In 1974, scientists transmitted the Arecibo message into deep space, containing a brief summary of humanity and Earth. To ensure intelligent extraterrestrials could decode it, the message was formatted as a stream of 1,679 binary digits. Because 1,679 is a semiprime number—the product of exactly two prime numbers, 23 and 73—an alien recipient would naturally realize the data should be arranged into a 23-by-73 grid, revealing the pixelated pictograms.
The Occasion and the Semiprime Architecture
On November 16, 1974, the Arecibo Observatory in Puerto Rico transmitted an interstellar radio message toward the globular star cluster Messier 13. The broadcast marked the completion of major upgrades to the observatory's 1,000-foot radio telescope, which included fitting a new high-precision reflector surface and a high-power radar transmitter. Astronomer Frank Drake, then at Cornell University and a pioneer in the Search for Extraterrestrial Intelligence (SETI), devised the message with assistance from Carl Sagan and several colleagues. Rather than sending continuous spoken language or an arbitrary picture, Drake designed a compact digital puzzle constructed strictly from mathematical principles.
The message consisted of exactly 1,679 binary digits, transmitted over roughly three minutes. The number 1,679 was chosen because it is a semiprime—a number that is the product of exactly two prime numbers, 23 and 73. If an extraterrestrial intelligence detected the transmission and recognized its mathematical property, there were only two ways to arrange the sequence into a rectangular grid without leftover elements: 23 rows by 73 columns, or 73 rows by 23 columns. When organized as 73 rows of 23 characters each, the binary pattern resolves into a coherent visual pictogram. Arranging it the opposite way produces an unreadable scatter of noise, providing an unambiguous visual cue for the correct layout.
Mathematical Foundations and the Chemistry of Life
The message is read from top to bottom and is divided into seven distinct sections. The opening section at the very top establishes a numerical system by displaying the numbers one through ten in binary format. Because binary notation in computer science can be read in different directions, Drake added a marker bit at the bottom of each column to indicate the start of the number and show which way the digits should be interpreted. By establishing numbers first, the message equipped the recipient with the mathematical key required to read all the dimensional values and quantities that follow in subsequent sections.
Directly below the numbers, the second section introduces the core chemical building blocks of terrestrial biology. It lists the atomic numbers of the five primary chemical elements found in deoxyribonucleic acid (DNA): hydrogen (1), carbon (6), nitrogen (7), oxygen (8), and phosphorus (15). Presenting these elements by their fundamental proton counts avoided any dependency on human naming conventions or Earth-specific units of measurement, relying instead on universal properties of atomic physics that any technologically capable civilization would share.
The Molecular Blueprint and the Human Form
The third and fourth sections build directly upon the chemical foundations to describe the mechanics of human genetics. The third section provides the chemical formulas for the sugars and bases that compose the nucleotides of DNA—specifically deoxyribose, adenine, thymine, guanine, and cytosine, linked by phosphate groups. Below these formulas, the fourth section depicts a stylized graphic of the DNA double helix. Running down the center of the helix is a binary number indicating the estimated number of base pairs in the human genome at the time of transmission, which was approximated at roughly 4.3 billion.
The fifth section transitions from the microscopic to the macroscopic by depicting the organism itself. At its center stands a simple stick figure representing a human being. Flanking the figure on the left is a representation of the average height of an adult human. This measurement was encoded not in meters or inches, but in relation to the physical wavelength of the transmission itself—12.6 centimeters. The graphic displays the binary number 14, which, when multiplied by the 12.6-centimeter carrier wavelength, yields an average human height of approximately 176 centimeters (5 feet 9 inches). On the right side of the human figure, another binary number conveyed the global human population in 1974, which stood at approximately 4.3 billion individuals.
Cosmic Origin and the Transmitting Instrument
The final two sections ground the transmission in physical space, showing where the message came from and how it was sent. The sixth section presents a sequential map of the solar system, starting with the Sun on the left and proceeding through all nine recognized planets of the era, from Mercury out to Pluto. To indicate which world was the source of the broadcast, the third planet—Earth—is drawn shifted upward out of alignment, positioned directly beneath the feet of the human figure in the section above it.
The seventh and concluding section depicts the Arecibo radio telescope itself. A curved graphic illustrates the primary reflector dish focusing incoming radio waves toward an overhead antenna feed. Directly beneath the graphic of the dish, another binary number indicates the physical diameter of the instrument: the number 2,430, which corresponds to 2,430 wavelengths of the 12.6-centimeter transmission signal, equaling roughly 306 meters (1,004 feet). This provided the recipient with a physical scale of the engineering apparatus responsible for the transmission.
Transmission Mechanics and Target Trajectory
The message was broadcast using frequency-shift keying on a main carrier frequency of 2,380 megahertz. The transmitter shifted the signal by 10 hertz back and forth to represent the zeros and ones of the binary stream. With a transmission rate of 10 bits per second, the entire sequence of 1,679 bits took approximately 169 seconds to transmit. When combined with the enormous focusing power and gain of the 305-meter dish, the effective isotropic radiated power of the broadcast reached roughly 20 terawatts, making it momentarily one of the brightest radio events in the galaxy within that narrow frequency band.
The transmission was aimed at Messier 13 (M13), a globular cluster containing hundreds of thousands of stars located near the edge of the Milky Way constellation Hercules. Messier 13 was selected largely because it was a dense, prominent target in the sky at the exact time and overhead coordinates required during the observatory's rededication ceremony. However, because M13 is located approximately 25,000 light-years from Earth, the signal will take 25,000 years to reach the cluster, and any potential response traveling at the speed of light would require an equal amount of time to return.
Demonstration, Legacy, and Scientific Context
Although the Arecibo message is frequently remembered as an earnest attempt at interstellar communication, its creators primarily intended it as a demonstration of human technical capability. The rededication event provided an opportunity to showcase the immense power of the upgraded dish rather than initiate a practical two-way dialogue. Because globular clusters are ancient stellar environments with low concentrations of heavy elements, modern astronomers consider M13 an unlikely location for terrestrial-type planets. Furthermore, because M13 is moving through space, the cluster will have shifted its position by the time the radio beam arrives in 25 millennia, meaning the beam may miss the core of the cluster altogether.
Despite its symbolic nature, the Arecibo message established a foundational framework for Active SETI—also known as METI (Messaging to Extraterrestrial Intelligence). By avoiding arbitrary language and deriving scales of time, length, and biology directly from universal constants, prime numbers, and fundamental chemistry, Frank Drake created a template for how civilizations might establish a common cognitive framework across interstellar distances.
Key takeaways
•The Arecibo message was encoded as a 1,679-bit stream whose semiprime length forces an alien recipient to arrange it into a 23-by-73 visual grid.
•Physical units like human height and dish diameter were expressed as multiples of the transmission's 12.6-centimeter radio wavelength rather than arbitrary human units.
•The 169-second transmission was broadcast on November 16, 1974, toward Messier 13 primarily to demonstrate the power of the newly upgraded Arecibo Observatory.