The telescope is not upside down but rather "up" with respect to the symbol for the planet Earth.

At the very end of the message, the size of the telescope is indicated. Here, it is both the size of the largest radio telescope on Earth and also the size of the telescope that sent the message (namely, the radio telescope at the Arecibo Observatory). It is shown as 2,430 wavelengths across, or roughly 1,000 feet (305 m). No one, of course, expects an alien civilization to have the same unit system that we use here on Earth—but physical quantities, such as the wavelength of transmission, provide a common reference frame.

This interstellar message was transmitted at a rate of 10 characters per second, and it took 169 seconds to transmit the entire information package. It is interesting to realize that just one minute after completion information about the current world situation or a collection of welcoming speeches from various terrestrial politicians and leaders. But we may quickly lose our extraterrestrial audience with such boring, domestic trivia. Even the hottest news items of today will in all likelihood become rather insignificant in the overall context of planetary history.

So what do the people of Earth say to the "little green men" (LGMs) who have been so patiently waiting for radio messages from other star systems. One scientist feels that humans could talk about mathematics, physics, and astronomy. The long pauses between exchanges might give subsequent generations of humans something to look forward to—as they await the next message from the stars. Another scientist has suggested that Earth send samples of human-created music, and, if advanced radio frequency or optical frequency communications techniques permit, even images of great works of art. Perhaps the creation and appreciation of beauty might serve as a common basis of communication between civilizations throughout the galaxy.

Along these lines, the first communication sequence between beings from Earth and an extraterrestrial civilization could boil down to something like:

EARTH: Hello! Is anyone there?

THEM: Yes, we're here. Who are we talking to?

EARTH: We are intelligent beings, called humans, from a place called Earth; listen to some of our civilization's most beautiful music. . . .

THEM: You've got a wrong number. (Click!)

Seriously, however, before the human race attempts to reach out and touch someone or something across interstellar space, people, acting in unison as a planetary society, must think about how we might respond to an interstellar message that invites Earth's civilization to "chat." Who should speak for the people of planet Earth? What information should they provide about Earth? And what questions should they ask about the alien civilization?

of transmission, the interstellar greetings passed the orbit of Mars. After 35 minutes, the message passed the orbit of Jupiter, and after 71 minutes, it silently crossed the orbit of Saturn. Some 5 hours and 20 minutes after transmission, the message passed the orbit of Pluto, leaving the solar system and entering "interstellar space." The Arecibo interstellar message will be detectable by telescopes anywhere in the Milky Way galaxy of approximately the same size and capability as the terrestrial facility that sent it.

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Responses

  • JASMINE
    How much power did the Arecibo telescope use to send an interstellar message?
    5 months ago

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